Grounding Fundamentals Course Presentation

Grounding Fundamentals

Instructor: Allan Bozek, P.Eng.www EngWorks cawww.EngWorks.ca

1 5 EICCEUs1. 5 CEUs

Introduction

• Introductions• Introductions• Please introduce yourself – name, job title and

experienceexperience• Sign-in sheet circulated, everyone please sign

in and return• Emergency response requirements• Please turn off all cell phones or turn to silentPlease turn off all cell phones or turn to silent

mode• Washrooms and Breaks

2www.EngWorks.ca Grounding Fundamentals 2

Safety TopicStatic Electricity and Refuellingy g

www.EngWorks.ca Grounding Fundamentals 3

Safety TopicStatic Electricity and Refuellingy g Some statistics Petroleum Equipment Institute reports 175 fires since

19921992 50% of the accidents occurred when the refueler returned

to their vehicle Women account for 75% of all static ignition firesWomen account for 75% of all static ignition fires

Safety Guidelines when refueling Turn off engine

D 't k Don't smoke Never re-enter your vehicle while refueling. Do not overfill or top off your tank

If a fire starts Do not remove the nozzle from the vehicle or try to stop

the flow of gasoline. Immediately leave the area and call g yfor help


Learning Objectives1. To understand why we ground2 To describe the difference between grounding and2. To describe the difference between grounding and

bonding3. To apply the safety requirements as defined by the3. To apply the safety requirements as defined by the

Canadian Electrical Code and the IEEE as they relate to grounding

4. To select the appropriate systems grounding scheme for an industrial facility Sizing of components How it impacts the overall design of a facility


Learning Objectives5. To implement a static electricity control and

lightning protection systemg g p y6. To avoid the problems typically associated with the

grounding of sensitive electronic systemsg g y Ground loops Methods of noise mitigation

7. To design a ground grid for a high voltage industrial substation Concept of ground potential rise and touch and step

potential


Agenda Overview1. History of Grounding2 System grounding2. System grounding Generator and UPS systems grounding

3 Equipment bonding3. Equipment bonding4. Static Protection 5 Lightning Protection5. Lightning Protection6. Grounding of Electronic and Instrumentation

SystemsSystems7. Station Ground Grid Systems Design8 T t i l8. Tutorialwww.EngWorks.ca Grounding Fundamentals 7

Introduction

Section 1

Edison's Pearl Street Generation Station

Pearl Street Generation station was initiallystation was initially constructed in 1882 to provide DC current for lighting systems in New York's financial district


Edison’s Floating Approach to DC Systemsy Original design used an earth ground for DC

lighting systemslighting systems Several incidents associated with “stray

currents” forced Edison to revise his plancurrents forced Edison to revise his plan One dead horse

W k b th ti t ti ld f l Workers nearby the generating station could feel the currentBelieved the there was a “devil in the wire”Believed the there was a devil in the wire


Edison’s Floating Approach to DC Systemsy

Current Flow

G LL

+

Gen LLIntendedReturnPath

-

UnintendedReturn Path


Human Sensitivity to Electricity

Physiological Reaction to y gElectric Current Range from minor muscular

contraction to ventricular fibrillationFunction of body weighty gCurrent magnitudeCurrent duration

H b d b id d 1000Ω i t


Human body can be considered a 1000Ω resistor

Human Sensitivity to Electricity

Direct Current Alternating Current

Human Response (ma)g

(ma)Men Women Men Women

Slight Sensation on Hand 1 0.6 0.4 0.3“Let Go” Threshold 6.2 3.5 1.1 0.7Shock – Not Painful 9 6 1.8 1.2Painful Shock – Muscular 62 41 9 6Control LossSevere Shock –Breathing Difficult

90 60 23 15


Edison's’ Isolated 3 Wire System Edison later adopted a 3-

wire system that did not Positiveyrely on a earth path for return G1 LL

+100V

Allowed two circuits to be run with three wires

Circuit was isolated from

G1-

+

Neutral200V

Circuit was isolated from ground

All currents within the circuit G2 LL

+

-100V

could be measured and accounted for Negative


Shock Current Path A shock current path

requires two pointsSingle point ofcontactq p

One point for the current to enter and the second to exitto exit

Voltage difference is required for current to flo

G1 L

N lflow An isolated system under

normal operating conditions

Neutral

Isolated GroundS tinsures a single point of

contactSystem

No Shock Current Exists


Shock Current Path Under fault conditions,

an isolated system Single point oft tA id t lan isolated system

ground creates a shock hazard

contactAccidentalGround

G1 LNeutral

Alternate circuit pathleads to shock hazardleads to shock hazard


Ground Fault Detection An isolated system cannot detect the presence

of a ground faultof a ground fault

Fuse

AccidentalGroundCircuit protection

cannot detectG1 L

Neutral

the accidentalground


System Overvoltage and Surges An isolated system cannot dissipate a high

voltage surgevoltage surge Usually results in equipment damage

Lightning

Equipment insulationis stressed as the

Fuse

LightningStrike

is stressed as the high voltage surge finds its way to

G1 L

Neutralground


The Intentional Grounding of Circuits Elihu Thompson Founder of Thompson-Houston

IndustriesLater merged with Edison General and

became General Electric Author of over 700 patents

Advocated AC systems should be intentionally earthedbe intentionally earthed Proposed as a safeguard against a

breakdown in insulation of a primary circuit conductor

Proposal created a large amount of controversyy


Grounding Premise An intentionally grounded circuit provides a circuit

path back to the source in the event of an paccidental ground

Allows the circuit protective devices to function ppreventing the circuit from becoming a safety hazard

Low Impedancepath to source G1 L

Fuse

AccidentalGround

path to sourceallows fuse tooperate

G1 L

Neutral


History of Grounding

Practice of earthing the secondary (neutral) conductor was banned by the New York Board ofconductor was banned by the New York Board of Fire Underwriters Speculation that Thomas Edison was behind the p

scenes with his patented 3 wire un-grounded circuit AIEE (Precursor to the IEEE) recommended that

low voltage AC systems be grounded where alow voltage AC systems be grounded where a reliable ground connection could be secured Advocated a solid connection without a fuse on the

t l ineutral wire


History of Grounding NFPA later resolved that grounding the secondary

circuit was the only way of absolutely insuring the f t f th i itsafety of the circuit The debate continued from 1903 – 1913 when it was

passed into law Secondaries of all circuits 550V or less must be groundedRecommended that all circuits 300V or less be grounded

Original rule has not been changed in substance Original rule has not been changed in substance since the original 1913 rule in the NEC Section 10 of the CEC Part 1 also adheres to the

fundamental premise of the rule


Canadian Electrical Code - Part 1 CSA C22.1-06

Minimum safety standards for installation and maintenance of electrical equipment Compliance will ensure a

safe installationSection 10 deals Section 10 deals specifically with grounding and bondingand bonding Significant re-write in 2006 Minor updates in 2009p


Canadian Electrical Code - Part 1 CSA C22.1-06Scope and object: Rules 10-000 and Rule 10-002Protect life from the danger of shockgLimit the voltage on a circuitFacilitate operation of protective devices

System and circuit grounding: Rules 10-100 to 10-116All circuits must be grounded with the exception of:Electric Arc furnacesCranes installed in Class III locationsCranes installed in Class III locations Isolated systems in patient care areasCircuits less than 50V


CEC Handbook Provides background

information and commentary th R l f th C dion the Rules of the Canadian

Electrical Code, Part I Intended to provide a clearer Intended to provide a clearer

understanding of the safety requirements of the CodeI t i f ti Incorporates information on: Rational IntentIntent Field Considerations


IEEE Standard 142(Green Book)( )

Recommended practices and methods associated with grounding Systems grounding Equipment grounding and bonding Equipment grounding and bonding Static and lightning protection Grounding electrode design Grounding of electronic equipment

Applies to industrial and commercial power systemscommercial power systems Utility grounding methods are not

covered Recommended Purchase


Commonly Used Grounding Terms and Definitions

Neutral Point Neutral Conductor

Metallic

Neutral GroundDevice

MetallicEnclosure

Grounding Conductor

Bonding ConductorStray Current

EarthGroundingElectrode


Earth Conducting body of varying resistance Earthing – A connection to earthEarthing A connection to earthInterchangeable with the term ground

Earth


Earth

Ground A conducting connection by which an electrical circuit

is connected to earth Grounding Electrode – a conductor buried in earth and

used for collecting or dissipating ground current to earth Grounding Conductor – conductor used to connect the g

service equipment to a ground electrode

Grounding G gConductor

Grounding


GroundingElectrode

Bonding Low impedance path created by joining all non-

current-carrying metal parts to ensure electrical ti itcontinuity

Bonding Conductor – conductor that connects the non-current carrying parts of electrical equipment, raceways or enclosures

B di C d tBonding Conductor(Equipment ground conductor)


Neutral Point The point of a symmetrical system which is normally

at zero voltageg Neutral Conductor – a system conductor, other than a

phase conductor that provides a return path for current to the sourcethe source

Neutral Point

Neutral GroundNeutral ConductorDevice


Definitions Ground Fault Current – ground current resulting

from any phase-conductor-to-earth faulty p Normal – brief flow of current that occurs until the

protective device opens Abnormal – continuous flow of current from a phase

conductor to ground Often referred to as the Zero Sequence Current Often referred to as the Zero Sequence Current

Neutral grounding devices include grounding resistors, grounding transformers, ground-faultresistors, grounding transformers, ground fault neutralizers, reactors, capacitors, or a combination of these components


Ground Fault Current

Metallic

IntendedG d F lt

MetallicEnclosureNeutral Ground

Device Ground FaultCurrent Path Ground Fault

Earth


Stray Current The uncontrolled flow of current over and through

the earth results in undesired safety and system performance

characteristics

Stray Current

Earth

Stray Current


Earth

Systems Grounding

Section 2

Purpose of a Systems Ground

“System grounding, or the intentional connection f h t l d t t th i fof a phase or neutral conductor to earth, is for

the purpose of controlling the voltage to earth, or ground within predictable limits”or ground, within predictable limits

Most system faults are ground fault related

IEEE 142 Green Book

Most system faults are ground fault related


Systems Ground A systems ground will: Control the voltage to ground to prevent stressing g g p g

equipment insulation Allow the operation of ground fault detection protection

d idevices Reduce the risk of fire and shock hazard to persons who

might come in contact with live conductorsmight come in contact with live conductors In some cases provide service continuity

Allow the ground fault to be isolated and repaired at a convenient titime


Concept of a System Ground A grounding system

consists of all SystemsGround 1

interconnected grounding connections in a specific

YGround 1

SystemsGround 2

power system and is isolated from adjacent; grounding systems through Y

YYSystemsgrounding systems through

a high impedance Isolation occurs via an M M M

Y Y

Y

SystemsGround 3

Isolation occurs via an ungrounded transformer winding connection

PPSystemsGround 4


Transformer Winding Connections ∆ (delta) Connections Isolates the power system p y

from ground Important is creating “zones of

protection”

Y (wye) Connections Y point provides a neutral point

for managing ground faultsfor managing ground faults Opportunity for multiple

voltages


System Grounding Classifications

Ungrounded Solid Ground

Resistance Ground Reactance Ground


System Grounding Classifications

Systems Grounding

Ungrounded Grounded

ImpedanceGrounded

SolidGrounded

Resistance Reactance

LowResistance

HighResistance

Reactance TunedReactance


Reactance

Ungrounded Historically was used on power systems where a

high level of process continuity was requiredg p y q Exists in many process facilities designed prior to 1980

Advantagesg Single ground fault does not does not allow current to flow

Allowed for a controlled shutdown for fault repairs

f Eliminates the need for elaborate protection schemes Grounding system cost is minimized A

NG B

C


Ungrounded Disadvantages On a ground faults, the voltage to ground for the remaining g , g g g

phases is elevated by 73%Higher insulation rating required for system components

T i t lt b bl Transient overvoltages can be a problemVoltages up to 6X system voltage stresses insulation eventually

leading to a second ground fault and subsequently a phase to phase fault


Ground Fault Voltage Shift

Normal Operating Conditions A

A

IA

N

VAG

IA IB

BCIB

I

N

A

BC

VCG VBGIC

CA CB CC

IC

NVANVCN

A

VCAVAB

VAG

GVBN

If CA = CB = CC then IA+ IB + IC = 0 BC

GVCA AB

VBC

VCGVBG

N


A B C A B C

Ground Fault Voltage Shift

Ground Fault Phase C A

VAG

IB

IG

A

VBGB

C G VCG=0

IA

B

VB

IA

I

N

A

VCA

VANC

CA CB

IBNVBN

IG

IA + IB = IGBC

CAVAB

V

VCG=0

VAG NG

VCN


BCG VBC

VBG

Intermittent Ground Faults Intermittent or restriking

type ground faults on A yp gisolated grounded systems can cause severe

A

B

IA

N

system overvoltages Up to 6 or 8 times line to

line voltage

BC

CA CB

IB

IG line voltage Will eventually lead to an

insulation failure resulting in G Breakdown in insulation ga phase to phase fault

Must be detected and corrected ASAP

results in phase to phase fault IG = ISC

corrected ASAP


Ungrounded System Ground Fault Detection Scheme10-106 Alternating-current systems (see Appendix B)(2) Wiring systems supplied by an ungrounded supply shall ( ) g y pp y g pp ybe equipped with a suitable ground detectiondevice to indicate the presence of a ground fault.

Ground

L L L 0VLight DimsOrExtinguishes

Fault

Extinguishes


Ground fault Detection Scheme

Solid Ground A solid grounded system is one in which the neutral

points have been intentionally connected to earth points have been intentionally connected to earth ground with a conductor having no intentional impedance Often referred to as effective grounding

N

A

NG B

C


Solid Ground

Uniground SystemUsed in Industrial Systems

Multi-grounded SystemUsed by Utilities in Rural

Distribution SystemsDistribution Systems


Solid Ground Advantages Partially reduces the problem of transient over-voltages

R d d i l ti l l i dReduced insulation level required Ground faults do not shift the system neutral Simple ground relay schemes provide for circuit protection

Disadvantages Damage at the fault may be excessive Arc flash hazard due to high ground fault current levels Difficult to coordinate ground fault protection

Magnitude of the fault current is unknown


CEC DefinitionEffective Grounding - a path to ground from circuits,

equipment, or conductor enclosures that is q p ,permanent and continuous and has carrying capacity ample to conduct safely any currents liable to be imposed upon it

CEC Rule 10-500 in Appendix B states that the complete fault path of the circuit conductor together with the bonding returnpath of the circuit conductor, together with the bonding return, should have an impedance that allows at least five times the current setting of the overcurrent device to flow when a fault of negligible impedance occurs


Solid Ground

A

NBC

N

IO/C fuse may notVAN

NVBN

VCN

IGO/C fuse may notclear arcing ground fault

HighG

HighImpedanceGround fault


High Resistance Ground System is grounded through a high-impedance

resistor High-impedance grounding typically limits ground fault

current to 25 A or less Typically used on low voltage (600V or less) systems

under 3000 Amps

N

A

G BC

2 - 25A


High Resistance Ground Scheme

1000 KVA Xfmr25kV 600V

51G AL25kV – 600V5.75% ZY 5 Amp

NGR

NGR5A

75kVAStarter

45kVA

5A

M MMLP HTPU

/H

25HPInjection

Pump

75HPRecyclePump30kWLighting

150HPCooling

Fan Heat Trace

75kVA

X 2X 2

Ground


u pPumpUnit

HeaterPanel

Fan Heat TracePanelFault

High Resistance Ground Advantages Allows system to operate under a ground fault conditiony p g Reduces arc flash energy associated with a ground fault Insures a ground fault of a known magnitude

Aids in protective relay coordination and limiting equipment damage

Reduces transient ground fault overvoltagesReduces transient ground fault overvoltages Allows easy identification and isolation of the ground fault

location Disadvantages Neutral shift on ground fault


Low Resistance Ground System is grounded through a low-impedance

resistor Low-resistance grounding typically limits ground fault

current to 400A or less for a short period of time (10 sec) Typically used on medium and high voltage industrial

power distribution systems

N

A

G BC

25 - 400A


Low Resistance Ground Advantages Allows protective relay devices to quickly clear a ground p y q y g

fault Limits damage to equipment and reduces overheating and

h i l t d tmechanical stress on conductors Disadvantages

Ne tral oltage shift of limited d ration Neutral voltage shift of limited duration


Low Resistance Ground Scheme

Trip Upstream Breaker

Y

51

NGR

Trip setting ~ 20%of NGR rating

Y400A NGR

13.8kV

NGR400A Trip Downstream Breaker

M51

Y600V5A NGRSGR

(Secondary GroundResistor)

XFMR


M MAlternate Arrangement

LR Grounding Resistor

Connection to Neutral Point on Transformer

Connection to ground

Resistors

Current Transformer

51

NGR400A


LR Grounding Transformer

Ground resistor

SGRXFMR

51

SGR(Secondary Ground

Resistor)

XFMR

Grounding Transformer


Alternate Grounding Schemes Corner-of-the-Delta System Applicable to low-voltage I

A

pp gsystems

Not widely used in industrial t

B

Csystems

Delta One Phase Grounded

I

Delta One Phase Grounded at Midpoint Applicable to single phase

240VApplicable to single phase 120/240V loads

G 240V

120V120V


Reactance Grounding Ground fault current is a

function of the neutral reactance Typically results in higher

N

ground fault currents than a resistance grounded system25 – 60% of three phase fault

51

Reactor5 60% o t ee p ase au tcurrent

Primarily used by Utilities on multi grounded systems on

IG

multi-grounded systems on systems above 5kV

Seldom used in industrial plant applications


Resonant Grounding Tuning reactor is used to ground

the neutral point to ground Reactor is tuned to match the system

capacitanceResults in a very low value of ground

N

fault current 75% of line to ground faults are self-

extinguishingC l t l i d t

51

Reactor Complex controls are required to

constantly match the reactance to the system capacitanceP i il d h d d I

Ground FaultNeutralizer

Primarily used on overhead and transmission lines above 15kV

Rarely used in industrial applications

IG


Grounding System Comparison

Condition Un-grounded

Solid Ground

Low Resistance

High Resistanceg

Immunity to transient overvoltages Worst Good Good Best

Arc Fault DamageArc Fault Damage Protection Worst Poor Better Best

Safety to Personnel Worst Better Good Best

Service Reliability Worst Good Better Best

Continued operation Better Poor Poor Bestafter initial ground fault Better Poor Poor Best

Ground fault locating Not Possible Good Better Best


Ground Fault Sensing


Ground Fault SensingGround ReturnGround fault current isGround fault current is

measured in the neutral to ground connection Phase A

Applicable only at a source transformer or generator 51G Phase B

Phase C

Neutral

gOften used for ground

fault alarm sensing on LV di t ib ti tdistribution systems


Ground Fault SensingZero Sequence RelayMeasures zero sequence orMeasures zero sequence or

ground currents by sensing the magnetic fields surrounding th h d t l

Phase A

the phase and neutral conductorsShould cancel under normal

Phase BPhase C

Neutral

conditions

Often used in motor protection and feeder breaker relays 51Gand feeder breaker relays


Ground Fault SensingDifferentialPhase current and neutralPhase current and neutral

current values are measured and ground fault current is

l l t d th diffPhase A

calculated as the differenceUsed in applications where

current transformers are Phase BPhase C

Neutral

required for phase overcurrent relaysHi h i d t ti

51G

Phase C

High accuracy in detecting ground faults


High Resistance Ground Detection Scheme

1000 KVA Xfmr25kV 600V

51G AL

25kV – 600V5.75% ZY 5 Amp

NGRPulsing ResistorNGR5A

Pulsing readingon phase indicates 75kVA

Starter45kVA

ClampOn CTon phase indicates

ground faultM MM

LP HTPU/H

25HPInjection

Pump

75HPRecyclePump30kWLighting

150HPCooling

Fan Heat Trace

75kVA

X 2X 2

Ground


u pPumpUnit

HeaterPanel

Fan Heat TracePanelFault

High Resistance Ground fault Detection Systemy


Ground Fault Relay SettingsAlarm only on continuous rated ground resistor

applicationsppAlarm setting at 80% of maximum current level allowed by

ground resistorAbove system charging current level

Trip on short time duty ground resistor applicationsHigh resistance ground applicationsHigh resistance ground applicationsTrip at 80% of maximum current level allowed by resistor

Low resistance ground applicationsg ppTrip at 20% of maximum current level allowed by ground resistor


Low Resistance Ground Detection Scheme

Trip Upstream Breaker

Y51G

NGR

p pTrip setting ~ 20%of NGR rating

Y400A NGR

NGR400A Trip Downstream Breaker

Trip

13.8kV51G

Trip Trip Trip51G

Trip

ZCT

ZCT

M

51G 51G 51G M

51G

ZCT - Zero Sequence CTs

ZCTZCT ZCT

ZCT


GF Relay Time Coordination Curves

Settings for ground-fault relays can berelays can be determined during the relay coordination studyy y

GF curves are plotted on the coordination diagrams Set parameters include

ti d t l ltime and current level

Ground Fault coordination curves


Ground Fault coordination curves

CEC Requirements AssociatedCEC Requirements Associated with Systems Grounding


CEC Code Requirements10-1102 – Installation of Neutral Grounding Devices1) Neutral grounding devices can only be installed on1) Neutral grounding devices can only be installed on

systems where line to neutral loads are not servedNo single phase loads from a resistance grounded system

2) S t ith lt 5kV h ll b d i d2) Systems with voltages > 5kV shall be de-energized on detection of a ground fault

a) Electrical systems operating at 5 kV or less are permitted to remain ) y p g 5 penergized if the ground fault current is controlled at 10A or less

i. Audible alarm is required


CEC Code Requirements3) Where line-to-neutral loads are served, the

system must be de-energized on occurrence of a:system must be de energized on occurrence of a:1) Ground fault2) Grounded neutral on the load side of the NGR3) Break in the continuity of the conductor connecting the

NGR to ground

Apparent conflict between subsection 1) and subsection 3)


NGR with Isolated System Neutral

AHTCkt

HTCkt

HTCkt N

HT HT HT HT

Trip main breaker

51 BC

HTCkt

HTCkt

HTCkt

HTCkt

NGR

IG

Rule 10-1102 requires the system tobe de-energized on detection of G gground current


Neutral Ground Devices10-1104 NGRs must be approved for the application

CAN/CSA-C22.2 No. 0.4 – Bonding and Grounding of Electrical E i (P i G di )Equipment (Protective Grounding)

CAN/CSA-C22.2 No.14 – Industrial Control Equipment CAN/CSA-C22.2 No. 94 – Special Purpose Enclosures

Must be continuously rated where provisions are not made to interrupt the fault Maximum temperature allowed is 375°C Maximum temperature allowed is 375°C

Where not continuously rated, the time rating of the device must be coordinated with the protective devices of the systemy

Must have an insulation voltage equal to the line-to-neutral system voltage


Location of Grounding Devices10-1106 All live parts must be enclosed Must be placed in a location accessible to qualified Must be placed in a location accessible to qualified

personnel Must be placed in a location where it can dissipate Must be placed in a location where it can dissipate

the heat under ground fault conditions Warning signs must be provided indicating the g g p g

system is impedance grounded and located at: Transformer or generator, or both Consumers service switchgear Supply authorities metering equipment


Grounding Conductors System grounding conductors must be copper Solid grounded systems sized as per CEC Table 17g y p

Based on the ampacity of the largest service conductor

No splicing is permitted


CEC Code Requirements


NGR Conductors10-1108 conductors connecting the NGR to the Neutral

point of the system must be identified as white or p ygrey Must not be grounded Sized to conduct the rated current of the device

No less than #8 AWG

Conductor connecting the NGR to the system ground Conductor connecting the NGR to the system ground electrode may be insulated green or bare

Made of copperpp


NGR Conductors


Bonding of Conduit Enclosing a Grounding or Bonding Conductorg g Magnetic effect of metal conduit can increase the impedance

of the grounding circuit by a factor of 40! Not an issue with PVC or aluminium conduits Not an issue with PVC or aluminium conduits

Problem can be mitigated by bonding the grounding conductor to the metal conduit at both ends Allow the metal conduit to carry a portion of the ground current Allow the metal conduit to carry a portion of the ground current New CEC rule 10-806 makes this mandatory


Sizing and Specification of NeutralSizing and Specification of Neutral Ground Resistors

NGR Sizing Criteria NGRs are sized based on the following criteria Charging currentCharging currentHRG - Maximum ground current must be greater than

3X the charging current for the systemLRG – Charging current not a factor

Temperature riseBased on how long the fault is allowed to persist

– Continuous E t d d ti (1 i t )– Extended time (1 minute)

– 10 seconds


NGR Sizing Criteria

RNGR =VLL

√RNGR

√3IG

RNGR ≤XCO

3IG ≥ 3ICO

NGR ≤ 351

NGRI

RNGR = Resistor Size (Ohms)

WNGR = IG2RNGR

IG

NGR ( )IG = Maximum Ground Current (Amps)ICO = System Charging Current (Amps)W = Resistor Size (Watts)


WNGR = Resistor Size (Watts)

NGR Sizing CriteriaSecondary Ground Resistory

RSGR = RNGR

N2

51 N =VLN(Pri)VLN(Sec)

SGRXFMR ISGR = NIG

KVA = PNGR = IGVLN(Pri)

RNGR = Equivalent Primary Resistance (Ohms)RNGR = Equivalent Primary Resistance (Ohms) I M i G d C t (A )IG = Maximum Ground Current (Amps)ISGR = Maximum Ground Current (Amps)N = Turns ratio


PNGR = Resistor Power Rating (Watts)

Charging Current - Estimation Resistor must be sized to ensure that the ground

fault current limit is greater than the system's total g ycapacitance-to-ground charging current

System Voltage

Charging Current (3ICO) Amps per 1000 kVA of System Capacity

480 0.1 – 2.0

600 0.1 – 2.0

2400 2.0 – 5.0

4160 2.0 – 5.0

13800 5.0 – 10.0


Typical Charging Currents based on Voltage Level

Charging Current – More Detailed Analysis

System Voltage

Component Type Typical Charging Current

600V Cable 3/C - 250 – 500MCM 0.15A/1000ft

3/C - #1 – 4/0AWG 0.02A/1000ft

Transformers 0.02A/MVA

Motors 0.01A/1000HP

4160V Cable 3/C - 500–1000MCM Shielded 0.58A/1000ft

3/C – 1/0 – 350MCM Shielded 0.23/1000ft

Non Shielded 0.1A/1000ft

T f 0 05A/MVATransformers 0.05A/MVA

Surge Suppressor. 1.35A per Set

Motors 0.1A/1000HP

13800V Cable 3/C - 600–1000MCM Shielded 0.65A/1000ft

3/C – 250 – 350MCM Shielded 0.75/1000ft

3/C - #1 – 4/0AWG Shielded 0.65A/1000ft

Transformers 0.05A/MVA

Surge Suppressor 2 25A per SetSurge Suppressor 2.25A per Set

Motors 0.15A/1000HP


Charging Current CalculationExamplepROT → IG ≥ 3ICO

I 3(4 78A) 14 34A 15A NGR more

Component Charge C t

Qty Total Ch i Y

SurgeSuppressor

12 MVA

IG ≥ 3(4.78A) = 14.34A 5 G o eappropriate size

Current Charging Current

Transformer 0.05A /MVA

17.5 0.875A

Y10A NGR

4160V

12 MVA

500MCM Cable

0.58A /1000ft

4200 ft 2.43A

250MCM 0.23A 600 ft 0.13A1 5MVA

1200ft500MCM600ft

250MCMSurge2MVA

1500

ft50

0MC

M

1500

ft50

0MC

M

2MVACable /1000ftSurge Suppressor

1.35A /Set

1 1.35A

T t l 4 78A

M600V2A NGRY

1.5MVA

3000HP

SurgeSuppressor

2MVAYY

2MVA

Total 4.78A


M M

Charging Current Test Procedure

Connect an ammeter to ground through a resistance, switch and g ,a fuse

Increase the resistance to maximum level and close the di t

A

IA

Ndisconnect

Slowly reduce the resistance to zero Ammeter will indication charging

BC

CA CB CC

IB

IC A Ammeter0-10A Ammeter will indication charging

current (3ICO) All three phases should be

measured and the average used

CA CB CC

G

0 10A

gas the system charging current

G


Cable Insulation Ratings on Resistance Grounded Systemsy Low Voltage Systems (≤ 600V) 100% insulation rating acceptable for all applications% g p pp Refer to Standata CEC 12

Medium Voltage Systems (IEEE Recommendations)g y ( ) 100% insulation level required where clearing time will not

exceed one minute 133% insulation level required where clearing time will not

exceed one hour 173% insulation level required where clearing time173% insulation level required where clearing time

exceeds one hour


NGR Ratings Based on the criteria defined in IEEE 32 - Standard

Requirements, Terminology, and Test Procedure for Neutral Grounding DevicesCurrentCurrent through the device during a ground fault conditionCurrent through the device during a ground fault condition

VoltageV = IR at 25ºCMay need to be de-rated at elevations above 1000m

FrequencyCircuit Voltage of SystemCircuit Voltage of System

ServiceNEMA Type 1 for Indoor ApplicationsNEMA Type 3 for Outdoor Applications


NGR RatingsBasic Impulse Insulation Level

System Insulation Class

Class BILClass BIL

1.2kV 452 5kV 602.5kV 605kV 75

8 7kV 958.7kV 9515kV 10023kV 15023kV 150


NGR Ratings Time Rating and Permissible Temperature Rise under fault

conditionsTime Rating Permissible Temperature Rise

(Rise Above 30ºC Ambient)

Ten Seconds (Short Time) (NGRs 760ºCTen Seconds (Short Time) (NGRs used with Protective Relay)

760 C

One Minute (Short Time) 760ºC

Ten Minutes (Short Time) (seldom specified)

610ºC

Extended Time (GF 610ºCExtended Time (GF allowed to persist > 10min)

610 C

Steady State (Continuous) 385ºC*


*CSA permissible rise is 375ºC over 40ºC Ambient

NGR Monitoring

Broken Spot Weld

NGR Thermal Failure


Broken Resistor Wire

NGR Monitor The NGR monitor

measures changes in NGR resistance, current in the neutral, and neutral-to-ground voltageg g Anomalies are detected and

an alarm or trip signal is activatedactivated


NGR Sizing Tutorial

NGR Sizing Tutorial Modular Substation incorporating

5 kV Switchgear and MCCs 600 V Switchgear and MCCs 600 V Switchgear and MCCs

Grounding system consists of: Power Distribution System Ground

5kV L i t d t 5kV Low resistance ground system 600V High resistance ground system

Objective Size the grounding resistors for the 5kV LRG system and the

600V HRG system Assume 1.5A charging current for the 600V System Assume 8A charging current for the 5kV System


Substation Single Line

YLRG NGR

M

5kV

MM M

Y600V

HRG NGR

MM

M M

~=

=~


UPSPP

NGR Sizing TutorialAnswers

Sizing the NGRs

RNGR =VLL

√RNGR

√3IG

RNGR ≤XCO

3IG ≥ 3ICO

NGR ≤ 351

NGR

RNGR = Resistor Size (Ohms)

WNGR = IG2RNGRIG

NGR ( )IG = Maximum Ground Current (Amps)ICO = System Charging Current (Amps)W = Resistor Size (Watts)


WNGR = Resistor Size (Watts)

HRG Sizing

ROT → IG ≥ 3ICO ICO = 1.5AG CO

IG ≥ 4.5 → Choose 5A as the HRG Current RatingCO

RNGR = VLL

√3IGWNGR = IG

2RNGR

RNGR = 600V√3 x 5A

WNGR = 5A2 x 69.3Ω

RNGR = 69.3Ω WNGR = 1733watts


LRG Sizing

ROT → IG-Trip Setting ≥ 3ICO to avoid nuisance tripping ICO = 8AT i i i hl 20% f h LRG i i

I 120A Ch 125A h LRG C R i

Trip setting is roughly 20% of the LRG resistor sizeIG-Trip Setting ≥ 24A to avoid nuisance tripping

IG ≥ 120A → Choose 125A as the LRG Current Rating

RNGR =VLL

√WNGR = IG

2RNGRRNGR √3IG

RNGR =4160V

WNGR IG RNGR

WNGR = 125A2 x 19.2ΩRNGR √3 x 125A

RNGR = 19.2Ω

NGR

WNGR = 300kW


NGR

System Grounding Application Summary

Solid Systems Ground Industrial applications 208V or lesspp Commercial Applications 600V or less

High Resistance Ground (5-15A)g ( ) Industrial applications 600V or less

CEC allows HRG to be used on applications up to 5kV

Low Resistance Ground (100 – 400A) Industrial applications 5kV – 34.5kV

G d f lt t ti id d b Z S CTGround fault protection provided by Zero Sequence CTs on individual equipment items

GF relays set to trip at 10 -20% of maximum ground fault current


Obtaining a Systems Neutral

Application of GroundingGrounding

Transformers

Obtaining a Systems Neutral Often there are cases

where a systems neutral point must be established for the purposes of: Servicing line to neutral

Y Servicing line to neutral

loads Establishing a systems

d i t t d th13.8kV

ground point to ground the system through a HRG, LRG or solid ground

ti

M

connectionExample: Conversion of a

isolated ground system to a high resistance ground systemhigh resistance ground system


Grounding Transformers Grounding transformers are the standard means of

obtaining a systems neutral Provide a low impedance path for ground fault currents

Zig-Zag transformerOft f d t di t f Often referred to as a grounding transformer

Specialized transformer with no secondary winding Wye-delta transformer configuration Wye delta transformer configuration Delta winding is left unconnected


Grounding Transformer Schemes

ABBC

I

I

G

I

GG

Zig Zag TransformerWye-DeltaTransformer


Transformer Connection The grounding transformer

is connected to the main bus and serves as the return path for any unbalanced or ground fault

Yunbalanced or ground fault currents A NGR is then connected to

th t l i t f th13.8kV

the neutral point of the grounding transformer establishing a connection to

d

M

ground


LRG

Specifying a Grounding Transformer Parameters for specifying a grounding transformer Primary Voltagey g BIL (Basic Impulse Level) rating

Defined by IEEE standards (refer to IEEE 141 Red book)

Transformer impedanceTypically very high (up to 100%) to minimize magnetizing current

flows

Continuous neutral current ratingApplicable to four wire application

F l d d i Fault current and duration If a LRG scheme of limited duration is used, ( typically 10 – 60

seconds) the grounding transformer does not need a continuous duty rating


G di f G tGrounding of GeneratorsSection 3

Generator Grounding Generators differ from transformers in several ways Less able to withstand the heating and mechanical effects g

of a short circuit Will have a higher initial ground fault current than three

h d tphase ground current Can develop third harmonic voltages and currents Less able to withstand voltage surges Less able to withstand voltage surges


Objective of Generator Neutral Grounding

Minimize the damage associated with internal ground faults

Limit mechanical stresses in the generator for external ground faults

Limit temporary and transient overvoltages on the

t i l ti tgenerator insulation system Provide a means of system

ground fault protectionground fault protection


Systems Ground Incorporating Generation

SystemGround #1

YNGR

M

5kV

SystemGround #2

M

Y600V Normal Bus

G G

M M G

SystemGround #3


600V Emergency Bus

Generator Ground Fault

IGF

400ANGR

2 X IGF2 X IGF

Stator Ground Fault near

400ANGR

IGF

Breaker Closed

Stator Ground Fault nearGenerator terminals

Initial ground fault current results in 2 X 400A flowing into fault


Generator Ground Fault

IGF

400ANGR

IGFIGF

Stator Ground Fault near

400ANGR

Breaker Open

Stator Ground Fault nearGenerator terminals

Upon breaker trip, ground fault current continues to flow due to the residual magnetism and inertia of the machine


the residual magnetism and inertia of the machine

Ground Fault Magnitude Magnitude of a ground fault is determined by the

impedance of the generator or transformer windingp g g Maximum ground fault will occur on the system bus Maximum theoretical ground fault current in the Maximum theoretical ground fault current in the

generator will occur at the generator terminals Closer the stator fault is to the generator terminals, the

higher the fault Resulting damage is a function of current and time


Solid System and Generator Ground

NOT RECOMMENDED NOT RECOMMENDED Results in very high ground fault currents resulting in

extensive damageg Risk of abnormal third-harmonic currents when more than

one generator is connected in parallel Increased magnetic core losses in both generator and transformer Increased magnetic core losses in both generator and transformer


Low Voltage Emergency Generator Scheme

BondingConductor

Normal Bus Emergency BusNormal Bus Emergency Bus

Gnd Gnd


Single Unparalleled Generator GroundingSolid Grounded with Neutral

EquipmentGround NeutralGroundconductor

NeutralConnectedTo ground

3 pole

Normal Bus Solid Neutral

TransferSwitch

NeutralConductor

Emergency Bus51G

Emergency Bus

GndNN GndZero SequenceCT is bypassedresulting in falset i


trip

Single Unparalleled Generator GroundingSolid Grounded with Neutral Connection of the neutral to ground at the generator

can cause problemsp Allows stray current to flow between the neutral and the

ground conductors Allow zero sequence (ground fault current) to flow in the

neutral causing nuisance tripping of the main breaker Prevent ground fault relays from detecting a ground fault Prevent ground fault relays from detecting a ground fault

A neutral should not be connected to ground on the load side of a service disconnectload side of a service disconnect


Option 1 – Switch Neutral

EquipmentGround NeutralGroundconductor

NeutralConnectedTo ground

4 Pole

Normal Bus Neutral SwitchedWith loadconductors

TransferSwitch

NeutralConductor

Emergency BusGFP

Emergency Bus

GndNN Gnd


Option 2 – Connect Generator Neutral with Transformer Neutral

EquipmentGround

G tGroundconductor

3 pole

GeneratorNeutral connectedto transformerneutral in transfer

Normal BusTransferSwitch

neutral in transferswitch

NeutralConductor

Emergency Bus51G

Emergency Bus

GndNN GndZero SequenceCT read fullneutral current

l


value

Additional References IEEE 446 Orange Book Provides application information pp

for the system grounding and transfer switching of standby generators 600V or lessgenerators 600V or less


Single Unparalleled Generator GroundingHigh Resistance Groundedg

HRG

HRG

HRG

BondingConductor

Normal Bus Emergency BusNormal Bus Emergency Bus

Gnd Gnd


HRG Source and Generator Grounding

HRG

HRG

HRG

Advantages Ground fault current limited to a very low valuey

Disadvantage Selective tripping on downstream breakers is not practical


LRG Source and Generator Grounding

LRG

LRG

LRG

Advantages Allows selective tripping of downstream feeders

Disadvantage Damage can occur to the generator from high ground fault currents Variations in fault current can cause relay coordination problems Variations in fault current can cause relay coordination problems


LRG Source and HRG Generator Groundingg

HRG

LRG

HRG


R d d l l f f lt t t th t i i i i d Reduced level of fault current to the generators minimizing damage Disadvantage

System is high resistance grounded when the generator is operating alone – makes selective tripping impossible


Source and Generator Grounded with Artificial Neutral

LRG


All ith t t id Allows either source or generator to provide power Disadvantage

Damage can occur to the generator from restriking and intermittent ground faults


LRG Source and Hybrid LRG/HRG Generator Groundingg

HRGLRG

LRG

HRGLRG

Ground fault will causet b k tgenerator breaker to

trip and open LRG circuit

Advantages Allows selective tripping of downstream feeders Allows generator to operate without the source transformer energized

Disadvantage Additional complexity in the grounding and relaying system Additional complexity in the grounding and relaying system


Unit Connected Generator Grounding

HRG

LRGLRGLRG

Advantages Allows selective tripping of downstream feeders Allows generator to operate without the source transformer energized

Disadvantage Cost of the additional transformer Cost of the additional transformer


E i t B diEquipment BondingSection 4

System Grounding Grounding and bonding have distinct meanings

within the context of the CEC Grounding refers to a conductive path direct to the

grounding electrodeg g Low impedance path to ground Conductors are sized to carry the expected fault current Insure the operation of protective devices in the circuit

should a fault occur


Equipment Bonding Refers to the interconnection and connection to earth

of all normally non-current carrying metal partsy y g p Insures that all metal parts remain at ground potential Reduces the shock hazard to personnel Provides a low impedance return path for ground currents

Allows the circuit protection device to operate

Minimize the fire and explosion hazard Minimize the fire and explosion hazard Reduce accumulated static charges


Ground Return Path through Earth

Insufficient current to operate protection device

~Line

Metallic Enclosure

S V

Neutral

GroundFault

Neutral

Short circuit musttake high impedance

High Impedance Ground Path

take high impedancepath to source


Ground Return with Metallic Path

High Current Operates Protection Device

~Line

Metallic Enclosure

S V

Neutral

GroundFault

Neutral

Low Impedance Path through Bonding Conductor

High Impedance Ground Path

Low Impedance Path through Bonding Conductor


Bonding Fundamentals To reduce electrical shock exposure: the impedance of the bonding conductor must be capable p g p

of carrying the fault current Must provide a lower impedance than all other parallel

thpaths For fire protection:

M st be able to cond ct the a ailable gro nd fa lt c rrent Must be able to conduct the available ground fault current without excessive temperature rise or arcingJoints and connections are critical components

Overcurrent Protection Operation: Provide a low impedance current path back to the source


Bonding – CEC Requirements 10-400 All exposed non-current carrying metal parts of fixed

equipmentq p Supplied by a conduit wiring system Supplied by a wiring system that contains a bonding

conductor Located in a wet location In a hazardous location In a hazardous location Operates at more than 150V to ground

Examples Examples Distribution equipment, motor and generator frames Lighting fixtures housingsLighting fixtures housings


Bonding Methods Bonding conductor in a cable or raceway Rigid metal conduit

Bonding conductor is required if the conduit is in underground service or installed in concrete slabs

EMT conduit EMT conduit Bonding conductor required if installed in concrete or masonry slabs

Sheath of a mineral insulated cable if manufactured of copper or aluminum

CEC Not acceptable Metal armor of liquid tight flex or cable assemblies Metal armor of liquid tight flex or cable assemblies Conduit made of stainless steel


Bonding Methods - Effectiveness Cable or Conduit DC Resistance

Ω/1000ftVoltage Drop V/1000A/100ft

1-1/4” Rigid Steel Conduit 0.0108 11

1-1/4” EMT 0.0205 22

1-1/4” Flexible Conduit 0.435 436

3/C St l A d C bl 553/C Steel Armored Cable 55

3/C Steel Armored Cable with Ground Conductor

11Conductor

3/C Aluminum Armored Cable 0.286 151

3/C Aluminum Armored Cable with 123/C Aluminum Armored Cable with Ground Conductor

12


Bonding Conductors Bonding conductors may be: Be copper or other corrosion resistant material

Aluminium conductors are acceptableMay be insulated or bare Insulated bonding conductors shall be coloured greeng g

May be spliced or tapped as required If installed to supplementary bond a raceway: Must be insulated Must be run in the same raceway

M st be protected against mechanical inj r if Must be protected against mechanical injury if: Copper - Smaller than #6 AWG Aluminum – Smaller than #4 AWGAluminum Smaller than #4 AWG


Bonding ConductorsEquipment and Racewaysq p y


Bonding of Cable Trays Rule 12-2208 of the CEC requires that cable trays be

bonded to groundg If the metal supports for cable tray are in good contact with

the grounded structural metal frame of a building, the tray h ll b d d t b b d d t dshall be deemed to be bonded to ground

If not in direct contact, a bonding conductor must be installed and the tray bonded to the conductor at intervalsinstalled and the tray bonded to the conductor at intervals not exceeding 15mSized as per CEC table 16 based on the largest ungrounded

conductor in the trayconductor in the tray

A bonding conductor may also be required in the cases that the tray supports single conductor cables of a three phase system


Bonding of Single Conductor Cables Separate ground conductor required to bond the metallic

equipment at either end Must follow the same routing as the phase conductors


Bonding Considerations Bonding connections require a clean surface Paint must be removed from connection points

Connections between dissimilar metals should be avoided Potential for deterioration of the connection due to galvanic Potential for deterioration of the connection due to galvanic

action Mechanical strength may often determine the size of

d tconductor Electrical continuity of expansion joints Cable tray connections Cable tray connections


Equipotential Bonding Practice of bonding all exposed and extraneous

conductive parts (Ref CEC 10-406)p ( ) Purpose is to ensure that under fault conditions, all

conductive parts remain at the same potential Applies to Metallic water and sewer piping

G Gas piping HVAC ducting Exposed metal equipment and structures Exposed metal equipment and structures Raised computer floors


Equipotential Bonding CEC requires a minimum #6 AWG conductor


Bonding of Portable Equipment Non-current carrying metal parts of portable

equipment must be bonded when:q p Equipment is used in a hazardous location Equipment is used in wet or damp locations Equipment operates at more than 150V to ground When the equipment is provided with a grounding means

Th l ith dThree prong plug with ground


Grounding of Portable Equipment Exceptions apply to double insulated equipment

productsp Additional insulation barrier added to the electrical device Will be marked with a double insulated symbol

Ground may omitted if a Class A ground fault circuit interrupter is used


GFCI Schematic

Designed to provide protection against electric shock Designed to provide protection against electric shock from leakage current flowing to ground

Provide supplementary protection but are not a pp y psubstitute for insulation and grounding protection


Ground Fault Circuit Interrupters GFCI Class A Primarily used for personnel protection Typically trip at 5ma Time to trip based on the formula

T =20I

1.43 T in secondsI fault current between 4mA and 260 mA

GFCI Class B (Ground Fault Equipment Protectors) Used for equipment protection

Heat trace circuits in hazardous locationsHeat trace circuits in hazardous locations 30ma trip level


GFCI – Where Required Outdoor receptacles Wet locations Wet locations Health care facilities Panels supplying power for buildings or projects Panels supplying power for buildings or projects

under construction Heat trace systems Heat trace systems


Static Grounding

Section 5

Did the Cellphone Cause the Ignition?


Static Hazards in Industry Aviation Industry Static charges are built up during flight and on the ground

Manufacturing Paper and Printing

P d b lt i llPower and conveyor belts moving over pulleys Paint operations

Transfer of fluids

Coal, Flour and Grain Industry Movement and accumulation of dust and particles

P h i l P i R fi i d Petrochemical Processing, Refining and Transportation Movement of materials Movement of materials


Reasons for Static Grounding Reduce the risk of fires and explosions Improve process and quality control Improve process and quality control Reduce the operating costs associated with storing

flammable materialsflammable materials Minimize the potential for damage to sensitive

electronic equipmentq p Loss of electronic data

Comply with hazardous goods transport and storage p y g p gregulations

Reduce the cost of insurance


Energy from a Static Discharge

10

CH4/AirH2/Air

Typical range ofE = CV2 X 10-9

)

Typical range of spark dischargeenergy from a human body Where

C = Capacitance in pFV = Voltage in V

Material Dust Dust

1.0

Ene

rgy

(mJ)

Stoichiometric

V = Voltage in VE = Energy in mJ

Cloud LayerCoal 60 mJ 560 mJ

0.1Igni

tion Stoichiometric

CH4/Air Mixture0.274 mJ

Grain 30 mJ -

Sulfur 15 mJ 1.6 mJ020 40 60 80

StoichiometricAir/H2 Mixture0.017 mJ


20 40 60 80Fuel (% Volume) Energy Required for Dust Ignition

Typical Values of Static Voltages and Capacitancesp

Equipment Voltage Object Capacitance Energy

Carpet Walk

12 kV Human Being 200 pF 28.8 mJWalkFabric on Fabric

25 kV Automobile 500 pF 312.5 mJ

Tank Truck 25 kV Tank Truck 1000 pF 625 mJ

Tank Truck 25 kV 3.6m Tank with Insulated Lining

100000 pF 62,500 mJ

Lining


Static Charge GenerationStatic electricity is generated by the movement of

dissimilar poor conducting materials in close contactdissimilar poor conducting materials in close contactNon conductive fluids or powders in motion are a frequent cause of static

Static charge increases as the velocity of movement is increased. Anything which generates eddies, turbulence or

discontinuities in flowFiltersFiltersChanges in piping cross sectional area


Static Charge GenerationTriboelectric Effectcontact electrification in which certain materials become

electrically charged when coming into contact with another and are then separated


Electrostatic Charge Dissipation Electrostatic charges continually leak away from a

charged bodyg y Termed electrostatic dissipation

Determined by a materials conductivityMeasured in pS/m (picosiemens per meter) for petroleum products

Electrostatic charges accumulate when they are generated at a higher rate than they are dissipatedat a higher rate than they are dissipated

Function of the relaxation time constantTime required for a charge to dissipate to approximately 37% of its

i i l loriginal value


Conductivity and Time Constants for Typical Materialsyp

Product Conductivity Relaxation Time (pS/m) (Seconds)

Benzene 0.005 >>100

Toluene 1 21

Gasoline 10 – 3000 0.006 - 1.8

Diesel 0.5 – 50 0.36 - 36

Fuel Oil 50 1000 0 018 0 36Fuel Oil 50 - 1000 0.018 – 0.36

Crude Oil > 1000 < 0.018


Static DischargeFor an electrostatic charge to be a source of ignition,

four conditions must be present:pA means of generating an electrostatic chargeA means of accumulating an electrostatic charge capable

of producing an incendiary sparkA spark gapAn ignitable vapor air mixture in the spark gapAn ignitable vapor-air mixture in the spark gap


Static Charge Generation


Static DischargeSpark discharges occur between conductive objects

that are at different voltagesgBrush discharges can occur between a grounded

conductive object and a charged low conductivity j g ymaterial

Incendive discharge is a discharge that has enough energy to cause ignition


Industrial Materials Prone to Static ElectricityyNonconductive glassNonconductive conveyor beltsNonconductive conveyor beltsRubberPlastic resinsPlastic resinsDry gasesPaperPaperPetroleum fluidsOil water mixturesOil water mixtures


Sources of Static Electricity Dry materials handling

equipment Flammable liquid pumps and

handling equipmentMultiphase flow enhances

Charge Separation in a PipeMultiphase flow enhances

charge generation

Liquid filling operations Plastic piping systems Conveyor Belts

Liquid motion in tanks Liquid motion in tanks


Sources of Static Electricity


API 2003

Spark Promoters A spark promoter will provide

the necessary conditions for a spark gap to occur Loose floating conductive

objects Conductive downspoutsGage tapes, thermometers or

sample containers lowered into pa tank “tank gauging rod, high-level

sensor, or other conductive ,device that projects into the cargo space of a tank truck”

API 2003


Static Sparks in Kanses


Static Control Ignition hazards from static sparks can be eliminated

by controlling the generation or accumulation of static y g gcharges

Static removal involves recombining separated g pchargesUsually met by bonding all electrically conducting parts


Methods of Static Control Piping Systems Keep fluid velocities low

Max 15 ft/sec

Filling Operations Filling Operations Eliminate splash filling and free fall of materials Reduce filling velocity to less than 3 ft/sec

Fluid Storage Non conductive material storage containers are not Non-conductive material storage containers are not

allowed for NFPA Class I, Class II and Class III materials


Methods of Static Control Humidity Control 65% or higher will prevent static discharge

Antistatic treatmentsAdditi f b bl k t t i l Addition of carbon black to materials

Use bonding and grounding to prevent build-up of Use bonding and grounding to prevent build up of potential differences between conductive parts Small gauge conductors generally sufficient to prevent the

b ild p of staticbuild-up of static


Static Grounding

Vehicle Connected to GroundVehicle Bonded Together

Vehicle Bonded together andTo Ground


Static Grounding

Drum Container Storage Scheme


Static Grounding

Bulk Fluid Transfer Operation


Static Grounding

Bonding connections should be less than 10Ω for static control


Bonding connections should be less than 10Ω for static control

Railcar Loading Bonding Scheme


API RP 2003 Provides guidance on how to protect

against hydrocarbon ignition from static, lightning and stray current discharges

Discusses how static charges are accumulated and how they can be safely dissipated

Lightning protection for metallic tanks equipment and structures

Identification and mitigation of stray currents resulting from fault currents and cathodic protection applications


NFPA 77 Applies to the identification,

assessment, and control of ,static electricity for purposes of preventing fires and explosions

Provides guidelines for t lli t ti l t i it icontrolling static electricity in

selected industrial applicationsapplications


Lightning Protection

Section 6

Lightning Strikes


Lightning No such thing as a standard lightning strike Highly complex phenomenong y p p Described by statistical means

+ + ++ +- -- -- -

Charge Separation in Cloud

Corresponding charge

High electric field causes ionization of air

++ +

+

+

+

+

Induced in groundCurrent flow in metallic pathways


Lightning Strike Initiation

+ + ++ + ++ +- -- -- -

--- - Downward leader

+ + ++ +- -- -- -

--- - Upwards

+ + ++ +- -- -- -

--- - Charge flows

+

Downward leader

Upward leader+

++

--Upwardsleader meetsdownwards leader

-

--

--Charge flowsto ground through structure

++ +

+

+

+

+++ +

+

+

+

+++ -

-

+

-

--


Lightning Discharge Direct effects of lightning

Heat energy and large h i l f

Lightning Stroke

Cumulative Frequency

98% 95% 80% 50% 5%mechanical forces

Direct ignition of flammable materials

First negative kA 4 20 90

Subsequent 4 6 12

Indirect effects of lightning Incendive sparks Electromagnetic pulse

kA 4.6 12

Typical Lightning Current Value

Electromagnetic pulse Earth current transients Bounded charges

Cur

rent

C

Time


TimeTypical Lightning Discharge

Difference between Lightning and High Voltage ElectricityHigh Voltage Electricity

Factor Lightning High Voltage

Energy Level 25 kA typical, millions of volts

Usually much lower

Time of Exposure Brief, instantaneous

Prolonged

Pathway Flashover, orifice Deep, internal y , p,

Burns Superficial and minor

Deep with major injury j y

Cardiac Primary & secondary arrest, asystole

Fillibration


asystole

Incidence of Lightning Lighting varies with Terrain Altitude Latitude Time of the year

Number of flashes per square kilometre per year


Lightning Protection Lightning strikes cannot be stopped but their energy can be

diverted in a controlled manner Strike frequency goes up with the square of the height above the

average terrain Damage is caused by the lightning energy taking a random – high g y g g gy g g

impedance path to ground

3 components to a lightning protection system Air terminal or electrode the intercepts the surge Air terminal or electrode the intercepts the surge Low impedance conductor system to ground Ground electrode to dissipate the energy

If all equipment within an elevated potential area is bonded together, the potential for damage is minimized


Inherent Grounding Inherent grounding

Metallic equipment, tanks and structures in direct contact with the ground do not require additional grounding if: The thickness of tanks, vessels and process equipment is greater than

5mm and are capable of withstanding a direct lightning strike without damagedamage

Indirect contact with the ground (self grounded) Sealed to prevent the escape of liquids, vapours or gas

M t t h i l f iliti i h tl d d d Most petrochemical facilities are inherently grounded and require no additional lightning protection

Equipment that may require special consideration Equipment that may require special consideration Open floating roof tanks Tank farms incorporating a containment liner


Bounded Charge Dissipation Bounded Charges Occurs when a storm cell induces an electricalOccurs when a storm cell induces an electrical

charge on everything beneath it Consideration with open floating roof tanks

Floating Roof- - - - - - - - - - - - - - -

Floating RooftankTeflon seal isolates

roof from tank + + + + + + + + + + + + + + Bounded Charge+

++

+++

Flammable Product++++

++++


+ + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Bounded Charge Dissipation

Floating Rooftank

+ + + + + + + + + + + + + +Bounded Charge- -

Flammable Product

+ + + + + + + + + + + + + + Incendivedischarge toground

----

----

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

ground- -


Protection against Lightning Floating Roof Lightning

ProtectionBoundedcharge

Floating roof cable connection Grounding Shunts (Not Recommended)

charge Dissipated withLightning strike

Cable connection tofloating roof

Groundingshunt

- - - - - - - -

floating roof

Flammable Product


Methods of Lightning Protection Conventional air terminal Provides a low impedanceProvides a low impedance

path to groundLightning rods (sometimes

called Franklin rods)

+ + ++ +- -- -- -

--called Franklin rods)Conducting mastsOverhead wires

- -

R

+

RA

RARA = 0.84 x h0.6 x I0.74

Att ti di i t

++ +

+

++

RA = Attractive radius in metersh = height of lightning mast in metersI = Peak lightning current in kA


Dissipative Array System (DAS) Claim of the technology is to

dissipate a charge before a lightning strike occurslightning strike occurs No scientific proof that this in fact

occurs Renamed the Charge Transfer Renamed the Charge Transfer

System (CTS) technology in recent years Still considered ineffective


Early Stream Emission Air Terminals Consist of lightning rods

incorporating a device that triggers the early initiation of a lightning strike Effectiveness is also

questioned


Lightning Surge Protection Transient Overvoltages can damage

electrical equipment Result in insulation breakdown and eventual

failure

Mitigated byg y Surge arrestors Equipment insulation standards

Lightning Strike EMFTravelling Wave

Line

Surge Voltage Wave


Line

Surge Protection

Diminished Surge Voltage Wave

35kV 13 8kVTransient Voltage Surge

Surge Arrestor

70kV

35kV 13.8kV

25kV

Is

Transient Voltage Surge

Surge ArrestorIsSurge Voltage isInduced on secondarywinding by capacitive

Surge suppressorreduces surge voltage winding by capacitive

coupling effectg g

to below BIL of transformer


Equipment Insulation Voltage Withstand Requirementsq Basic Impulse Level (BIL) is used to describe the

insulation class of electrical equipmentq p Based on the voltage rating of the equipment Based on specified crest value kVp Specified in the various

equipment standards


Surge Voltage and Current Wave

Surge Arrestors

Surge arrestor must have a high i t d l ditiresistance under normal conditions

and a very low resistance under surge conditions

Metal oxide arrestor is the industry standard

Consist of a series connection of zinc Consist of a series connection of zinc oxide elements


Surge Arrestors Class of Surge Arrestors (IEEE Std C62.11)

Station Class Intermediate Class Distribution Class – Heavy duty Distribution Class – Normal dutyy Secondary


600V Secondary Surge Arrestor Distribution Class Surge Arrestor

Surge Arrestor Installation Considerations

Should be mounted as close as possible to the transformer bushings

Arrestor must be coordinated with the BIL of the equipment it is protecting

A dedicated “down lead” conductor to ground required for A dedicated down lead conductor to ground required for each arrestor

Down lead conductor should be mechanically and thermally y ycapable of handling the surge voltage to ground

Down lead should be as short as possible with no changes in directiondirection Minimum radius of 200mm No bends greater than 90º


Lightning Arrestors – CEC Requirements

10-1000 Lightning Arrestors on Secondary Services1) Grounding conductor shall be as short (and straight) as possible2) The lightning arrestor grounding conductor may connected to the:

a) Grounded service conductorb) Common grounding conductorc) Service equipment grounding conductord) Separate grounding conductor

Common ground conductor

i i di d

Grounded service conductor


Service equipment grounding conductor

Lightning References NFPA 780 “Standard for the

Installation of Lightning Protection g gSystems” provides detailed guidance on the design of lightning protection systems”

API 2003 “Protection against I iti A i i t f St tiIgnitions Arising out of Static, Lightning and Stray Currents”IEC 61024 “P t ti f St t IEC 61024 “Protection of Structures Against Lightning – Part 1”


NFPA 780 Installation standard for

lightning protection systems for building structures and facilities handling flammable vapors gases and liquidsp g q

Does not apply to electric generating, transmission

d di t ib ti tand distribution systems


Electronic Equipment Grounding

Section 7

History Grounding principles for communication systems

were developed to meet the operational h t i ti f th i tcharacteristics of the equipment Early telegraph systems used a two wire circuit path Later systems used the earth return as the signal pathLater systems used the earth return as the signal path


Morse Landline Telegraph System

History Earth return offered several advantages Iron wire was used for telegraph conductors

The use of an “earth” ground doubled the distance a circuit could be run

Eliminated one wire from the circuit Problems endured: Quality of the signal was effected by weather

Leakage current to ground during wet weatherLeakage current to ground during wet weatherResistance of the return ground path varied with soil conditionsPresence of “foreign” voltages


History Early development of the

telephone system also relied th lid DC fon the solid DC reference

ground as the return path lines were particularly noisy, p y y,

picking up electrical noise from power lines, adjacent telephone lines, telegraph lines, streetcars, g pand machinery

The grounded system was later replaced with a systemlater replaced with a system employing two wires per telephone line eliminating most of the noisemost of the noise


Electronic Equipment Grounding Terms

Signal Common Grounding referred to as the “DC Signal Common” Zero reference system for data lines Very sensitive to transient voltages

DC Power Supply Reference Ground Bus DC Power Supply Reference Ground Bus -ve terminal on a DC power Supply

Equipment Ground Bus Used for equipment chassis bonding Often referred to as the safety ground bus

Variety of other terms used (depending on manufacturer) Variety of other terms used (depending on manufacturer) AC Safety Ground, Computer Reference Ground, DC Signal Common,

Earth Common, DC Ground Bus


Electronic Systems Grounding Most electronic computer systems employ a DC

reference ground Required for logic circuits

Problems occur when the DC reference ground is tied to the AC safety groundtied to the AC safety ground With the logic circuits referenced to the equipment chassis

ground, any small amount of chassis potential caused by current flow in the grounding of the device could cause reference error in the equipment.


Noise in Sensitive Circuits Errors result when the noise is greater than the

actual signalg Results in parity check errors

signal is ignored if check fails

30-50VL iLogicSignal

Noise does not impact signal

3-5VLogicSignal

Noise cause parity check errors


Circuit Noise Sensitivity Measurement Signal to Noise Ratio Measure of the interference in a communications circuit Measured in dB

Bit Error RateSNR = 10log dBS

N

Measure of the number of bits received to those in error

10 610-6

10-7

10 8Erro

r Rat

e

10-8

10-9

Bit

E


0 10 20 30 40 50 60SNR (dB)

Categories of Noise Traverse Mode Noise A disturbance that appears between two active conductors pp

in an electrical system Measurable between two line conductors or from line to

t lneutral Originates from within the power system

A

V

A

B

C

N

G


Categories of Noise Common Mode Noise Appears simultaneously in each active conductorpp y The term "common" refers to the fact that identical noise

appears on both the active and neutral wires Generally involves the ground conductor

A

V

A

B

CV

N

G


Typical Problems Associated with Electronic Systems Groundingy gElectronic Equipment Symptoms Electrical Condition

Temporary or chronic data hang-upsSlow data transfers, multiple retries

Different signal reference levelsI d d t blI/O Damage Induced currents on cable

Intermittent lock-upsCorrupted SignalsI/O damage

Transient voltages and currents

I/O damageRandom data errorsSlow transfer in analog circuits

Stray currents and common mode noise in equipment gro nding cond ctorgrounding conductor


Electronic Equipment Grounding Computers require a “quiet” ground where no voltage

transients or electromagnetic noise occursg Stabilize input voltage levels Act as a zero voltage reference point for circuits

Led to the practice of installing an “Isolated Ground” system specifically for electronic equipment

This practice was in direct conflict with the CEC which requires that all grounding systems be i t t dinterconnected CEC is concerned with safety – not with performance


Principles of Noise Mitigation For noise to be a problem Requires a noise source of sufficient magnitudeq g Some means of communicating the unwanted noise to the

electronic circuitGalvanic couplingElectrostatic / Capacitive couplingMagnetic or Inductive couplingg p g

Solving the problem involves either reducing the amplitude of the noise voltage or effectively isolating the circuit from the noise source


Source of Electrical Noise Motor starts High current in-rush is impressed on the communications g p

circuit Fluorescent lighting High frequency noise associated with the ballast operation

Switching power supplies or VFD systems High frequency noise associated with switching power

suppliesHi h lt d t li ht i t ik d High voltage surges due to lightning strikes and electrical faults


Galvanic Coupling Occurs when two circuits

share a common conductor Examples: Telephone circuits

that used the same common t th DC t li ireturn path as DC tram lines in

the early days Easily solved by separating Easily solved by separating

the circuits by using separate return conductors


Electrostatic/Capacitive Coupling Form of coupling that is

proportional to the p pcapacitance between the noise source and the signal

R1

I1wires Function of:

C1 RL

I1

I2I3

Distance from the noise source to the signal wires

Length of the signal wires E

C2R2

Noise source dVg g Strength of the noise voltage Frequency of the noise voltage

ENoise source dT


Electrostatic/Capacitive Coupling Mitigation Shielding of the signal wiresg g Separating the source from the noise Reducing the amplitude of the noise voltage Reducing the frequency of the noise voltage Twisting of the signal wires


Shielding Conductor shield provides a lower impedance path for the

noise current to flow

R1 C3

I2Copper braid (85% coverage) provides a noise reduction ratio of

RL

I3

I4I5

noise reduction ratio of 100:1Aluminum Mylar tape with drain wiren provides a

R2C4

I1

noise reduction ratio of 6000:1

E

C2

Noise sourcedVdT

C3 and C4 are 1/100 C1 and C2

C1


dT

Shield Grounding Shielding on instrumentation and communication

circuits eliminates the electrostatic induction into wires carrying low signal voltages

Shielding method may beg y Braided copper wire Metalized foil, with a copper drain wire Metal conduit (if steel conduit, this also serves as a

magnetic shield)Shields must be grounded Shields must be grounded One end only for frequencies up to 1 Mhz Two or more locations for frequencies > 1 Mhz Two or more locations for frequencies > 1 Mhz


Magnetic or Inductive Coupling Depends of the rate of

change and the mutual inductance between the source of noise and the signal wiresg

Influenced by:Magnitude of the noise

currentcurrentFrequency of the noise

currentA l d b h i lArea enclosed by the signal

wiresDistance between the noise

d th isources and the wires


Magnetic or Inductive Coupling Mitigation Twist the signal conductorsg

This results in lower noise due to the smaller area for each loop. This means less magnetic flux to cut through the loop and consequently a lower induced noise voltageconsequently a lower induced noise voltage

Noise voltage that is induced in each loop tends to cancel out the noise voltages from the next sequential loop

Inductive coupling is reduced by ratios varying from 14:1 for a four-inch lay to 141:1 for one-inch lay


Magnetic or Inductive Coupling Mitigation Enclose the signal wires with a magnetic shieldg g

The magnetic flux generated from the noise currents induces small eddy currents in the magnetic shield which then create an opposing magnetic flux Ø1 to the original flux Ø2opposing magnetic flux Ø1 to the original flux Ø2

Galvanized steel conduit is an effective magnetic shield

Placing parallel (untwisted) wires into a steel conduit will provide a noiseprovide a noise reduction of approximately 22:1


Physical Segregation Separate the noise sources from the noise sensitive

equipmentq p Cable spacing based on susceptibility levels defined by

IEEE 518 Level 1 – High: Analog signals less than 50V and digital

signals less than 15V Level 2 – Medium: analog signals greater than 50V Level 2 – Medium: analog signals greater than 50V Level 3 – Low: Switching signals greater than 50V Level 4 – Power: Voltages 0 – 1000V; Currents 20–800Ag ;


Physical Segregation

Level Level Separation1 2 2 30

Level Level Separation1 2 2 – 30mm1 3 3 – 160mm1 4 4 670mm

1 2 2 – 30mm1 3 3 – 110mm

1 4 4 – 670mm 1 4 4 – 460mmCables Contained in Separate Trays One Cables in Conduit and the other

In Tray

Level Level Separation1 2 2 – 30mm

y

1 2 2 30mm1 3 3 – 80mm1 4 4 – 310mm


Both Cables in Conduits

Electrical Segregation

Y Y Y

Shielded IsolationTransformer UPS

M M

SensitiveLoads

Worst Case

MMY

MM ~=

=~

Worst CaseSensitive Loads are subject

to voltage fluctuations causedby motor loads Better


Best

Separately Derived AC Power Distribution System using an Isolation Transformery g Isolates power to the control system from the rest of

the AC distribution systemy Provides good line regulation and transient filtering

Transformers should be of a shielded designg Provide superior noise isolation using the same concepts

used for shielded cables Input power to the transformers should be sourced

from the highest line voltage availableK f t t f h ld b id d if th K factor transformers should be considered if the control system load employs a large number of switching power suppliesswitching power supplies


Isolation Transformer Grounding


Separately Derived AC Power Distribution System using an UPSy g Provides a continuous power supply to the control

system in the event of a power interruptiony p p Protects the control system from power system

surgesg Isolation transformers cannot prevent surge events from

being transmitted to the load without additional surge protectionprotection

Provides a “conditioned” AC power supply to the control systemcontrol system Completely disconnects the control system power supply

from the source providing superior isolation from power system transients and noise


1 Phase UPS


3 phase UPS


UPS Grounding

UPS configuration with common source for UPS and bypass circuityp Does not meet the definition of a separately derived circuit Common mode noise attenuation may be a problem


UPS Grounding

Addition of a bypass transformer meets the definition of a separately derived sourcedefinition of a separately derived source Improved common mode noise attenuation Neutrals in UPS and bypass transformer are connected Power distribution center must be within 15m of the UPS


UPS Grounding

Best configuration for common mode noise attenuation No restriction on distances Allows more flexibility in UPS voltages


Multiple UPS Grounding Scheme


Ground Loops Occurs when there is more than one ground connection path

between two pieces of equipment ground current may take more than one path to return to the grounding ground current may take more than one path to return to the grounding

electrode form the equivalent of a loop antenna which very efficiently picks up

interference currents Conductor lead resistance transform the currents into voltage

fluctuations Consequences

Ground reference in the system is no longer a stable potential Signals ride on the noise Noise becomes part of the program signal

Example Audible 60hz noise in your stereo system


Ground Loop

U it A

+

-Input

+

-Output

CommunicationCable

InternalConnection

Unit A Unit B

PowerGround

PowerGround

1A Current Flowing

Low Resistance 0.1Ω0.2V0.1V

1A Current Flowing

Stray Current in Ground Causes Current to Flowin communication conductors


Ground Loop – Motor Start

+Input

+Output

CommunicationCable

InternalConnection

Unit A Unit B

--

PowerGround

PowerGround

Connection

FromFromElectricalSources

MMotor Start


Motor frame bondedTo ground

Ground Loop Mitigation

Add one or more separate groundsN t CEC d li t Not CEC code compliant

+

-Input

+

-Output

CommunicationCable

InternalConnection

Unit A Unit B

PowerGround

C t Fl i Mi i i d Separate

High Resistance

Current Flowing Minimized pInstrumentationGround

www.EngWorks.ca Grounding Fundamentals 244Motor frame bondedTo ground

Ground Loop Mitigation Interrupting the continuity of the grounding conductor Shielded communication cables

Interrupt ground

++

CommunicationCable

Unit A Unit Bpath here

-Input

-Output

PowerGround

PowerGround


Ground Loop Mitigation• Control the path of the ground current

• Use an insulated ground receptacle

CommunicationUnit A Unit B

+

-Input

+

-Output

Cable

PowerGround

PowerGround

IsolatedInstrument ground

Single point groundInsulated ground conductor


Isolated Ground Receptacle Helps to limit electrical noise introduced into a circuit via the

grounding conductor Establishes a dedicated ground path connected to ground at

one point only


Conventional Receptacle Isolated Ground Receptacle

Isolated Ground Receptacle

B h Ci it

NEMA IG#5-15R2Isolated Ground Receptacle

Branch CircuitPanelboard

JunctionBox

Power Transformer

Metal Device Box

Conduit or CableInsulatedIsolatedIsolatedGround

wireSystemGround

Bare BondingConductor or Conduit


Concept of a Single Point Ground System

Poor or faulty grounds are the most common causes of control system faultsy

The best way of insuring the performance and reliability of a control system is to employ a single y y p y gpoint ground network system Consists of an organized system of ground wiring that

t i t i i l d di t d i t th l t dterminates in a single, dedicated point on the plant ground grid

Provides a clean reference for control signalsProvides a clean reference for control signals


Single Point Ground System


Single Point Ground System Multiple Enclosures


Instrument Tri-Ground System

GroundC d t

Main Transformer

LightningArrestor

NGR

Conductor

System GroundConnection to

system ground may be temporarilydisconnected to

AC Ground

isolate ground loop

InstrumentTri Ground


Tri Ground

Intrinsically Safe Circuit Grounding

H d A Non Hazardous Area

I i i C l

Hazardous Area Non Hazardous Area

Field Device

IntrinsicSafe

Barrier

ControlSystem

Interface

AssociatedApparatus

IS Apparatus

InterconnectingWiring System

pp


Wiring System

Intrinsic Safety – Simple Field Devices

Th l

Non Hazardous Hazardous Location

ControllerSimple

Thermocouple

Device

Controller

InternalFault

Controller

ExplosionInternalFault

ControllerFault

ISBarrier

Device isConsideredSafe under FaultC di i


IS GroundConditions

Intrinsic Safe Barrier Circuit

Protects Zener from Destruction Limits the output

t

Safe Area HazardousArea

Limits input current current

FieldCurrentLimiting

Control

Fuse

Z FieldDevice

LimitingResistorSystem

Interface

ZenerDiodes

IS GroundLimits the output

lt


voltage

Intrinsic Safety - Grounding

Incorrect Ground Scheme Correct Ground Scheme


Intrinsic Safety - Grounding Extremely important for the safe operation of an IS

wiring systemsM t b i ibl id tifi d d ibl Must be visibly identified, secure and accessible

Must be capable of carrying the maximum fault currentcurrent #12 AWG minimum conductor size

Total resistance must not exceed 1Ωf Must be insulated from ground in all places except at

the point of connection to the ground electrode Duplicate ground conductors requiredup ca e g ou d co duc o s equ ed

Aluminium must not be used as a ground conductor material Potential for electrolytic corrosion Potential for electrolytic corrosion


IEEE Standard 1100(Emerald Book)( )

Recommended engineering principles and practices for power and grounding p p g gsensitive electronic equipment Provides consensus in an area

where conflicting information haswhere conflicting information has prevailed

Excellent reference that describes the many challenges associated withthe many challenges associated with grounding electronic equipmentPower related noise controlSignal related noise controlSignal related noise control


Station Electrode Design

Section 8

Ground Grid Design


Ground Grid Design Fundamentals In the event of a fault or transient phenomena

(lightning or switching transients) the ground grid ( g g g ) g gmust Ensure personnel safety Protect equipment against damage

Design Considerations Grid must be able to withstand the maximum ground

current without damage Limit the ground potential rise between two points to a safe Limit the ground potential rise between two points to a safe

value


CEC Code Requirements Section 10 “Grounding and Bonding” addresses

grounding electrodes for facilities operating at less g g p gthan 750V to ground Requirements are minimal

Section 36 “High Voltage Installations” addresses the grounding of facilities operating at more than 750V to

dground Requirements are in addition to those defined in Section 10

More substantial in nature and therefore require a deeper More substantial in nature and therefore require a deeper understanding of ground electrode theory


CEC Section 10 Requirements CEC Section 10 does not specify a minimum

ground resistance for a grounding electrode but g g gspecifies the acceptable methods of obtaining a grounding electrode NEC specifies a ground resistance of 25Ω or less

G f10-700 Grounding Electrodes shall consist ofa) Manufactured grounding electrodesb) Fi ld bl d di l t db) Field assembled grounding electrodesc) In-situ grounding electrodes


Manufactured Ground Electrode

Must be certified to CSA C22.2 No.41 “Grounding and Bonding Equipment”g q p Rods must be driven to their full length and separated by a

minimum of 3m Connected by a bonding conductor sized by Table 17

R d El t d


Rod ElectrodePlate Electrode

Field Assembled Ground Electrode

Min 6m bare copper conductor buried or encased in concrete

conductor must be encased within the bottomencased within the bottom 50 mm of a concrete foundation footing, with the footingfooting

in direct contact with the earth, at not less than 600 ,mm below finished grade




In-situ Ground Electrode Copper water pipe Metal reinforcement of concrete slabs, concrete pilings,

and concrete foundations Iron pilings, when they are in significant contact with earth

600 mm or more below finished grade600 mm or more below finished grade


In-situ Grounding Electrode

10-700 Grounding Electrodes(5) Where local conditions such as rock or permafrost

prevent a rod or grounding plate from being p g g p ginstalled at the required burial depth, a lesser depth shall be permitted


Horizontal Ground Rod Installation

Other Section 10 Requirements Lightning rod systems must be connected to ground

using a separate grounding electrode that is not used g p g gas the grounding electrode for any other system

Where a facility incorporates more than one ground y p gelectrode for lightning, communication or other systems Must be separated by a minimum of 2m Bonded together by a minimum #6AWG conductor


CEC Section 36 Requirements36-302 Station ground electrodeEvery outdoor station shall be grounded by means of a station

ground electrode that shall meet the requirements of Ruleground electrode that shall meet the requirements of Rule 36-304 and shall

a) consist of a minimum of four driven ground rods not less than 3 ma) consist of a minimum of four driven ground rods not less than 3 m long and 19.0 mm in diameter spaced at least the rod length apart and, where practicable, located adjacent to the equipment to be grounded;

b) have the ground rods interconnected by ground grid conductors not less than No. 2/0 AWG bare copper buried to a maximum depth of 600 mm below the rough station grade and a minimum depth of 150 mm below the finished station grade; anddepth of 150 mm below the finished station grade; and

c) have the station ground grid conductors in Item (b) connected to all non-current-carrying metal parts of equipment and structuresall non current carrying metal parts of equipment and structures


Distribution Utility Standard Ground Electrode Designg


Distribution Utility Standard Ground Electrode Designg


CEC Requirements for Station Ground Resistance36-304 Station ground resistance (see Appendix B)1) The maximum permissible resistance of the station1) The maximum permissible resistance of the station

ground electrode shall be determined by the maximum available ground fault current injected g jinto the ground by the station ground electrode or by the maximum fault current in the station, and the

d i t h ll b h th t d ll ilground resistance shall be such that under all soil conditions that exist in practice (e.g., wet, dry, and frozen conditions)frozen conditions) …..


CEC Section 36 – Ground Potential Rise

36-304 Station ground resistancethe maximum ground fault current conditions shall….the maximum ground fault current conditions shall

limit the potential rise of all parts of the station ground grid to 5000Vg g

2) In addition to subrule (1), the touch and step voltage at the edge, within, and around the station grounding electrode…..shall not exceed the tolerable values specified in Table 52


CEC Section 36 – Tolerable Touch and Step Voltagesp g


IEEE Standard 80

IEEE Standard 80 Defines the safe limits

for touch and step potentialspotentials Provides guidance on

the design of groundthe design of ground systems for outdoor substationsP i il d b tiliti Primarily used by utilities for grounding on high voltage substationsg


Ground Potential Rise (GPR)

Ground potential rise is a function of the current magnitude injected into the earth and the soil g jresistivity Measured with respect to a remote point

May vary from a few meters to several hundred meters awayMay vary from a few meters to several hundred meters away

5000V criteria specified in the CEC is based on the maximum GPR communication circuits are d i d t h dldesigned to handle

GPR = IG X RGPR IG X RgIG = Maximum Grid CurrentRg = Grid Resistance


Ground Potential Rise

3 i t i i ith

N∆

G

∆ N

A3 wire transmission with noMetallic return path

N∆ ∆ G BC

NGR EGGroundGeneratorTransformer IG

E th

EG

GPR

GroundFault

G

EarthRg Ground Path


System Ground Fault Return Path

Non Metallic conduitWith no bonding conductorWith no bonding conductor

Y

0.1Ω Ig=5kA

500V Ground Return Path

Lack of bonding conductor forces ground fault return path through the earth creating personnel hazard


path through the earth creating personnel hazard

System Ground Fault Return Path

Metallic conduitwith bonding conductorwith bonding conductor

Low ImpedanceGround Return Path

YIg=5kA

High ImpedanceGround Return PathGround Return Path

Bonding conductor provides low impedance path to source: Stray current is minimized with improved


source: Stray current is minimized with improved safety

Current GF Path with Local Source

YY

Multiple low impedance ground pathslimit the ground potential rise withinth t ti


the station

Current GF Path with Remote Source

Overhead ground wire currentpath

YY

Multiple high impedance ground current paths back to source

Stray Current Paths


Multiple high impedance ground current paths back to source

Ground Potential Rise (GPR) Grid system must limit the ground potential rise

(GPR) between two points to a safe value GPR can cause hazardous voltage in the form of Step &

Touch PotentialsMay occur in location remote to the actual fault

Safe values of GPR, Touch and Step Voltages are determined by the human tolerance to shock currentsFunction of current magnitude, duration and frequencyg , q y

GPR = IG X RgIG= Maximum Grid CurrentRg = Grid Resistance


Step Voltage

I

Potential Rise aboveremote earth during short

R1

IF

RF

ESTEPcircuit

IF

R2RKESTEP

R0RF

RKK


R1 R2 R0

Touch Voltage

IFETOUCH

Potential Rise aboveremote earth during shortcircuit

R1

RFRK

ETOUCH

circuit

IF

RK

1

RF/2

TOUCH

RK

R0

RRF/2


R1 R0

Touch and Step Potential IEEE 80

ρ = resistivity of earth beneath surfaceρs = surface material resistivity (Ω . m)


ρs surface material resistivity (Ω m)hs = thickness of surface material in m

Relationship between GPR, Touch and Step Potentialsp

YY

EtEs

Emesh GPR

Remote Earth


Mesh Voltage Defined as the maximum

surface voltage potential g pdifference between a grid conductor and and a point between two grid conductors

Th ti l i Theoretical maximum touch voltage found within a ground grid


Touch and Step Potential Two ways of making a grounding system safe1 Minimize the touch and step voltages that may1. Minimize the touch and step voltages that may

appear at any point within the substation and around its perimeterp

2. Increase the tolerable touch and step voltages by placing a high resistivity material over rough grade Asphalt Crushed rock

Both methods are typically used together


Ground Surface Potential Gradients

10kA High ground resistanceincreases step potential

GroundRod

R1

Re10 VoltsInfinite Earth

R = R1+Re1 = 3 ohm

Infinite Earth

nd S

urfa

cePo

tent

ial

30kV

8kV Step Potential


Gn P

0 Distance from Rod

8kV Step Potential

Ground Surface Potential Gradients

Multiple ground rods reduce the10kA

Multiple ground rods reduce the ground surface potentiallow resistance surface reducesstep potentialstep potential

4kAR1

Re10 VoltsInfinite Earth

R1+Re1 = 3 ohms R2

Re26kA

0 ohms R2+Re2 = 2 ohms

Infinite Earth

nd S

urfa

cePo

tent

ial

12kV


Gn P

0Infinite Earth (0V)

Symmetrical Grid Current The current that causes the ground potential rise in a

grid is from a remote sourceg Only a portion of the current is responsible for the

ground potential riseg p Multiple return paths include Overhead ground conductors Cable shields

The current flowing into the ground that is responsible for the GPR is adjusted by a split factor to incorporate the effect of the multiple paths


Symmetrical Grid Current

Ig = Sf x If

Ig = RMS Symmetrical Grid CurrentS = Split factor (Current Division Factor)Sf = Split factor (Current Division Factor)If = RMS value of the symmetrical ground fault current

Sf may be estimated using computer programs or by graphical analysisby graphical analysis

Typically ranges between 10 – 70% of If

Refer to IEEE 80 for more information Refer to IEEE 80 for more information


Split Factor

Split factor accounts for the multiple current paths thatwill occur in a fault situation Overhead ground wire current

path

will occur in a fault situation

YY IfIf

Ig = Sf x If


Split Factor Graphical EstimateIEEE 80 Annex C

Curve most likelyCurve most likelyto be used for a single circuitcustomer owned28%substation

The symmetrical grid current (I ) would becurrent (Ig ) would be approximately 28% of the total fault current for a substation with a2.5Ω grid resistance


2.5Ω Substation

Grid Current for Design

IG = Cp x Df x Ig

IG = Maximum Grid CurrentCp = Estimated growth factor during station life span

Cp = 1 for zero growthp f gDf = Decrement factor for the duration of the faultIg = RMS value of the symmetrical ground fault currentg f y g f


Decrement FactorDecrement factor accounts for the total asymmetric fault current flowing between the grounding system and the surrounding earth

Fault Duration tf Df

Sec CyclesSec Cycles0.008 0.5 1.650 1 6 1 250.1 6 1.250.25 15 1.10.5 30 1.0

S b i T i N k S S ( 30 )

0.5 30 1.0

296www.EngWorks.ca Industrial Power System Protection and Control 296

SubtransientNetwork (0-5 cycles)

Transient Network (5-30 cycles)

Steady State Network (>30 cycles)

Design Information Provided by Utility

Current values to be used in the design of the station

d idground grid


Ground Resistance The “WildCard” in the grounding design

Ground grid resistance varies with soil Ground grid resistance varies with soil conditions and may change over time

Ch i t t bl Changing water tableResistivity of the ground will change in drying or drought

conditionsconditions Chemical content of soilPresence of salts decrease resistivityPresence of salts decrease resistivity

Frozen ground or permafrost conditionsConsideration in all Canadian grounding situationsg g


Soil Resistivity (Ω . cm) = 100 x (Ω . M)

MediumResistivity (Ω . cm)

Minimum Average MaximumSurface Soil, Loam 100 5,000Clay, Shale, Gumbo 300 4,000 20,000Sand and Gravel 5 000 10 000Sand and Gravel 5,000 10,000Limestone 500 400,000Granite, basalt 1,000,000Low Hills, Rich Soil 3,000Medium hills, Medium Soil 20,000St Hill R k S il 50 000Steep Hills, Rocky Soil 50,000Sandy, dry coastal country 30,000 50,000 500,000Freshwater Lake 10,000 20,000 20,000,000Sea water 2,000 10,000 20,000


Effects of Moisture on Resistivity

Moisture Resistivity Ω-cmMoisture Content

Resistivity Ω cmTop Soil Sandy Loam

0 > 109 > 109

2.5 250,000 150,0005 165,000 43,00010 53,000 18,50015 19,000 10,50020 12,000 6,30030 6,400 4,200


Effects of Temperature on Resistivity for Sandy Loamy

Temperature ºC

Resistivity Ω cmºC Ω-cm

20 7,200

10 9,900

0 (Water) 13,800

0 (Ice) 30,000

-5 79,000

-15 330,000


Seasonal Variation in Earth Resistance

19mm Rod in Stony Clay SoilCurve 1 – 1m below surfaceC 2 3 b l fCurve 2 – 3m below surface

Moisture and temperature is more stable at greater depthsp g pbelow the surface


Effect of Chemicals on Earth Resistivity for Sandy Loamy

Effect of Temperature on Resistivity of Soil with Salt (20%

Effect of Salt on Resistivity of Soil (Moisture 15% Temp 17ºC) Resistivity of Soil with Salt (20%

Moisture 5% Salt)

Temperature ºC

Resistivity (Ω-cm)

( p )

Added Salt % by Weight of

Moisture

Resistivity (Ω-cm) C (Ω-cm)

20 11010 142

Moisture

0 10,7000.1 1,800 10 142

0 190-5 312

0.1 1,8001.0 4605 190 5 312

-13 1,4405 19010 13020 100


Electrode Resistance

Rg (rod) = ρ(Ω.cm)335 cm

Ω


Applies to 3m ground rod and is accurate within 15%

Multiple Ground Rod Resistance Resistance of a grounding system of 2-24 rods

placed on rod length apart will provide a grounding p g p p g gresistance divided by the number of rods multiplied by the factor F taken from Table 14 IEEE Std 142


Alternate Formulas for Ground Rod Resistance

Contact resistance of one ground rodρ 4Lρ

R = 2πL X Ln 4La - 1

ρ = Soil resistivity in Ω-cm Ground Rod Separationρ Soil resistivity in Ω cmL = rod length in cma = rod diameter in cm

D = 2.2 X L

Contact resistance of multiple ground rods

Rn = Rn X 2 – e-0.17(n - 1)

n n

n = number of ground rods


Smoking Ground Rod Current loading capacity of a ground rod is a factor Current passing through an electrode will have a direct p g g

impact on the temperature and moisture conditions immediately surrounding the ground rodM t b h k d Must be checked

I =34,800 X d X L

I =√ ρ X t

I = Current loading per foot of rod lengthg p gd = rod diameter in metersL = Length in metersρ = ohm metert = seconds (3 0 seconds is the value recommended by IEEE)


t = seconds (3.0 seconds is the value recommended by IEEE)

Ground Rod Resistance to EarthGround Rod Resistance to Earth Tutorial

Ground Rod Resistance to Earth Tutorial

1. Determine the resistance to earth for a ground rod system consisting of Qty 4 – 10 foot long 5/8” y g y g(16mm) ground rods spaced at 10’ intervals and interconnected connected together and placed in clay soil

2. Calculate the resistance to earth for a ground rod t i ti f Qt 4 20 f t 5/8” (16 )system consisting of Qty 4 – 20 foot 5/8” (16mm)

ground rods interconnected together and placed in clay soilclay soil

3. Calculate the current loading capacity of the system


Ground Rod Resistance to EarthGround Rod Resistance to Earth Tutorial

Answers

4 - 10’ Ground Rods

Rg (rod) =ρ(Ω.cm)335 cm

Ω Average ρ for clay soil = 4000 Ω·cm 335 cm

Rg (rod) = 4000 Ω.cm335 cm

= 11.94 Ω

For 4 ground rods

11 94ΩRg (4 rods) =

11.94Ω4

X 1.36 = 4Ω

IEEE 142Table 14


4 – 20’ Ground RodsContact resistance of one ground rod

ρ = Soil resistivity in Ω-cmρ 4L

4000 4(610)

L = rod length in cma = rod diameter in cm

ρR = 2πL X Ln 4L

a - 1

4000R = 2π(610) X Ln 4(610)

1.6 - 1 = 6.649Ω

Contact resistance of multiple ground rods

Rn = Rn X 2 – e-0.17(n - 1) n = number of ground rods

n n

Rn = 6.6494 X 2 – e-0.17(4 - 1) = 2.32Ω


Current Carrying Capacity of Ground Rod Systemy

I =34,800 X d X L

I = Current loading of the ground rod systemd = rod diameter in metersL = Length in meters

√ ρ X tL Length in metersρ = ohm metert = seconds (3.0 seconds typical value)

34 800 X 0 016 X 3 048 X 44 – 10ft Ground Rods I =

34,800 X 0.016 X 3.048 X 4

√ 40 X 3= 620 Amps

4 – 20ft Ground Rods I =34,800 X 0.016 X 6.1 X 4

√= 1240 Amps

√ 40 X 3


Primary and Auxiliary Ground Electrodes

Primary Ground Electrode Installed specifically for grounding purposesp y g g p p

Ground rods Interconnecting wire mesh

A ili G d El d Auxiliary Ground Electrode Installed for purposes other than grounding

T picall ha e limited c rrent carr ing capacit Typically have limited current carrying capacity Examples

Steel building pilesg pSteel reinforced concrete foundationsRebar grounding


UFER Ground First used by the US Army to ground a series of bomb storage

vaults in the vicinity of Flagstaff, Arizona Dry desert conditions made for a very poor ground electrode system Herbert Ufer developed an alternate electrode system based on using

the steel rebar used to reinforce concrete

Concrete is inherently alkaline and hydroscopic (absorbent) in nature The high pH provides a supply of ions to conduct current The high pH provides a supply of ions to conduct current

soil around concrete becomes “doped” by the concrete has an effective resistance of 3000 Ω-cm

Bases for the development of concrete encased electrodes


Concrete Encased Electrodes Can be used as part of an effective low resistance grounding

system Will typically lower the overall resistance of the ground

Very cost effective! Adds very little cost to the installation Adds very little cost to the installation Reduces the amount of buried conductor required for an installation Aids in reducing the amount of construction re-work

B i d d d t l it t di d i t hi Buried ground conductors are a popular item to dig up during trenching operations

The 2006 NEC requires that rebar encased in concrete be incorporated into the system ground No equivalent CEC requirement


Methods of Connecting Rebar to Building Steel

Option 1 – Connect structural steel and rebar using ground wire Requires electrical trade to be on site during pouring of Requires electrical trade to be on-site during pouring of

foundations

C Wi

Bolted Connection to Steel

GroundWell

Copper Wire

GroundingCompression

To groundgrid

pConnection orCadweld


Methods of Connecting Rebar to Building Steel

Option 2 - Tie the vertical rebar to anchor bolts and the steel columns are grounded through the bolts g gand nuts

Rebar welded To anchor boltsGround

Well

To ground grid


Rebar Grounding Installation Considerations

Rebar must be bare or zinc coated

Surge Current Conductivity of Rebar in Concrete

Minimum length – 6m Minimum diameter 13mm

I t ll d i i i f

Rebar Diameter (in.)

Surge Ampere per Foot

0 375 3400 Installed in a minimum of 50mm of concrete Preferably located near the

0.375 3400

0.5 4500ybottom of the foundation

Concrete must be in direct contact with earth

0.625 5500contact with earth

0.75 6400

1 0 81501.0 8150


Ground Conductors For a given application, ground rods are more

effective than ground grid conductorsg g Ground rods will penetrate the frost level injecting current

into unfrozen ground Basic Requirements for the selection of a Ground

Grid conductorH ffi i t d ti it Have sufficient conductivity

Resist fusing and mechanical deterioration under fault conditionsconditions

Be mechanically reliable and rugged Resist corrosion


Ground Grid Conductor Options Copper Highest conductivityg y Highest cost Subject to theft Commercial hard drawn specification most often used

Copper-clad steel Good option where theft is a problem

Aluminum Not recommended as is subject to corrosion

Steel Poor conductivity limits use


Sizing of Grid Conductors Based on the design ground fault current and the

fault duration time

Akcmil = I · Kf √tc

Akcmil = area of conductor in kcmilI = Fault current in kAtc = current duration in seconds (IEEE recommends 3.0 seconds)Kf = constant based on the material (Refer to table 2 IEEE 80)


Material Constants(Excerpt from Table 2 IEEE 80)( p )

Material Conductivity KfCopper, Annealed Soft Drawn 100% 7.00C C i l H d D 97% 7 06Copper, Commercial Hard Drawn 97% 7.06Copper Clad Steel Wire 40% 10.45Aluminum 6201 Alloy 52.5% 12.47Steel 1020 10.8% 15.95Stainless Steel 304 2.4% 30.05


Grid Conductor Sizing Example Fault current is estimated at 6kA Commercial hard drawn copper selected as the grid Commercial hard drawn copper selected as the grid

conductor What size of grid conductor is appropriate to handle What size of grid conductor is appropriate to handle

the maximum fault current for 3.0 seconds?

A = I · K √tAkcmil = I · Kf √tc

Akcmil = 6 · 7.06 √3.0Akcmil 6 7.06 √3.0

Akcmil = 73.37kcmil → #1 AWG


Grid Conductor Sizing Table

kcmil AWGCurrent Carrying Capacity (kA)

HD Copper Copper Clad Steel 1020pp ppSteel Wire

500 - 40 27.62 18.1250 20 4 13 81 9 05250 - 20.4 13.81 9.05212 4/0 17.34 11.71 7.67133 2/0 10.88 7.35 4.8183.7 #1 6.84 4.62 366.4 #2 5.43 3.67 2.4

Based on a 3.0 Second Fault Duration


Grounding Connections All grounding

connections must be selected to withstand the short circuit forces and heating effects associated with an extended groung faultextended groung fault

Resist the effects of corrosioncorrosion

High pullout resistanceresistance


Time-current curves for ground grid conductors and connectors

Typical Connections found within a Grid Designg

Conductor to Ground Rod

Conductor toEquipment to

G id C d tSubstation

F t G id


Conductor to Conductor

Grid ConductorFence to Grid Conductor

Cadweld Connections Thermite welding process is used to fuse the connection Suitable for high current applications Will meet the requirements of IEEE 837 ““IEEE Standard

for Qualifying Permanent Connections Used in Substation Grounding”Grounding

Cadweld Connection


Grounding Connections Compression Connections Acceptable alternative to Cadweld connections in p

substation applications


Compression Tool

Corrosion Considerations A basic corrosion cell consists of the following: Anode – An electrode losing metalg Cathode – An electrode gaining metal Electrolyte – chemicals in solution in contact with the

anode and cathode Connecting conductor

V

0.78V+-

COPPERIRON

V

COPPERIRONAnode Cathode

Earth

G l i C ll


Galvanic Cell

Corrosion Considerations When iron or steel is connected to copper with a low

impedance conductor, it corrodesp , Rate of corrosion is dependent on the current flow Each ampere-year of current flow will result in 20lbs of

steel being lost

Soil CorrosivityμA – 5AOhm-Cm Corrosivity

<2000 Very HighA +-

μ 5

2-5000 High5-10000 Moderate10 25000 Mild

IRON COPPERIRONAnode Cathode

Earth 10-25000 Mild


Earth

Corrosion Mitigation Use a UFER ground system whenever possible Do not use an UFER ground system in conjunction with a g y j

copper ground rod and grid system in the same vicinityWill result in deterioration of the concrete rebar

I i i h id i i l i i In situation where a grid system is in close proximity to large amounts of steel and corrosive soil conditions existconditions exist Consider using galvanized ground rods and insulated

ground conductorsgConductor insulation should have a high resistance to chemical

degradation

Consider installation of a cathodic protection system Consider installation of a cathodic protection system


Other CEC Requirements36-312 Grounding of metallic fence enclosures of

outdoor stations(1) The fence shall be located at least 1 m inside the perimeter of the station ground electrode area.p g(2) The station ground electrode shall be connected to the fence by a tap conductor at each end post, corner post, and gate post, and at intermediate posts at intervals not exceeding 12 m by a conductor of not l th N 2/0 AWGless than No. 2/0 AWG copper


Grounding of Fence Enclosures

Grounding Detail Gate Grounding Detail Fence


Other CEC Requirements36-304 Station ground resistance(4) After completion of construction, the resistance of the ( ) p ,

station ground electrode at each station shall be measured and changes shall be made if necessary to verify and ensure that the maximum permissible resistance of Subruleensure that the maximum permissible resistance of Subrule (1) is not exceededExceptionp Station phase to phase voltage is less than 7500V Ground surface has a 150mm layer of crushed rock or asphalt

(5) d f i l h ll t d t l t 1(5) ground surface covering layer shall extend at least 1 m beyond the station grounding electrode area on all sides.


Measurement of Ground Resistance

Current IA is passed through auxiliary probe A IA

Voltage between L and P is measured

R is then calculated based on

VA

1 – 10A

R is then calculated based on IA and VLP

Several ground resistance t t k dmeasurements are taken and

the results are averaged Moisture and temperature

L P A

IEEE Std 81 addresses Ground data should also be recorded Resistance testing


IEEE Standard 81

Describe the techniques d t dused to measure ground

resistance and impedance Factors that impact earth Factors that impact earth

resistivity choice of instruments and

techniques purpose of the measurement accuracy required Potential sources of error


Design of a Station Ground Grid Typical plant ground grid should have a resistance

of 10Ω or less For satisfactory lightning protection, grounding

network resistance must be less than 5Ω IEEE 80 Guidelines IEEE 80 Guidelines 1Ω or less for transmission substation 5Ω or less for distribution substation5Ω or less for distribution substation


Options Available for the Design of a Station Ground Grid 1. Design the GPR for the station to be less than the

Touch Potential Specified in the CECp Suitable for situations where the ground fault current is of

limited value Station resistance is determined by selecting the

appropriate number of ground electrodes using the formulas provided in IEEE 142 Green Bookformulas provided in IEEE 142 Green Book

Ground electrodes are then connected together using 4/0 AWG grid conductors

Results in a very conservative design


Options Available for the Design of a Station Ground Grid 2. Use the publication “Simplified Rules for Customer

Owned Substations” (CEA Report 249 D541) Publication is referenced as a referenced in CEC Section

36-304(3) and appendix B Outlines a procedure for design of a pre-approved stationOutlines a procedure for design of a pre approved station

electrode designMethod 1 based on simple calculations, tables and curves

– Takes into account frost penetration in winter– Takes into account frost penetration in winterMethod 2 based on design curves

– Valid only for fault durations less than 0.5 seconds and where frost penetration is negligiblep g g

Examples are provided Was developed primarily to simplify the ground grid design

process prior to the availability of computer aided groundprocess prior to the availability of computer aided ground grid design programs


Options Available for the Design of a Station Ground Grid 3. Use the procedure outlined in

IEEE 80 Calculation intensive but will result

in an appropriate and safe design Suitable for the design of Utility andSuitable for the design of Utility and

Large scale customer owned substations where ground fault current levels are highcurrent levels are high

Most computer aided ground grid design programs use the methods outlined in IEEE 80outlined in IEEE 80


Station Ground Grid Design

GPR < Touch Voltage MethodMethod

Relationship between GPR, Touch and Step Potentialsp

YPremise of the methodIs to reduce the GPR

Y To less than the tolerableTouch voltage

EtEs

Emesh GPR

Remote Earth


GPR < Touch Voltage PremisePremise of the method is to reduce the GPR to less than the tolerable touch voltage as defined in Table 52 of the CEC

YY

Etouch maxEstep max

Emesh GPR

Remote EarthGPR = IG X RgIG= Maximum Grid Current


GRg = Grid Resistance

Step 1 Ground Fault Current1. Determine the maximum ground fault current (IG)

that might be injected into the station ground g j gelectrode Reference CEC 36-304

Specifies the maximum ground fault current or: The maximum fault current for the station

Information is typically provided by the UtilityInformation is typically provided by the Utility


Step 2 Ground Resistivity2. Determine the resistivity of the ground in the area

of the substation Should be determined by test Often done as part of the geotechnical survey for civil works Ground resistivity under all conditions (wet, dry, frozen earth)

Determine the type of surface layer to be used in the vicinity of the substationvicinity of the substation 150mm of crushed stone generally used


Step 3 Tolerable Touch Voltage

3 Determine the tolerable

Touch Voltage Criteria

3. Determine the tolerable touch voltage from CEC table 52

Typical Value Used basedon Table 52 Note 2


Step 4 Required Ground Resistance

4 Calculate the required ground resistance for the4. Calculate the required ground resistance for the station ground electrode The use of 150mm of crushed stone over the surface of

the substation will help raise the Etouch and the overall required ground resistance and simplify the design

R = Etouch/IGR = Grid ResistanceIG = Maximum Grid CurrentEtouch = Tolerable touch voltage from CEC table 51


Step 5 Number of Ground Rods5. Estimate the number of ground rods required to

obtain the specified ground resistancep g Estimate the ground resistance of one ground rod Divide out the resistance of one ground rod by the

number of rods required to achieve the target station ground resistance

IEEE 141

Rg (rod) =ρ(Ω.cm)33

Ω

IEEE 141

Rg (rod) 335 cm


Valid for 5/8” 10 foot ground rods

Step 6 Ground Rod Layout6. Space out the ground

rods in a symmetrical tt th h t th

Substation fencepattern throughout the substation Minimum spacing of one

Min. 1 rod length

p gground rod distance apart

Ground rods on the peripheral are more 1m Minperipheral are more effective than ground rods in the interior of the substation

Grid design should extend beyond the fence of the substation a minimum of 1 meter


Step 7 Interconnect Ground Rods7. Interconnect the ground rods using a minimum

2/0AWG bare copper ground wire Conductors shall be buried to a minimum depth of

150mm below finished grade to a maximum of 600mm below the rough station grade

G t D t il d d i→ Go to Detailed design

2/0AWG CU Wire


Station Ground Grid Design

IEEE 80 Design ProcedureProcedure

IEEE 80 Ground Grid Design Appropriate for both Utility and Large customer

owned substation facilities Large generation facilities should reference IEEE 665

“IEEE Guide for Generator Station Grounding”Generating station typically cover a much larger physical area and

have numerous large buried structures and foundationsWorkers generally work indoors and are not in direct contact with

fthe earth or layer of crushed gravel

Defines the safety criteria which establishes the basis for design and then provides a procedure forbasis for design and then provides a procedure for the design of a practical grounding system


Step 1 Field Data Obtain a field map of the location

and determine the area that may be used for the installation of a ground grid

Conduct a soil resistivity test Conduct a soil resistivity test Determine the soil resistivity profile of

the area in concern Select and determine the resistivity of Select and determine the resistivity of

the surface layer material to be used in the design of the substation


Step 2 Conductor Size Select the ground grid conductor material

and calculate an appropriate conductor size

Ak il = I · Kf √tAkcmil I Kf √tc

Akcmil = area of conductor in kcmilI = Maximum 3 phase fault current in kAtc = current duration in seconds (IEEE recommends 3.0 seconds)Kf = constant based on the material (Refer to table 2 IEEE 80)

Maximum 3 phase fault current value is provided by the Utility


Step 3 Touch and Step Criteria Calculate the tolerable touch and

step voltage criteria for the stationp g

i i i f h b h fρ = resistivity of earth beneath surfaceρs = surface material resistivity (Ω . m)hs = thickness of surface material in m


Step 4 Initial Design Preliminary design should incorporate a

conductor loop surrounding the available area with cross conductors to provide convenient access for equipment grounds

Recommend ground rodsRecommend ground rods be placed around the perimeter of the grid


Step 5 Grid Resistance Estimate the resistance of the initial design

ρRg = +√20ALT

111 +

11+h√20/A

Rg = Substation resistance in Ωρ = Soil resistivity in Ω . mA = Area occupied by the grid in m2A Area occupied by the grid in mh = Depth of the grid in mLT = Total length of conductors and rods in m


Step 6 Grid Current Determine the maximum ground fault

current expected within the substation This value can usually be obtained from the

Utility If not, calculate the fault current using the

following:

IG = Cp x Df x IgG p f g

IG = Maximum Grid CurrentCp = Estimated growth factor during station life span

C = 1 for zero growthCp = 1 for zero growthDf = Decrement factor for the duration of the faultIg = RMS value of the symmetrical ground fault current


Step 7 GPR < Touch Voltage? Determine if the GPR is less than the

acceptable touch voltage for the station

GPR = IG X Rg < Etouch

I = Maximum Grid CurrentIG = Maximum Grid CurrentRg = Grid ResistanceEtouch = Etouch50 or Etouch70

If YES → Grid Design is completeGo to Detailed Design

If NO → go to step 8


Step 8A Calculate Mesh Voltage Calculate the MESH voltage for the grid

design

ρ · Km· Ki · IGEm = LMLM

EM = Mesh Voltageρ = Soil resistivity in Ω·mKm = Geometrical correction factor for grids of varying dimensionKi= Correction factor for grid geometry IG = Maximum grid currentL = Effective length of grid conductors and ground rods in mLM = Effective length of grid conductors and ground rods in m


Km = Geometrical correction factor

K =1

lD2 D + 2 · h 2 h Kii+ l

816 · h · d

Km = 2 · π

· ln + 8 · D · d-

4 · d +Kh

· π(2 · n – 1)ln

For grids with ground rods along the perimeter and thoughout the grid area: K = 1For grids with ground rods along the perimeter and thoughout the grid area: Kii = 1For grids with no ground rods: Refer to IEEE 80 Formula 82 for calculation of Kii

Kh = 1 +√ ho

ho = 1m (grid reference depth)h

D = Spacing between parallel conductors in mh = Depth of ground grid conductors in md = Diameter of grid conductor in m

√ o

d Diameter of grid conductor in m n = Effective number of parallel conductors in a given grid


Km = Geometrical correction factorn = na · nb · nc · nd

2 L LC = is the total length of the conductor in

nb = 1 for square grids1 f d t l id

Lp

2 · LCna =

C g fthe horizontal grid in m

Lp = is the peripheral length of the grid in m

nc = 1 for square and rectangular gridsnd = 1 for square, rectangular and L – shaped grids

Otherwise0 7 A

nb =4 · √A

Lp nc = ALx · Ly Lx · Ly

0.7 ·A

nd =Dm

√Lx2 · Ly

2

Ki= Correction factor for grid geometryKi = 0 644 + 0 148 · n


Ki 0.644 + 0.148 n

LM = Length of Grid Conductors and Ground Rods

For grids with no ground rods or very few rods scattered throughout the grid – but none on the corners or on the perimeter

LM = LC + LRLC = is the total length of the conductor in the horizontal grid in mC g f gLR = is the total length of all ground rods

For grids with ground rods in the corners, as well as along the perimeter and throughout the gridthroughout the grid

LM = LC + 1.55 + 1.22Lr

√Lx2 · Ly

2 LR√ x y

Lr = is the length of each ground rod in metersLx = is the maximum length of the grid in the x direction in mL i th i l th f th id i th di ti i


Ly = is the maximum length of the grid in the y direction in m

Step 8B Calculate Step Voltage

ρ · Ks· Ki · IGE

Es = Step Voltageρ = Soil resistivityKs = Spacing factor for step voltageEs = LS

s p g f f p gKi= Correction factor for grid geometryIG = Maximum grid currentLS = Effective buried conductor length m

LC = is the total length of the conductor in the horizontal grid in m

LR = is the total length of all ground rods LS = 0.75 · LC + 0.85 · LR

1 1 1 1 2

D = Spacing between parallel conductors in m

h = Depth of ground grid

2 · hKS = 1

π1

+ D + h1

+ D1

1 – 0.5n-2 conductors in md = Diameter of grid conductor

in m n = Effective number of parallel


n = Effective number of parallel conductors in a given grid

Step 9 Em < Etouch?

If YES go to step 10If NO → Modify design


Step 10 Es < Estep?

If YES go to Detailed DesignIf NO → Modify design


Detailed Design of the StationDetailed Design of the Station Ground Grid


Station Ground Grid Detailed Design1. Connect all non current carrying metal equipment to

the station ground gridg g Two 2/0 AWG connections for electrical equipment

apparatus Overhead transmission ground wires Metal structures

Pedestals Pedestals Security fence Substation building steel

Underground metal pipes and other metallic structures passing through the station Intervals not exceeding 12m Intervals not exceeding 12m


Station Ground Grid Detailed Design2. Connect the neutral ground resistor to the station

ground grid using a conductor sized to maximum current of the resistor Provide for the inspection of grounding connections

3 Connect the lightning arrestor to the station ground3. Connect the lightning arrestor to the station ground grid using a minimum 4/0 AWG conductor Must be short, straight and direct as possible, g p

4. Interconnect adjacent substations and the reinforced steel of adjacent plant structures using 4/0 AWG i t id d t4/0 AWG intergrid conductors

5. Connect isolated instrument ground to the station ground grid at one location onlyground grid at one location only


Intergrid Conductors

Reinforced Steel of Building or Plant

Substation Fence

Buried Substation Ground Electrode

Intergridconductors


Step by Step Instructions for the Design of a Station Ground Electrode6. Verify the following design parameters GPR Touch and Step voltages Current loading capacity of the ground rod

7 T t th i t f th t ti d l t d7. Test the resistance of the station ground electrode Procedures outlined in IEEE 81 Tests are not required if the phase to phase voltage is less than

7500V and a 150mm surface layer is installed– See CEC 36-306

8. Modify the design as requiredy g q Install additional ground rods or use longer ground rods to

reduce the station resistance Modify the resistance of the soil using chemical salts Modify the resistance of the soil using chemical salts


Grid Design Documentation Soil resistivity data Dates the measurements were made Temperature of the air and soil Wetness of the earth Measurement methodology Material used for earth surface covering layer

Value assumed for the maximum fault current injected into the station ground grid

I di t if l l l t d bt i d f th Indicate if value was calculated or obtained from another source


Grid Design Documentation Indicate what soil model was used and how it was

obtained Worst case scenario

Indicate the maximum GPR, touch and step voltages predicted for the systempredicted for the system Fault duration assumed

Incorporate the results of ground resistance testing Incorporate the results of ground resistance testing of the grid

Incorporate scale drawings of the ground rod p g gplacement and ground conductor interconnections Show all fences, underground pipes, intergrid conductor

connectionsconnections


Grounding Tutorial

Section 9

Substation Ground System Modular Substation incorporating

5 kV Switchgear and MCCs 600 V Switchgear and MCCs 600 V Switchgear and MCCs UPS power distribution system DCS Control System

Grounding system consists of: Grounding system consists of: Power Distribution System Ground

5kV Low resistance ground system 600V High resistance ground system 600V High resistance ground system

Equipment Ground Instrumentation ground Lightning surge arrestor ground


Substation Single Line

Surge Arrestor

Overhead lineWith no Neutral

YLRG NGR

Surge Arrestor

M

5kV

MM M

Y600V

HRG NGR

MM

M M

~=

=~


UPSPP

Substation Layout

600V MCC DCS

5 kVSwgr5kV MCC600V Swgr UPS

5kV

LRGNGR

HRG NGR

600VXFMR

5kVXFMR

Surge


SurgeArrestor

Tutorial Objectives Use the HRG and LRG NGRs sizing from the previous tutorial

5A NGR on the 600V System 125A NGR on the 5kV System

Design a station ground grid system based on the following informationinformation Maximum ground fault current = 0.4kA (Provided by utility) Assume a 150mm crushed rock surface layer over a clay subsurface

layer with a resistivity of 6000 (Ω cm) under normal conditionslayer with a resistivity of 6000 (Ω . cm) under normal conditions

Verify the GPR, touch and step potentials for the design


Grounding Tutorial

Answers

GPR < Touch Voltage MethodStep 1 – Determine the Maximum Ground Fault

Current 3phase fault current given as 5kA Maximum ground fault current injected into the grid is

given as 0.4kA Step 2 – Determine the resistivity of the soil in the

i i it f th b t tivicinity of the substation Resistivity of Clay given as 6000 (Ω . cm) or 60 Ω·m 150mm Surface layer of Crushed stone specified 150mm Surface layer of Crushed stone specified


Step 3 Touch Potential

Step 3 - Ground grid must be capable of handling 0.4kA and limit the touch and step potentials as p pspecified in CEC table 52

U l l thUse lower value as the Ground grid voltage Rise Criteria


Step 4 Required Ground Resistance Target grid resistance is based on limiting the station

GPR to less than the maximum touch voltageg

R = Etouch/IGtouch GR = Grid ResistanceIG = Maximum Grid CurrentEtouch = Tolerable touch voltage from CEC table 51

M i i d i t f d id 885V 2 21Ω

Etouch Tolerable touch voltage from CEC table 51

Maximum required resistance of ground grid =0.4kA

= 2.21Ω


Step 5 Number of ground rods required

Resistance of one ground rod

Rg (rod) =ρ(Ω.cm)335 cm

Ω

6000Ω.cmRg (rod) =

6000Ω.cm335 cm

= 17.91Ω

Estimate 16 ground rods to start Estimate 16 ground rods to startRg= Rn X Fn

2 14Ω

17.91Ω= Rn X 1.9216


= 2.14ΩRn

Step 6/7 Station Ground Grid3.3m

600V MCC DCS

3.3m Ground rods with

Inspection well


5A NGR

5kV

5A NGR

3 d d

Bare 2/0 AWG

600VXFMR

5kVXFMR

125ANGR

SurgeArrestor

3m ground rods(16 in total)


IEEE 80 Design Calculation Step 1 Field Data Maximum 3 phase fault current 3 p

given as 5kA Maximum ground fault current

i j t d i t th id i iinjected into the grid is given as 0.4kA

Resistivity of Clay given as 6000 (Ω . Resistivity of Clay given as 6000 (Ωcm) or 60 Ω·m

150mm Surface layer of Crushed t ifi dstone specified


Step 2 Conductor Size Select the ground grid conductor material

and calculate an appropriate conductor size

Akcmil = I · Kf √tc

Akcmil = area of conductor in kcmilI = Maximum 3 phase fault current in kAtc = current duration in seconds (IEEE recommends 3.0 seconds)K = constant based on the material (Refer to table 2 IEEE 80)Kf = constant based on the material (Refer to table 2 IEEE 80)

Akcmil = 5 · 7.06√3 = 61.14 kcmil

61.14 kcmil → Minumum #2 AWG


2/0 AWG Selected for Mechanical Strength

Step 3 Touch and Step Criteria

ρ = resistivity of earth beneath surfaceρ resistivity of earth beneath surfaceρs = surface material resistivity (Ω . m)hs = thickness of surface material in m

60

CS = 1 -3000

601 -0.09

2(.15) + 0.09= 0.773


( )

Step 3 Touch and Step Criteria

ETouch =116 + 0.174(0.773)(3000)

= 735V√0.5

EStep =116 + 0.696(0.773)(3000)

= 2446V√0.5√0.5


Step 4 Initial Design Preliminary design should incorporate a

conductor loop surrounding the available area with cross conductors to provide convenient access for equipment grounds

14m

13 3 d d 39

10m

13 - 3m ground rods = 39m5x14m + 5x10m = 120m #2/0 AWGArea = 10m X 14m =140m2

Grid depth = 450mmp


Step 5 Grid Resistance

ρRg = +√20ALT

111 +

11+h√20/A√20ALT 1+h√20/A

Rg = Substation resistance in Ωρ = Soil resistivity in Ω . mA = Area occupied by the grid in m2

h = Depth of the grid in mLT = Total length of conductors and rods in m

60Rg = +√20(140)159

11 1 +1

1+0.45√20/140

Rg = 2.48Ω


Step 6 Grid Current Maximum ground fault current

injected into the grid is given as 0.4kA


Step 7 GPR < Touch Voltage? Determine if the GPR is less than the

acceptable touch voltage for the station

GPR = IG X Rg < Etouch

I = Maximum Grid CurrentIG = Maximum Grid CurrentRg = Grid ResistanceEtouch = Etouch50 or Etouch70

GPR = 400A X 2.48Ω = 992V

GPR 992V > E 734V

Go to step 8 and Calculate Mesh

GPR = 992V > Etouch = 734V

Voltage


Step 8A Calculate Mesh Voltage Calculate the MESH voltage for the grid

design

ρ · Km· Ki · IGEm = LM

= 60 · Km· Ki · 400

LMM

EM = Mesh Voltageρ = Soil resistivity in Ω·m

M

ρ yKm = Geometrical correction factor for grids of varying dimensionKi= Correction factor for grid geometry IG = Maximum grid currentL = Effective length of grid conductors and ground rods in mLM = Effective length of grid conductors and ground rods in m


Km = Geometrical correction factor

K =1

lD2 D + 2 · h 2 h Kii+ l

816 · h · d

Km = 2 · π

· ln + 8 · D · d-

4 · d +Kh

· π(2 · n – 1)ln

For grids with ground rods along the perimeter and thoughout the grid area: K = 1For grids with ground rods along the perimeter and thoughout the grid area: Kii = 1

Kh = 1 +√ ho

ho = 1m (grid reference depth) = h 1 +√ 1

0.45 = 1.2

D = Spacing between parallel conductors in m = 3.5mh = Depth of ground grid conductors in m = 0.45md = Diameter of grid conductor in m = 0.0093m

√ o √

d Diameter of grid conductor in m 0.0093mn = Effective number of parallel conductors in a given grid


Km = Geometrical correction factorn = na · nb · nc · nd

2 L

LC = is the total length of the conductor in the horizontal grid in m = 120m

Lp = is the peripheral length of the grid in m 2 120

Lp

2 · LCna =

Lp

p= 48m

A = Area of Grid = 140m2= 482 · 120

= 5

1 0148

nc = 1 for square and rectangular gridsnd = 1 for square, rectangular and L – shaped grids

nb =4 · √A

p =4 · √140

= 1.01

K C ti f t f id t

n = 5 · 1.01 · 1 · 1 = 5.05

Ki= Correction factor for grid geometry

Ki = 0.644 + 0.148 · n = 0.644 + 0.148 · 5.05 = 1.39


LM = Length of Grid Conductors and Ground Rods

For grids with ground rods in the corners, as well as along the perimeter and throughout the grid

LLM = LC + 1.55 + 1.22

Lr

√Lx2 · Ly

2 • LR

Lr = is the length of each ground rod in metersLx = is the maximum length of the grid in the x direction in mLy = is the maximum length of the grid in the y direction in mL i th l th f d t i th id iLC = is the length of conductors in the grid in mLR = is the length of rods in the grid in m

3LM = 120 + 1.55 + 1.22

3√102 · 142 • 39 = 181.46


Mesh Voltage

16 · 0.45 · 0.0065Km =

1

2 · π· ln

3.52

+ 8 · 3.5 · 0.0065

3.5 + 2 · 0.45 2- 4 · 0.0065

0.45

1+

1 2· π(2 · 5.05 – 1)ln

8

K 0 70 1.2 π(2 5.05 1)Km = 0.70 Ki = 1.39L = 121 57

ρ · Km· Ki · IG 60 · 0.7· 1.39 · 400

LM = 121.57

ρ m i GEm = LM

= 181.46= 128.7V



ρ · Ks· Ki · IGE

Es = Step Voltageρ = Soil resistivity = 60 Ω·mKi= Correction factor for grid geometry = 1.39Es = LS

i f f g g yIG = Maximum grid current = 400ALS = Effective buried conductor length mKs = Spacing factor for step voltage

LC = is the total length of the conductor in the horizontal grid in m = 120

LS = 0.75 · LC + 0.85 · LR

C is the total length of the conducto in the ho i ontal g id in m 0LR = is the total length of all ground rods = 39m

LS = 0 75 · 120 + 0 85 · 39 = 123 15LS 0.75 120 + 0.85 39 123.15



2 · hKS = 1

π1

+ D + h1

+ D1

1 – 0.5n-2

D = Spacing between parallel conductors in m = 3.5h = Depth of ground grid conductors in m = 0.45

1 1 1 1 5 05 2

d = Diameter of grid conductor in m = 0.0065n = Effective number of parallel conductors in a given grid = 5.05

2 · 0.45KS = 1

π1

+ 3.5 + 0.45 + 3.51

1 – 0.55.05-2 = 0.513

ρ · Ks· Ki · IGEs = LS

= 60 · 0.513 · 1.39 · 400

123.15 = 139V


Step 9 Em < Etouch?

Emesh = 128.7V < 734V = Etouch

YES go to step 10

Emesh 128.7V 734V Etouch


Step 10 Es < Estep?

Es = 139V < 2446V = Estep

YES go to Detailed Design

Es 139V 2446V Estep


Detailed Design3.3m 2/0 AWG Insulated Gnd Wire

600V MCC DCS

3.3m Ground rods with

Inspection wellIsolated Inst. Gnd BusEqpt Gnd Bus


5A NGR

IntergridGrounding Conductors

5kV

5A NGR

3 d d

Bare 2/0 AWG

600VXFMR

5kVXFMR

125ANGR

SurgeArrestor

3m ground rods(16 in total)

2/0AWG toXo Terminal


Xo TerminalGnd Conductor as short as possible

Substation Ground System Instrumentation Ground Designed to insure that all components of the Designed to insure that all components of the

control system operate at the same potentialEliminate potential ground loopsEliminate potential ground loopsIsolate system noise on the ground systemAllow ground to be accessible for disconnect to assist in

isolating ground loops


Control System Grounding Scheme

SWGR UPS UPSPNLPNL

Main Transformer

InstrumentPanel Bonding

N

LightningA t

NGR

GroundConductor

Bonding ConductorPanel BondingConductors

Arrestor

System Ground

CP CP CPIsolated Ground Bus

Insulated Instrument Ground conductors


Computer Analysis GPR < TouchSummer Conditions


Computer Analysis GPR < Touch

Summer Conditions


Computer Analysis GPR < Touch

Winter Conditions


Computer Analysis IEEE 80


Computer Analysis IEEE 80


Computer Analysis Optimized


Computer Analysis Optimized


Summary and Wrap up

Section 10

Learning Objectives Review1. To understand why we ground Protect life from the danger of shock Protect life from the danger of shock Limit the voltage on a circuit Facilitate operation of protective devices Facilitate operation of protective devices

Low Impedancepath to source G1 L

Fuse

AccidentalGround

path to sourceallows fuse tooperate

G1 L

Neutral


Learning Objectives Review2. To describe the difference between grounding and

bondingg System grounding refers to the intentional connection of a

phase or neutral conductor to earth for the purpose of t lli th lt t d ithi di t bl li itcontrolling the voltage to ground, within predictable limits

Bonding or equipment grounding refers to the interconnection and connection to earth of all normallyinterconnection and connection to earth of all normally non-current carrying metal parts Insures that all metal parts remain at ground potentialReduces the shock hazard to personnelReduces the shock hazard to personnelProvides a low impedance return path for ground currents

– Allows the circuit protection device to operateMinimize the fire and explosion hazard


Learning Objectives Review3. To apply the safety requirements as defined by the

Canadian Electrical Code and the IEEE as they yrelate to grounding CEC Section 10 Defines when a system should be grounded and when equipment

should be bonded Describes the acceptable methods for grounding and bonding

and stipulates the size of grounding and bonding conductors Defines what an acceptable grounding electrode shall be

CEC Section 36CEC Section 36 Describes the grounding and bonding requirements for high

voltage substationsGPR < 5000\v– GPR < 5000\v

– Touch and Step potential as per table 52


Learning Objectives Review3. To apply the safety requirements as defined by the

Canadian Electrical Code and the IEEE as they yrelate to grounding IEEE 142 (Green Book) Recommended Practice for the Grounding of Industrial and

Commercial Power Systems

IEEE 1100 (Emerald Book)IEEE 1100 (Emerald Book) Recommended Practice for Powering and Grounding Electronic

Equipment

IEEE 80 IEEE 80 Guide for Safety in AC Substation Grounding Primarily concerned with outdoor AC substations


Learning Objectives4. To select the appropriate systems grounding

scheme for an industrial facilityyCondition Un-

groundedSolid

GroundLow

ResistanceHigh

Resistance

Immunity to transientImmunity to transient overvoltages Worst Good Good Best

Arc Fault Damage Protection Worst Poor Better Best

Safety to Personnel Worst Better Good Best

Service Reliability Worst Good Better Best

Continued operation after initial ground fault Better Poor Poor Best

Ground fault locating Not G d B tt B t


gPossible Good Better Best

Learning Objectives5. To implement a static electricity control and

lightning protection systemg g p y Static Control

Bond together and to ground

Lightning ProtectionLightning strikes cannot be stopped but their energy can be

diverted in a controlled mannerRequires a low impedance path to ground


Learning Objectives6. To avoid the problems typically associated with the

grounding of sensitive electronic systemsg g y Ground loops - use the single point grounding concept Methods of Noise Mitigation Physical Separation Electrical Segregation Harmonic Filtering Harmonic Filtering Shielding or screening of noise sources


Learning Objectives7. To design a ground grid for a high voltage industrial

substation Limit the ground potential rise between two points to a safe

value Limit the touch and step potentials to a safe value Must be able to withstand the maximum ground current

without damagewithout damage Important part of a safe and reliable electrical systems

design


Documents

Grounding Fundamentals Course Presentation