CHAPTER 5 - System Protection.pdf

7/28/2019 CHAPTER 5 - System Protection.pdf

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BEX 42803/ BEF 33203/ BEE 4213 Utilisation of Electrical Energy

Earthing System Protection Against Electric Shocks Earth Faults Protection Protection Against Voltage Surges

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Earth : The conductive mass of the Earth, whose e ec r c po en a a any po n s conven ona y taken as zero.

An earth is defined as a connection to the general mass of earth .

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(1) Earth electrode

(2) Earthing conductor

(3) Protective conductor

(4) Electrical installation

(5) Bonding conductor

(6) Main earthing terminal

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(7) Removable link


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A conductor or other metal is earthed when it s e ec ua y connec e o e genera mass o earth by means of a metal rod or a system of metal water pipes or other conducting object.

Solidly earthed or Bolted earthed when it is earthed without the intervention of a fuse switch, circuit breaker , resistor, reactor, or solenoid.

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Earthing is meant by having every item of apparatus an every con uctor s a e prevented from giving rise to earth leakage

currents . It is carried out by ensuring any metal liable to

become char e should be earth and ever art of the earthing circuit should be properly installed.

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Other alternatives to earthing that could be

construction, double insulation and by having an isolation.

Earthing protects people and equipment from potentially dangerous over voltages and

in homes, offices, retail outlets and industrial plant.

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Groundingonnec ng equ pmen an po n s on e ec r ca sys ems

to the earth or an earth substitute. Purpose is to limit overvoltages between the equipment

and the earth due to lightning, faults, etc.

Bonding

neutral point. Purpose is to limit voltages between equipment and to

provide a path for ground fault current.

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Reason (1) Minimise overvoltages

Lightning strike

Lightning arrester

transformerUtility phase conductor

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Service earth rodTransformer earth rod


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Reason (2) Limit voltage potential on equ pmen enc osures

415V

Ungroundedmotor frame

Insulation breakdown

Groundedmotor frame

Insulation breakdown

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400V motor

feeder conductors 240V

400V motor

Equipment earthingconductor

0V

Earthing surface Earthing surface


Reason (3) Provide a low impedance path for au t current

415V

Ungrounded

motor frame

Insulation

breakdown

Grounded

motor frame

Insulation

breakdown

IF=

Overcurrentdevices

Overcurrentdevices

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400V motor

feeder conductors

400V motor

Equipment earthingconductor IF

IF F


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Designing a safe earthing system means prov ng e eas es an s or es pa or e fault current without exposing a person to electric shock .

Satisfactory earthing is the most important part of an electrical installation because o eration of all the protective devices depend upon it.

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The total impedance of the conductor,

to the earth electrode (earth continuity conductor) measured between the earth electrode and any other part of the installation at supply frequency should not exceed 1.0 .

. resistance), it is an indication of improper earthing.

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Earth lug terminal rusty Loose wire connection Layers of paint on the electric apparatus Loose connection of earth wire to the plug and

socket outlet

Loose connection between conduit and terminal box

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T = Terre (French word for earth)

supply connected to earth through an impedancefor isolation.

N = Neutral (in AC system, the earth point isnormally the neutral point).

provided by separate conductors).

C = Combin (neutral and protective functionscombined in a single conductor, PEN).

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TT system (earthed neutral)

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One point at the supply source is connected directly to earth. All exposed andextraneous conductive parts are connected to a separate earth electrode at theinstallation. This electrode may or may not be electrically independent of the sourceelectrode.


TNC system

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The neutral conductor is also used as a protective conductor and is referred to as a PEN (Protective Earth and Neutral) conductor.


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TNS system

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The protective conductor and the neutral conductor are separate. Onunderground cable systems where lead sheathed cables exist, the protectiveconductor is generally the lead sheath.


TNCS system

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In the TN CS system, the TN C (4 wires) system must never be used downstream of the TN S (5 wires) system, since any accidental interruption in the neutral on theupstream part would lead to an interruption in the protective conductor in thedownstream part and therefore a danger.


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IT system (isolated neutral)

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No intentional connection is made between the neutral point of the supply sourceand earth. Exposed and extraneous conductive parts of the installation areconnected to an earth electrode.


IT system (impedance earthed neutral)

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An impedance Zs (in the order of 1,000 to 2,000 ) is connected permanentlybetween the neutral point of the transformer LV winding and earth


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Simplest solution to design and install. Used ininstallations supplied directly by the public LV distributionnetwor .

Does not require continuous monitoring during operation.

Protection is ensured by special devices , the residualcurrent devices (RCD), which also prevent the risk of firewhen they are set to 500 mA.

Each insulation fault results in an interruption in thesupp y o power, owever e ou age s m e o efaulty circuit by installing the RCDs in series or in parallel.

Loads or parts of the installation which, during normaloperation, cause high leakage currents , require specialmeasures to avoid nuisance tripping .

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TNC system

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TNS system


Requires the installation of earth electrodes at regularintervals throughout the installation.

Requires that the initial check on effective tripping for thefirst insulation fault be carried out by calculations during thedesign stage, followed by mandatory measurements toconfirm tripping during commissioning.

Requires that any modification or extension be designed andcarried out by a qualified electrician.

May result, in the case of insulation faults, in greater damageto the windings of rotating machines.

May, on premises with a risk of fire, represent a greaterdanger due to the higher fault currents

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Solution offering the best continuity of service during operation.

Indication of the first insulation fault, followed by mandatory location and clearing, ensures systematic prevention of supply outages.

Generally used in installations supplied by a private MV/LV or LV/LV transformer.

Requires maintenance personnel for monitoring and operation.

Requires a high level of insulation in the network (implies breaking up the network if it is very large and the use of circuit separation transformers to supply loads with high leakage currents).

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An electric shock is the pathophysiological effect.

The degree of danger for the victim is a function of the magnitude of the current , the parts of thebody through which the current passes, and theduration of current flow .

through a human being from one hand to feet, theperson concerned is likely to be killed, unless thecurrent is interrupted in a relatively short time.

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30Zones time/current of effects of AC current on human body when passing from left hand to feet


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A direct contact refers to a person coming into con ac w a con uc or w c s ve n normal circumstances.

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Is = Touch current


An indirect contact refers to a person coming nto contact w t an expose con uct ve part

which is not normally alive, but has become

alive accidentally (due to insulation failure or some other cause).

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Id = Insulation fault current

Is = Touch current


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The earthing of all exposed conductive parts of

constitution of an equi potential bonding network.

Automatic disconnection of the supply of the section of the

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, way that the touch voltage/time safety requirements are respected for any level of touch voltage (Vc).


Maximum safe duration (disconnecting time ) of

Vc (V) 50 < Vc 120 120 < Vc 230 230 < Vc 400 Vc > 400

SystemTN or IT 0.8 0.4 0.2 0.1

TT 0.3 0.2 0.07 0.04

e assume va ues o ouc vo age n seconds) not exceeding 32 A:

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The impedance of the earth fault loop consists mainly . .

installation electrodes) in series.

The magnitude of the earth fault current is generally too small to operate overcurrent relay or fuses, and the use of a residual current operated device is

.

Protection by automatic disconnection of the supply used in TT system is by RCDof sensitivity:

38 A

n R

I 50

RA is the resistance of the earth electrode for the installation


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TNC


In all TN systems, any insulation fault to earth .

High fault current levels allow to use overcurrent

protection but can give rise to touch voltages exceeding 50% of the phase to neutral voltage at the fault position during the short disconnection

. The use of CB, fuses, and RCDs may be necessary

on TNearthed systems.

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A permanent monitoring of the insulation to earth ,

and/or flashing lights, etc.) operating in the event of a first earth fault.

During a phase to earth fault , the current passing through the electrode resistance RnA is the vector sum

.

The use of CB, fuses, and RCDs may be necessary on ITearthed systems

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For regular houses 40A / 63A, single phase or

capacity, ELCB's sensitivity is not exceeding 100mA .

For heater water , it should posses separate additional ELCB (other than no. 1 above) with

. For load which higher than 100A, 3 phases , ELCB's sensitivity must not exceed 100mA .

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To load fault

Circuit Breaker Tri coil

Current Transformer

earth

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3wire 3phase suppl y


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During normal operation the current from supply L flows through N1, to the load and then flow through N2 before returning to N.

and the current through them is also the same the resulting flux in the iron core is zero.

However, if there is a leakage from the load to the ground, a certain amount of current will flow to the ground.

As such current through N1 and N2 will not be the same resulting in some magnetic flux setting up in the core.

The fault sensing winding will trip the tripping device if

the leakage current falls within 10 to 100 milliampere. A test circuit consisting of a push button and a test resistor is normally included in the ELCB as shown. Its function is to create an unbalance current in the N1 and N2 windings, and the current flow through the test resistor is great enough for the fault sensing winding to trip the circuit breaker.

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There are two main reasons why RCDs are used: To provide additional and a higher level of protection than

that given by direct earthing, against electric shock and alsoagainst fire risk caused by earth leakage currents. Wherefuses and miniature circuit breakers (MCBs) are the only

means of earth fault protection, it is possible for earth faultcurrents to flow undetected and cause fire risk (or touchvoltage problems).

The use of an RCD will prevent the flow of a sustained

greatly reducing shock and fire risk. All live conductors inthe protected circuits should be disconnected in the eventof earth leakage current flowing .

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Terms associated with RCDs: RCCB:

Residual Current Circuit Breaker used in distribution boards to protect individual or groups of circuits

RCBO:Residual Circuit Breaker with overcurrent protection. This is a combined MCB/RCD and provides overload, short circuit and earth fault protection in one unit

Socket outlet with combined RCD

PRCD:This is a portable RCD unit with an inbuilt plug top and socket outlet

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Single Phase RCD

LOAD

Neutral

Phase

Detection Coil

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Relay

Test Button


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Three Phase RCD

EarthedMetalwork

L1

L3

L2

Test Resistor

LOAD

Test Button

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Amplifier

Magnetic Core

Trip Relay

u rDetection Coil


Regulation D22 (Basic Earthing Requirements)

States that earth leakage protection may be provided by means of fuses or excess current circuit breakers if the earth fault current

available to operate the protective device and so make the faultycircuit dead exceeds:1. 3 times the current rating of any semi enclosed fuse or any

cartridge fuse having a fusing factor exceeding 1.5 , used to protect the circuit, or

2. 2.4 times the rating of any cartridge fuse having a fusingfactor not exceeding 1.5 , used to protect the circuit, or

3. 1.5 times the tripping current of any excess current circuitbreaker used to protect the circuit.

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A single phase 240 V, 15 kW 50Hz motor circuit o eratin at 0.8 ower factor la in is rotected b a cartridge fuse having blowing current of 110 A. A fault occurs in the circuit causes a current of 220 amperes to flow through the earth continuity path. As a result of poor contact due to a lock nut and bush connecting a steel conduit to a metal box, the resistance of this conduit connection alone is 1.35 . State:a) whether the fuse will ruptureb) the amount of heat produced at the metal boxc) the degree of risk, if any, of a fire developing

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Yes. The rating current:

Fusing factor:

AV

I r 125.78

8.0240=

=

41.1125.78

110==

A

AF

F

D22, the protection is by a cartridge having a fusing factor

not exceeding 1.5. Then the maximum current in the fault is 2.4 x 78.125 A = 187.5 A.

I2R = (220) 2 x 1.35 Ohms = 65.34 kW. High Risk

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Atmospheric voltage surges Operating voltage surges Transient overvoltage at industrial frequency Voltage surges caused by electrostatic

discharge

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Lightning risk. Between 2,000 and 5,000 storms are .

are accompanied by lightning which constitutes a serious risk for both people and equipment.

Strokes of lightning hit the ground at a rate of 30 to 100 strokes per second.

Lightning affects transformers, electricity meters, household appliances, and all electrical and electronic installations in the residential sector and in industry.

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Lightning discharge values given by the IEC g tn ng protect on comm ttee:

Beyond peak

probability P%

Current peak,

I (kA)

Gradient,

S (kA/ s)

Total duration

(s)

Number of

discharges n

95 7 9.1 0.001 1

50 33 24 0.01 2

60

5 85 65 1.1 6


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A sudden change in the established operating con ons n an e ec r ca ne wor causes transient phenomena to occur.

These are generally high frequency or damped oscillation voltage surge waves.

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The opening of protection devices (fuse, circuit breaker), and the opening or closing of control devices (relays, contactors, etc.).

Inductive circuits due to motors starting and stopping, or the opening of transformers such as MV/LV substations.

Capacitive circuits due to the connection of .

All devices that contain a coil, a capacitor or a transformer at the power supply inlet: relays, contactors, television sets, printers, computers, electric ovens, filters, etc.

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These overvoltages have the same frequency as the network (50, 60 or 400 Hz).

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Phase/frame or phase/earth insulating faults on a ,

or by the breakdown of the neutral conductor . When this happens, single phase devices will be supplied in 400 V instead of 230 V.

A cable breakdown. For example, a medium voltage cable which falls on a low voltage line.

The arcing of a high or medium voltage protective spark gap causing a rise in earth potential during the action of the protection devices.

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The purpose of primary protection devices is to

lightning . They catch and run the lightning current into the

ground . The principle is based on a protection area determined by a structure which is higher than the rest.

There are three types of primary protection: Lightning conductors Overhead earth wires The meshed cage or Faraday cage

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The lightning conductor is a ape e o p ace on top of the building . It is

earthed by one or more conductors (often copper strips).

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These handle the effects of atmospheric , opera ng or n us r a requency vo age surges.

They can be classified according to the way they are connected in an installation: serial or

arallel rotection.

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This is connected in series to the power supply w res o t e system to e protecte .

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Transformers

certain harmonics disappear by coupling. This protection is not very effective .

Filters Based on components such as resistors, inductance

coils and capacitors they are suitable for voltage surges

cause y n ustr a an operat on stur ance corresponding to a clearly defined frequency band . This protection device is not suitable for atmospheric disturbance .

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Wave absorbers They are essentially made up of air inductance coils

which limit the voltage surges, and surge arresters

which absorb the currents . They are extremely suitable for protecting sensitive electronic and computing equipment.

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Main characteristics

correspond to the network voltage at the installation terminals.

When there is no voltage surge , a leakage current should not go through the protection device which is on standby .

When a voltage surge above the allowable voltage

, protection device abruptly conducts the voltage surge current to the earth by limiting the voltage to the desired protection level, Vp.

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When the voltage surge disappears, the protection

without a holding current. This is the ideal U/I characteristic curve : The protection device response time (t r) must be as short

as possible to protect the installation as quickly as possible.

e pro ec on ev ce mus ave e capac y o e a e to conduct the energy caused by the foreseeable voltage surge on the site to be protected.

The surge arrester protection device must be able to withstand the rated current , In.

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(1) Voltage limiters They are used in MV LV

substations at thetransformer output, inIT earthing scheme.

surges to the earth,especially industrialfrequency surges.

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(2) LV surge arresters

Low voltage surge arresters come in the form of modules to be installed inside LV switchboard .

They ensure secondary protection of nearby elements but have a small flow capacity.

ome a e even u n o oa s a oug ey cannot protect against strong voltage surges.

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CHAPTER 5 - System Protection.pdf