6-Earting TN

8/6/2019 6-Earting TN

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3/30/20

GroundingGrounding

Professor Ahdab Elmorshedy

The objective of a grounding system are:1. To provide safety to personnel during normal

and fault conditions by limiting step and touch

potential.

2. To assure correct operation of electrical/

electronic devices.

3. To prevent damage to electrical/electronicapparatus.

4. To dissipate lightning strokes.



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5. To stabilize voltage during transient

conditions and to minimize the probabilityof flashover during transients.

6. To divert stray RF energy from sensitive

audio, video, control, and computer

equipment.

1. CLASSIFICATION OF ELECTRICALSYSTEMS

• An electrical system comprises a source

of energy and an electrical installation.

• According to the relationship between

the source and Earth and between theexposed conductive parts and Earth, a

system can be classified as follows:



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a. TN System

• The system has one or more points of the

source of energy directly earthed and the

exposed and extraneous conductive parts of

the installation are connected only by

means of protective conductors to the

earthed point(s) of the source.

Extraneous: not forming an essential or proper part:



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TN systems are subdivided into the following:

• TN-C systems where the neutral and protective

functions are combined in a single conductorthroughout the system.

• TN-S systems where there are separate neutraland protective conductors throughout thesystem.

• TN-C-S systems where the neutral and

protective functions are combined in a singleconductor but only in a part of a system.

b. TT System• The system has one or more points of the

source of energy directly earthed and the

exposed and extraneous conductive parts of

the installation are connected to a local earth

electrode that are electrically independent of

the source earth(s).



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c. IT System

• The system has the source either unearthed

or earthed through a high impedance and

the exposed conductive parts of the

installation are connected to an electrically

independent earth electrode

2. PURPOSE OF GROUNDING

• The purpose of grounding is to connect

together all metalwork, other than that

intended to carry current, to the earth, so

that dangerous potential differences

cannot exist, either between different

metal parts, or between metal and earth.



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• The system is connected to earth at the

secondary winding of the supply

transformer, where one conductor, which

is usually the neutral, is connected to an

earth electrode, buried in the mass of

earth.

Advantages of Grounding

• The whole system is tied to the potential of the

general mass of earth, and cannot 'float' at

another potential.

• For example, we know that the neutral of our

system is at or very close to zero volts

(reference potential) and not above or below it

when becoming charged.



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• By connecting to earth metalwork not

intended to carry current, a path is provided

for leakage current which can then be

detected, and if necessary, can be cut.

Disadvantages of Grounding

• Cost - the provision of a complete system of

protective conductors, earth electrodes, etc., is

expensive.

• Safety - the argument is made that complete

isolation from earth will prevent shock from

exposed conductive parts because there is no

complete path for the shock current.



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• This approach ignores the effect of leakage

resistance and of phase-to-earth capacitance(the insulation behaves as the dielectric

between the lines and earth).

• In many situations, the combined impedance of

leakage resistance and earth capacitive

reactance is low enough to allow significant

shock current to flow.

GROUNDING FOR TN-S SYSTEM



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a. Principle

• The principle of 'earthed equipotential bondingand automatic disconnection of the supply in a

TN-S supply system' is adopted here to explain

the typical arrangement.

• In addition, the grounding practices are based

mainly upon the IEE Wiring Regulation (BS7671)

and the Code of practice for Grounding (BS7430).

• This method is generally applicable to preventthe occurrence of a voltage of such magnitude

and duration between simultaneously accessible

conductive parts that danger could arise.

• With this method, the characteristics of the

protective devices for automatic disconnection,

the earth arrangements for the installation and

the relevant impedances of the circuits

concerned shall be coordinated.



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• A system is satisfactorily earthed if the

protective gear operates to remove

danger in the event of a fault to any

metalwork having a continuous metallic

connection to the system neutral.

b. Earth Mat

• Earth mats are installed in each substation.

• The overall earth mat design including number

and location is to meet the maximum allowable

step and touch potentials.

• Earth electrodes are jointed together and brought

out to be the principle earth conductor.

• A bolted copper link is normally provided for

connecting and disconnecting the principle earth

conductor from the grounding network to

facilitate testing.



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c. Earth Electrode

Common types of earth electrodes or means

of grounding include:

(a) Vertical plates, rods & pipes

(b) Horizontal strip or round conductor

(c) Uninsulated metallic sheaths and armour of cables

(d) Underground structural steelwork

(e) Sheet steel piling and steel reinforcement of

concrete piling

•Water and service pipes, gas or drainage, should

not be used as the means of grounding but should be

bonded to the protective conductors.

• Material for an earth electrode shall be resistant to

corrosion in the type of soil in which it will be used.

• Copper is one of the better and commonly used

material.

• The resistance to earth of an electrode depends upon

the size and shape of the conductor and the

electrical resistivity of the soil in which it is installed.



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d. Grounding Network

• From the test link, a vertical grounding conductorrising main with an grounding terminal at aconvenient location in each level is provided.

• From the grounding terminal in each levelincorporating electrical equipment rooms, acopper tape protective conductor is routedthrough all electrical equipment rooms.

• For substation incorporating more than one earth

mat, a grounding conductor of copperinterconnecting the various earth mats is provided.

e. Equipotential Bonding

• Main equipotential bonding conductors shallconnect to the earth network for the largeextraneous conductive parts not alreadyearthed by circuit protective conductor.

• They include, but do not limited to, thefollowing:

(a) all external metallic service pipes.

(b) fire main pipes.

(c) ventilation, air conditioning and chilled waterductwork.

(d) steel floor plates.

(e) cable tray.



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f. Grounding Conductor

The grounding conductor of the earth mat should be(a) sufficiently sized and supported to carry without

danger the greatest earth fault currents

(b) sufficiently robust to withstand mechanical

damage and corrosion in the ground, and

(c) compatibility with the material of the earth

electrode

g. Residual Current Device (RCD)

• RCDs are to be installed when earth fault

current in a circuit is insufficient to cause

operation of the overcurrent protective

devices within the time required.

• RCD should be installed for every socket outlet

circuit.



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• The RCD should have the product of the ratedoperating current (in A) and the earth fault loop

impedance (in ) not exceeding 50 V and be

capable of disconnecting all the phase

conductors of the circuit.

• RCDs for socket outlet circuit, in addition to

requirements above, should have a rated

residual operating current not exceeding 30 mA.

• The RCD range offers both 2 and 4 pole devicesin 30mA - 500mA trip sensitivities, with main

current ratings of 25A, 40A, 63A and 100A

• The RCD protects against residual currents

(earth leaks), where a small current may leak

from the circuit due to bad insulation.

• Critically the residual current device will react to

leakage currents as low as 30mA.

• This is an essential protection requirement.



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Residual Current Device (RCD)

h. System Grounding

High Voltage Power Distribution

(1) The secondary star point of the distribution

transformers is solidly earthed by direct

connection to the substation earth network with

copper conductors of sufficient size.

(2) The sheath and armour of all high voltagedistribution cables is bonded to the earth bar or

terminal of the switchgear panel or transformer

at each termination.



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Low Voltage Power Supply

(1) The star point of the service transformers isearthed by connection to the local earth networkwith copper tapes of appropriate size.

(2) The phase and neutral conductors of thedistribution circuits are provided with a circuitprotective conductor.

The protective conductors are bonded to thegrounding terminal or earth bar of the

switchboard, control panel or to which they areconnected.

Equipment Grounding

• The objective of electrical equipment

grounding is to ensure effective operation of

the protective gear in the event of earth fault

currents that might otherwise cause damage

to property, and to protect against danger to

life through shock due to installation

metalwork being maintained at a dangerouspotential relative to earth.



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HV & LV Switchgear

(1) All switchgear should be provided with a copperearth bar of sufficient size, running the full length

of the switchboard.

(2) All metal parts, other than those forming part of

an electrical circuit should be connected to the

earth bar.

The protective conductors of incoming and

outgoing cables should be bonded to the earth bar.

(3) Busbar and circuit grounding devices are

provided for all HV switchgear.

4) The earth bars of HV and LV main switchboards

are bonded to the substation earth network via

separately routed copper grounding conductor

connected at each end of the earth bar.

(5) Fuseboards and MCB boards are equipped with a

single earth terminal for connection of all

conducting parts, which do not form part of an

electric circuit.

The terminal is bonded to the earth network by

copper conductor.



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5. SPECIAL APPLICATION AND

SITUATION OF GROUNDINGa. Lightning Protection

• Protection of electrical equipment in a building

against induced voltages and possible side flash

can be provided by bonding the grounding

conductor of the lightning system to the main

grounding terminal of the electrical installation.

b. Clean Grounding

• If required, it should be directly connected to the

principle earth conductor isolating link using

insulated copper conductor and separated from the

grounding network and power cables to minimize

interference.

• The distribution of the insulated clean earthconductor should be in the form of star topology

or alike in order to avoid electrical noise loop back

phenomenon.



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c. Corrosion Due to Interconnection With

Another Metallic Item

• The possibility of damage to underground

services in the vicinity of earth electrodes, to

which the grounding system is to be bonded,

due to electrolytic action between dissimilar

metals should be considered.

• The rate of corrosion depends on the metals

involved and to some extent on their relative

surface areas.

• Galvanized steel is strongly electronegative to

both copper and steel in concrete so that an

earth electrode of bare galvanized steel

should not be bonded to either of them.



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d. Grounding Cathodically Protected Structures

• Cathodic protection is normally applied to

wholly or partly buried ferrous structures in

order to counteract electrolytic corrosion.

• Such a protection system relies on the

metalwork being maintained at a slightly

more negative potential with respect to theground than it would exhibit if it were

unprotected.

• If the cathodically protected structure has to

be earthed for any reason, earth electrodes of

bare copper should not be connected directly

to the structure.

• The bare copper is strongly cathodic to

ferrous materials and may require a quite

unacceptable current drain if the protection

is to be maintained.



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• In the event of failure of the source of protective

current, a copper electrode will accelerate the

rate of corrosion of the structure.

• If copper electrodes are used, the connection to

the structure should be made through a

polarization cell.

• This will drain only a small current from the

cathodic protection source, but will pass

alternating current with a low voltage drop.

TESTING AND MONITORING

Tests should be carried out on the earthing system to

determine its effectiveness at the completion of

installation and periodically after installation at a preset

interval.

The tests include the following items:

(a) Earth resistance of each earth mat.

(b) Earth fault loop impedance measurement.

(c) Touch voltage and step voltage measurement.(d) Interference voltage measurement.

(e) Inspection on the integrity of earthing conductorsand all associated connections.

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6-Earting TN