anjuli chandra

8/19/2019 Anjuli Chandra

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Earthing configurationsfor

LT distribution systemsby

Anjuli Chandra

Director CEA


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Earthing for LT systems – Deficient A earthing system should be installed in a manner

that it will limit the effect of ground potential

gradients to such voltage and current level that willnot endanger the safety of people or equipmentunder normal and fault conditions, as well as assurecontinuity of service.

Earthing systems adopted at Generating Stations,Substations are well designed for safety

Substations usually have ground grid system withground mat as extra protection. Grid system has the

form of horizontally buried grid conductor,supplemented by a number of vertical ground rodsconnected to the grid.

It is however observed that this area is most often

neglected especially in the LT Distribution networkand consumer premises.


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Earthing of LT networks

The earthing system for Distributionnetworks

Fixes the potential of live conductors withrespect to the earth in normal operation sothat it is consistent with the level of insulation.

Limits voltage between the frames of electrical equipment and earth should aninsulation fault occur

Enables having low zero phase sequenceimpedance to keep the higher fault currentand enable operation of relays for circuitbreaking under fault conditions

Limits rises in potential due to MV faults.


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LT earthing practices

The world has several different power Distribution

system grounding configurations.

In IEC standards, the various power systemconfigurations have designations based on theconnection of the Neutral wire to earth, and on theconnection of the Protective Conductor connection to

earth.

The IEC designation is comprised of two letters, thefirst for the Neutral conductor, and the second for theProtective Conductor.

T=Earth N = Neutral S = Separate C = Combined I = Isolated


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Types of Earthing Configurations

Three types of earthing

configurations or their combinationare generally followed:

1. TT

2. TN2.2 TN-C

2.3 TN-S

2.4 TN-C-S3. IT


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Neutral of the source connected and earthed .

Consumer or licensee must provide connection to

earth at consumer premises, i.e. by installing a earth

electrode local to the installation

The protective conductor is connected to its own

ground rod, remote from the neutral ground rod. In some

cases, the ground rod may be the steel frame of the

building. In any case, there is no direct copper connection between the consumer earth and the supply

system.

TT System of Earthing


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TN-S System

Neutral of the source of energy connected withearth at one point only, at or as near as isreasonably practicable to the source.

The protective earth conductor is connected to theneutral

The “S” in the designation means that theprotective earth conductor is a separate systemconductor.

Consumer’s earthing terminal is typicallyconnected to the metallic sheath or armour of thedistributor’s service cable into the premises or to aseparate protective conductor, for instance, in anoverhead supply


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L

L

L L

N

E

Source

Transfor

mer

TN-S System


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TN-S System

Unlike the TT and IT systems, in the TN-S systemthe equipment and man are grounded throughdifferent paths. If the current through the differentpaths is different, then a potential difference willoccur between the equipment and the man, andcurrent will pass through the man.

To minimize the potential difference due to thedifference between the equipment and the man, itis imperative to keep the equipment ground circuit

resistance as low as practicable.


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TN-S system

For a TN-S system, grounding does not provide anequipotential environment due to the finiteresistances of the equipment grounding circuit.

However, equipment grounding through itsprotective conductor does serve to limit the

voltage for low fault currents.

For higher fault currents, another scheme providesprotection against electric shock: limited durationof the current through the body by means of

automatic disconnection of the supply (operationof the circuit-breaker).

Consumers having dedicated transformers for theirinstallation and installed adjacent to or within

their premises. In such situations the usual form ofsystem earthing is TN-S.


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TN-S System


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TN-C system

The TN-C system has a single ground rod. Source star point is earthed and the function of

neutral and earth is combined throughout thesystem.

The protective earth conductor is connected to the

neutral in the equipment. There is no separate protective conductor.

The “C” in the designation means that theprotective earth conductor is combined with the

neutral conductor.


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TNC System

L

L

L L

N

Source

Transformer


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TN-C-S system earthing

A TN-C-S system has the supply neutral conductor of a

distribution main connected with earth at source and atintervals along its run. This is usually referred to as

protective Multiple earthing (PME).

With this arrangement the distributor’s neutral conductor is

also used to return earth fault currents arising in the

consumer’s installation safely to the source.

To achieve this, the distributor will provide a Consumer’s

earthing terminal which is linked to the incoming neutral

conductor.

In TN-C-S system neutral must be connected to the earth

only on supply side of the customer disconnecting switch.

So TN-C-S system is a combination of TN-C for the power

supply network and TN-S for the customer Network


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Protective Multiple Earthing (PME) systemWhere an PME earthing system is used, the following

conditions should be met:

Neutral conductor should be earthed at or near each

LV distribution centre

Neutral should be earthed at other points along the

distribution system.

Earthing should be arranged so as to ensure that

resistance of neutral to earth at any location does not

exceed prescribed limits.

In PME system the high voltage and low voltage

earthing systems should be kept separate.


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IT system

The distribution system has no connection toearth at all, or it has only a high impedanceconnection.

The neutral is connected through an impedanceto its ground rod in the second case

The protective conductor is connected to its ownground rod, remote from the neutral ground rod.In some cases, the ground rod may be the steelframe of the building.

In any case, there is no direct copper connection

between the enclosure and the supply system.


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One characteristic of the IT system is that the systemis tolerant of a fault to ground. That is, a fault to grounddoes not operate the circuit breaker, so the systemremains operational. (An alarm identifies the fault toground, but the system continues to operate.)

•Advantages are all voltages with respect to earth are

kept as low as possible and also allows an earth fault

to develop without causing a trip.

•Such systems must always have Earth Fault Warning

Devices, if it is imperative for a trip to occur on anearth fault, yet the advantage of reduced voltages with

respect to Earth are required.

IT system


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IT system


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Governed by Rule33

•The supplier shall provide and maintain on the

consumer’s premises for the consumer’s use, a suitableearthed terminal in an accessible position at or near the

point of commencement of supply as defined under rule

58.

•

Provided that in the case of medium, high or extra highvoltage installation the consumer shall, in addition to

the aforementioned earthing arrangement, provide his

own earthing system with an independent electrode.

•Provided further that the supplier may not provide any

earthed terminal in the case of installations already

connected to his system or before the 30th June, 1966

if he is satisfied that the consumer’s earthing

arrangement is efficient.

Earthing at consumer premises

I E RULES REGARDING SAFETY AND EARTHING


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I.E.RULES REGARDING SAFETY AND EARTHING

29: Rating sufficient for power, insulation andestimated fault current to construct in such a manner to

ensure safety of human beings, animals and property.

To follow relevant code of practice

30:The supplier shall ensure that all electric supply

lines, wires ,fittings and apparatus belonging to him or under his control, which are on a consumer premises

are in safe conditions and in all respects fit for supplying

energy and the supplier shall take due precautions to

avoid danger arising on such premises from such supply

lines, wires, fittings and apparatus.

The consumer shall also ensure that the installation

under his control is maintained in a safe condition.


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46: Periodical inspection and testing of installation

(1) Existing installations to be checked once in five years by

the inspector or any other appointed to assist the inspector

47: Testing of consumer installation

(1)The supplier shall inspect and test the applicant’s

installation for a new or additional supply of energy

48: Precautions against leakage before connection

The supplier to check for leakage before connection and

the insulation resistanceshall be at least one Mega Ohm or

as specified in relevant Indian standard.


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61 Connection with the earth

Neutral conductor shall be earthed by not less than two

separate and distinct connections with a minimum of

two different earth electrodes at sub-stations or such

large number as may be necessary to bring the earth

resistance to a satisfactory value both at generating

stations and sub stations.

The earth electrodes so provided, may also be

interconnected to reduce earth resistance.


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61 Connection with the earth

it may also be earthed at one or more points along the

distribution system or service line in addition to any

connection with earth which may be at the consumer’s

premises.

Limit earth resistance sufficiently low to permit

adequate fault current for the operation of protective

device in time and to reduce neutral shifting.


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61 A : Earth Leakage Protective device

The supply of energy to every electrical installations

other than low voltage installation below 5 KW and those

low voltage installations, which do not attract

provisions of section 30 0f the Indian Electricity act shall

be controlled by an earth leakage protective device so

as to disconnect the supply instantly on the occurrence

of earth fault or leakage of current.

Provided that the above shall not apply to overhead

supply lines having protective devices which are

effectively bonded to the neutral of supply transformers

and conforming to rule 91 of I.E. Rules.


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Draft safety regulations: ELPD:

The supply of electricity to every electrical installationother than voltage not exceeding 250 V below 5kW

and those installations of voltage not exceeding 250V

which do not attract provisions of Section 54 of the

Electricity Act, 2003,shall be controlled by an earth

leakage protective device so as to disconnect thesupply instantly on the occurrence of earth fault or

leakage of current:

Conclusion


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Under present I.E.Rules It is responsibility of DISCOM to

ensure safety

The three earthing systems of TT, IT and TN for LTnetworks have their own advantages anddisadvantages.

The choice depends upon the Distribution Company,type of installations, type of consumers to be served,

operation philosophies.

TT system are difficult to maintain in India

TN-S or TN-C-S system are better as it provide low

impedance path (except in situations like petrol pumps

etc).

TN-S system offers best properties for EMC

Conclusion


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