Earthing System - Earthing System - MaintenanceMaintenance

Engineer Abid Aziz

Earthing Systems:Earthing Systems:

Electrical earthing is designed primarily to render electrical installation safe. The purpose of earthing are :

1. Protection to the plant2. Protection to the personnel and3. Improvement in service reliabilityNon- current carrying parts with conducting surface such as tanks of Power

Transformers, and frame work of circuits breakers, structural steel work in switch yard instrument transformer cases, lightning arresters and armored cables armoring should be effectively grounded for protection of equipments and operating personnel. Earth connections of all equipments should be made in duplicate.

Connecting lead should have sufficient current carrying capacity.L A s should have independent earth electrode which should be inter connected

to the station grounding system.All paints, enamel, seals should be removed from the point off contact of metal

surfaces before earth connections are made.The resistances of earth system should not exceed 2 ohms for 33/11 KV Sub

Stations. But in the sub stations of Distribution companies Earth resistance Maximum of 1

Ohm is maintained.Suitable grounding mat should be provided in the sub station yard.

In a Sub Station the following shall be earthed.In a Sub Station the following shall be earthed.

The neutral point of the systems of different voltages which The neutral point of the systems of different voltages which have to be earthed.have to be earthed.

Apparatus, frame work and other non-current carrying metal Apparatus, frame work and other non-current carrying metal work associated with each system, for example transformer work associated with each system, for example transformer tanks, switch gear frame work etc.,tanks, switch gear frame work etc.,

Extraneous metal frame work not associated with the power Extraneous metal frame work not associated with the power systems, for example, boundary, fence, steel structures etc., systems, for example, boundary, fence, steel structures etc.,

The earthing Means connecting of Electrical equipment, The earthing Means connecting of Electrical equipment, machinery or an electrical system with the general mass of machinery or an electrical system with the general mass of earth is termed as earthing or groundingearth is termed as earthing or grounding

FUNCTION OF AN EARTHING SYSTEMFUNCTION OF AN EARTHING SYSTEM

The earthing system must provide an environment which is The earthing system must provide an environment which is free from the possibility of fatal electric shock.free from the possibility of fatal electric shock.

The earthing system must provide a low impedance path for The earthing system must provide a low impedance path for fault and earth leakage currents to pass to earth.fault and earth leakage currents to pass to earth.

The earthing conductors must possess sufficient thermal The earthing conductors must possess sufficient thermal capacity to pass the highest fault current for the required timecapacity to pass the highest fault current for the required time

The earthing conductors must have sufficient mechanical The earthing conductors must have sufficient mechanical strength and corrosion resistance.strength and corrosion resistance.

A Sub Station earthing system has to satisfy four A Sub Station earthing system has to satisfy four requirements:requirements:

Earthing can be broadly divided as :Earthing can be broadly divided as :

System Grounding ( System Earthing)System Grounding ( System Earthing) Equipment Grounding (Safety Grounding)Equipment Grounding (Safety Grounding)

System Grounding:System Grounding:

It is a connection to the ground of a part of the plant forming It is a connection to the ground of a part of the plant forming part of the operating circuits for example the star point of the part of the operating circuits for example the star point of the transformer or the neutral conductor. The grounding of the transformer or the neutral conductor. The grounding of the lighting, arrestors also comes under the head of system lighting, arrestors also comes under the head of system grounding. The provision of system ground reduces to grounding. The provision of system ground reduces to considerable extent the magnitude of the transient over considerable extent the magnitude of the transient over voltages and there by increases the life of electrical equipment voltages and there by increases the life of electrical equipment besides minimizing the services interruptions.besides minimizing the services interruptions.Thus the fundamental purpose of system ground is the Thus the fundamental purpose of system ground is the protection of installation and improvement in quality of protection of installation and improvement in quality of service. The system ground also will ensure the safety of the service. The system ground also will ensure the safety of the personnel to some extent, as it helps to clear the fault speedily.personnel to some extent, as it helps to clear the fault speedily.

Safety Grounds (Equipment Grounding)Safety Grounds (Equipment Grounding)

It is a connection to the ground of non-current carrying parts of the equipments like It is a connection to the ground of non-current carrying parts of the equipments like Motors, Transformer Tanks, Switchgear enclosures, Metallic enclosures of all Motors, Transformer Tanks, Switchgear enclosures, Metallic enclosures of all electrically operated equipments and also the installations used to carry/ Support electrically operated equipments and also the installations used to carry/ Support electrical equipments. The frames of the equipments, if not earthed when come electrical equipments. The frames of the equipments, if not earthed when come into contact accidentally with live parts will have potential with reference to the into contact accidentally with live parts will have potential with reference to the ground. The potential difference, when shunted between the hands and the feet of a ground. The potential difference, when shunted between the hands and the feet of a person touching the frame, produces current through the body which can result in a person touching the frame, produces current through the body which can result in a fatality. By connecting body which can result in a fatality. By connecting the fatality. By connecting body which can result in a fatality. By connecting the frames to a low resistance ground system, a sufficiently high current will flow into frames to a low resistance ground system, a sufficiently high current will flow into the ground when accidentally the live parts of the equipment / Machinery touch the the ground when accidentally the live parts of the equipment / Machinery touch the frames, and consequently saves the operating personnel from fatal accidents.frames, and consequently saves the operating personnel from fatal accidents.Thus the equipment grounding is basically intended to safeguard to a great extent Thus the equipment grounding is basically intended to safeguard to a great extent from the hazards of touch voltages. The safety ground is so designed that the from the hazards of touch voltages. The safety ground is so designed that the potential difference appearing between the frames and the neighboring ground is potential difference appearing between the frames and the neighboring ground is kept within safe limits.kept within safe limits.

Separation of system and safety grounds:Separation of system and safety grounds:

During ground fault conditions, the fault current flows via the During ground fault conditions, the fault current flows via the system ground. When the system and safety grounds are inter system ground. When the system and safety grounds are inter connected, the fault current flowing (via) the system ground connected, the fault current flowing (via) the system ground rises the potential of the safety ground. Also the flow of rises the potential of the safety ground. Also the flow of current to safety ground results in hazardous potential gradient current to safety ground results in hazardous potential gradient in and around sub station. In view of the above it is some in and around sub station. In view of the above it is some times suggested that separate system and safety grounds will times suggested that separate system and safety grounds will avoid the danger arising due to potential gradients. The idea is avoid the danger arising due to potential gradients. The idea is that by connecting the system ground to a separate earthing that by connecting the system ground to a separate earthing system situated in a in accessible spot, the ground fault current system situated in a in accessible spot, the ground fault current does not flow through the safety ground. However, this does not flow through the safety ground. However, this separate system of grounds has many disadvantages and can separate system of grounds has many disadvantages and can be more hazardous as mentioned below be more hazardous as mentioned below

With separate grounds we can avoid danger due to potentials With separate grounds we can avoid danger due to potentials only for faults outside the stations.only for faults outside the stations.

Short circuit currents will be more if the fault occurs in the Short circuit currents will be more if the fault occurs in the sub stations.sub stations.

The resistance may be more and in some cases sufficient The resistance may be more and in some cases sufficient currents may not flow to operate the relays.currents may not flow to operate the relays.

For effective separation of the earthing systems, the system For effective separation of the earthing systems, the system ground shall be installed at a distance of at least twice the ground shall be installed at a distance of at least twice the diagonal length of the sub station which is covered by safety diagonal length of the sub station which is covered by safety grounding. The neutral of the transformer has to be grounding. The neutral of the transformer has to be connected to this remote earthing by means of insulated connected to this remote earthing by means of insulated leads. Even with this arrangement one cannot always be leads. Even with this arrangement one cannot always be sure about the complete isolation of the two systems and sure about the complete isolation of the two systems and there is always a chance of inadequate electrical connection there is always a chance of inadequate electrical connection through buried neutral pipes etc., Hence, this is through buried neutral pipes etc., Hence, this is impracticable, complicated and costly. It is therefore a impracticable, complicated and costly. It is therefore a common practice to install a common grounding system and common practice to install a common grounding system and design the same for effective earthing and safer potential design the same for effective earthing and safer potential gradients.gradients.

System EarthingSystem Earthing

System earthing is governed by provisions of Rule - Of I.E System earthing is governed by provisions of Rule - Of I.E Rules, 1956. Unearthed systems have been tried and due to the Rules, 1956. Unearthed systems have been tried and due to the phenomenon of Arcing Grounds associated with them, theses phenomenon of Arcing Grounds associated with them, theses have been abandoned, excepting in a few cases of power have been abandoned, excepting in a few cases of power station auxiliaries supply systems where other arrangements station auxiliaries supply systems where other arrangements are made for indicating earth faults. In an ungrounded system are made for indicating earth faults. In an ungrounded system the insulation of all the equipments, lines etc, will have to be the insulation of all the equipments, lines etc, will have to be much higher values as compared to those of equipments and much higher values as compared to those of equipments and lines of a grounded system. This aspect greatly reduces the lines of a grounded system. This aspect greatly reduces the costs and ensures more safety.costs and ensures more safety.

Types of System Earthing:Types of System Earthing:

Earthing through a resistance.Earthing through a resistance. Earthing through a reactance.Earthing through a reactance. Earthing through a Peterson coilEarthing through a Peterson coil Earthing directly or solid earthing.Earthing directly or solid earthing.

Sub Station EarthingSub Station EarthingBecause of the difficulties and disadvantages involved in marinating the Because of the difficulties and disadvantages involved in marinating the system grounding and safety grounding separately it is the common system grounding and safety grounding separately it is the common practice now to have a combined grounding system at the sub stations. practice now to have a combined grounding system at the sub stations. Provision of adequate earthing in a sub station is extremely important for Provision of adequate earthing in a sub station is extremely important for the safety of the operating personnel as well as for proper system the safety of the operating personnel as well as for proper system operation. The Primary requirements of a good earthing system in a sub operation. The Primary requirements of a good earthing system in a sub station are.station are.The impedance to ground should be as low as possible. The impedance The impedance to ground should be as low as possible. The impedance of the earth system shall not exceed the following limits in the sub of the earth system shall not exceed the following limits in the sub stations stations

Power StationsPower Stations 0.5 Ohms0.5 Ohms

Major Sub stations above 110 KVMajor Sub stations above 110 KV 1.0 Ohms1.0 Ohms

Minor Sub Stations below 110 KVMinor Sub Stations below 110 KV 2.0 Ohms2.0 Ohms

Distribution Transformer StationDistribution Transformer Station 5.0 Ohms5.0 Ohms

Transmission line supportsTransmission line supports 10.0 Ohms10.0 Ohms

The Step and touch potentials should be within safe limits

Touch Potential :Touch Potential :Touch potential is the potential difference between the ground surface potential Touch potential is the potential difference between the ground surface potential where a person is standing and the potential of his outstretched hand (s) which where a person is standing and the potential of his outstretched hand (s) which are in contact with an earthed structure. It is normally assumed that a person’s are in contact with an earthed structure. It is normally assumed that a person’s maximum reach is 1.0 meter.maximum reach is 1.0 meter.Step PotentialStep Potential : :Step Potential is the potential difference between outstretched feet, at a spacing Step Potential is the potential difference between outstretched feet, at a spacing of 1.0 meter without the person touching any earthed structureof 1.0 meter without the person touching any earthed structureMesh Potential Mesh Potential The maximum potential difference between the centre of a mesh in an earth The maximum potential difference between the centre of a mesh in an earth grid, and an earthed structure connected to the buried grid conductors. It is grid, and an earthed structure connected to the buried grid conductors. It is worst case scenario of a touch potential.worst case scenario of a touch potential.Transferred potentialTransferred potentialThe transferred potential is a touch potential which is transferred some distance The transferred potential is a touch potential which is transferred some distance by an earth referenced metallic conductor. For example, consider a screened by an earth referenced metallic conductor. For example, consider a screened cable connecting two sub stations which are some distance apart. If a person cable connecting two sub stations which are some distance apart. If a person disconnects the earthed termination at one end of a screened cable he may be disconnects the earthed termination at one end of a screened cable he may be subjected to the full ground potential rise occurring due to an earth fault. This subjected to the full ground potential rise occurring due to an earth fault. This can be a very high touch potential.can be a very high touch potential.

To keep the ground impedance as low as possible and also to To keep the ground impedance as low as possible and also to have satisfactory step and touch voltages, an earthing mat will be have satisfactory step and touch voltages, an earthing mat will be buried at a suitable depth below the ground and it is provided buried at a suitable depth below the ground and it is provided with grounding electrode at suitable points. All the non-current with grounding electrode at suitable points. All the non-current carrying parts of the equipments in the sub stations are carrying parts of the equipments in the sub stations are connected to this grid so as to ensure that under fault conditions, connected to this grid so as to ensure that under fault conditions, none of these parts are at higher potential than the grounding none of these parts are at higher potential than the grounding grid. Under normal conditions, the ground electrode make little grid. Under normal conditions, the ground electrode make little contribution to lower the earth resistance; they are, however, contribution to lower the earth resistance; they are, however, desirable for marinating low value of resistance under all desirable for marinating low value of resistance under all weather conditions, which is particularly important where the weather conditions, which is particularly important where the system fault currents are heavy.system fault currents are heavy.Earthing in a sub station must conform to the requirements of the Earthing in a sub station must conform to the requirements of the Indian Electricity Rules and follow the directives laid down in Indian Electricity Rules and follow the directives laid down in section I and III of IS : 3043-1966. the earthing system has to be section I and III of IS : 3043-1966. the earthing system has to be designed to have a low overall impedance, and a current carrying designed to have a low overall impedance, and a current carrying capacity consistent with fault current.capacity consistent with fault current.

The factors which influence the design The factors which influence the design are:are:

Duration of fault.Duration of fault. Magnitude of the fault current.Magnitude of the fault current. Resistivity of the underlying strata.Resistivity of the underlying strata. Resistivity of the surface materialResistivity of the surface material Material of the earth electrode.Material of the earth electrode.

1.1. 100 X 16 mm and 75 X 8mm size MS steel flats are being ordered for forming the earthing 100 X 16 mm and 75 X 8mm size MS steel flats are being ordered for forming the earthing system for EHT Sub station and 33/11 KV Sub Stations respectively system for EHT Sub station and 33/11 KV Sub Stations respectively

2.2. Earth mat shall be formed with the steel flats buried in the ground at a depth of 500mm.Earth mat shall be formed with the steel flats buried in the ground at a depth of 500mm.3.3. The earth mat shall extend over the entire switchgear yard and beyond the security fencing The earth mat shall extend over the entire switchgear yard and beyond the security fencing

of structural yard by at least one meter.of structural yard by at least one meter.4.4. The outer most peripheral earthing conductor surrounding the earth mat shall be of 100 x 16 The outer most peripheral earthing conductor surrounding the earth mat shall be of 100 x 16

mm size MS flat.mm size MS flat.5.5. The intermediate earthing conductors forming the earth mat shall be of 75 x 8 mm size flat.The intermediate earthing conductors forming the earth mat shall be of 75 x 8 mm size flat.6.6. All the risers used for connecting the equipment steel structures etc., to earth mat shall be All the risers used for connecting the equipment steel structures etc., to earth mat shall be

of 50 x6 mm size excepting for earthing of L A s and transformer neutrals for which 100 x of 50 x6 mm size excepting for earthing of L A s and transformer neutrals for which 100 x 16 mm or 75 x 8 mm size shall be used.16 mm or 75 x 8 mm size shall be used.

7.7. All Junctions (crossing of the steel flats while forming the earth mat and taking risers from All Junctions (crossing of the steel flats while forming the earth mat and taking risers from the earthmat for giving earth connections to equipments, steel structural conducts, cable the earthmat for giving earth connections to equipments, steel structural conducts, cable shearths shall be propersly welded.shearths shall be propersly welded.

8.8. Proper earthing lugs shall be used for connecting the earth terminals of equipments to the Proper earthing lugs shall be used for connecting the earth terminals of equipments to the earthing steel flat.earthing steel flat.

9.9. Provisions shall be made for thermal expansion of the steel flats by giving suitable bends.Provisions shall be made for thermal expansion of the steel flats by giving suitable bends.10.10. The earth mat shall be formed by placing 75 x8mm MS flat at a distance 5 meters along the The earth mat shall be formed by placing 75 x8mm MS flat at a distance 5 meters along the

length & breadth of the sub station duly welding at crossing.length & breadth of the sub station duly welding at crossing.11.11. All the equipments, steel structural, conduits, cable sheaths shall be solidly grounded by All the equipments, steel structural, conduits, cable sheaths shall be solidly grounded by

connecting to the earthing mat at least two places for each.connecting to the earthing mat at least two places for each.12.12. The ground mat of the switchyard shall be properly connected to the earth mat of the The ground mat of the switchyard shall be properly connected to the earth mat of the

control house at least at two points.control house at least at two points.13.13. welding is done shall be given a coat of black asphalitic varnish and then covered with welding is done shall be given a coat of black asphalitic varnish and then covered with

hessain tape to avoid rusting.hessain tape to avoid rusting.

14.14. All paints, enamel and scale shall be removed from point of contact in metal surfaces All paints, enamel and scale shall be removed from point of contact in metal surfaces before applying ground connections.before applying ground connections.

15.15. The risers taken along the main switchyard structures and equipment structures up to The risers taken along the main switchyard structures and equipment structures up to their top) shall be clamped to the structure at an interval of not more than one meter their top) shall be clamped to the structure at an interval of not more than one meter with ground connectors.with ground connectors.

16.16. 75 X 8 mm ground conductor shall run in cable trenches and shall be connected to the 75 X 8 mm ground conductor shall run in cable trenches and shall be connected to the ground amt at an interval of 5 meters.ground amt at an interval of 5 meters.

17.17. Grounding electrodes 2.75 Mtrs length 100 mm dia 9 mm thickness CI Pipes shall be Grounding electrodes 2.75 Mtrs length 100 mm dia 9 mm thickness CI Pipes shall be provided at all their peripheral corners of the earthiong mat and also at Distance of 10 provided at all their peripheral corners of the earthiong mat and also at Distance of 10 Mtrs along length & width of switch gearand in the entire switch yard.Mtrs along length & width of switch gearand in the entire switch yard.

18.18. The grounding electrodes shall be drived into the ground and their tops shall be welded The grounding electrodes shall be drived into the ground and their tops shall be welded to a clamp and the clamp together with the grounding shall be welded to the ground to a clamp and the clamp together with the grounding shall be welded to the ground conductor.conductor.

19.19. The switchyard surface area shall be covered by a layer of crushed rock of size 25 x 40 The switchyard surface area shall be covered by a layer of crushed rock of size 25 x 40 mm to a depth of 100mmmm to a depth of 100mm

20.20. Transformers and L A s and single phase potential transformer shall be provided with Transformers and L A s and single phase potential transformer shall be provided with earth pits near them for earthing and these earth pits in turn shall be connected to the earth pits near them for earthing and these earth pits in turn shall be connected to the earth mat.earth mat.

21.21. Power Transformers neutral shall be provided with double earthing. Neutral earthing Power Transformers neutral shall be provided with double earthing. Neutral earthing and body earthing of power transformers shall be connected to separate earth electrode.and body earthing of power transformers shall be connected to separate earth electrode.

22.22. the entire earthing system shall be laid with constructional conveniences in the filed, the entire earthing system shall be laid with constructional conveniences in the filed, keeping in view the above points.keeping in view the above points.

23.23. The joints and tap-offs where welding is done shall be given a coat of black asphalitic The joints and tap-offs where welding is done shall be given a coat of black asphalitic varnish and then covered with hessain tape to avoid rusting.varnish and then covered with hessain tape to avoid rusting.

THE PERMISSIBLE LIMITS OF STEP POTENTIAL AND TOUCH THE PERMISSIBLE LIMITS OF STEP POTENTIAL AND TOUCH POTENTIAL SHALL BEPOTENTIAL SHALL BE

Maximum Acceptable step VoltageMaximum Acceptable step Voltage

Fault clearance timesFault clearance times

Fault clearance timesFault clearance times 0.2 Seconds0.2 Seconds 0.35 Seconds0.35 Seconds 0.7 Seconds0.7 Seconds

On soilOn soil 1050 V 1050 V 600 V600 V 195 V195 V

On chippings 150mm)On chippings 150mm) 1400 V 1400 V 800 V 800 V 250 V250 V

Maximum Acceptable Touch VoltageMaximum Acceptable Touch Voltage

Fault clearance timesFault clearance times

Fault clearance timesFault clearance times 0.2 Seconds0.2 Seconds 0.35 Seconds0.35 Seconds 0.7 Seconds0.7 Seconds

On soilOn soil 3200 V 3200 V 1800 V1800 V 535 V535 V

On chippings 150mm)On chippings 150mm) 4600 V 4600 V 2600 V 2600 V 815 V815 V

EARTH GRID- MATERIALEARTH GRID- MATERIALThe following are the minimum sizes of materials to used.

S. NoS. No ItemItem Material to be usedMaterial to be used

11 Grounding ElectrodesGrounding Electrodes CI pipe 100 mm (inner dia) CI pipe 100 mm (inner dia) Meters long with a flange at Meters long with a flange at the topthe top

22 Earth matEarth mat 75 X 8mm MS Flat 75 X 8mm MS Flat

33 Connection to between Connection to between electrodes and earthmatelectrodes and earthmat

75 X 8mm MS Flat75 X 8mm MS Flat

44 Connection to between Connection to between earth mat and equipment earth mat and equipment (Top Connections)(Top Connections)

50 x 6mm MS Flat50 x 6mm MS Flat

The size of trench for burying earth mat shall be 300mm X The size of trench for burying earth mat shall be 300mm X 500mm. The earth mat shall be buried in the ground at a depth 500mm. The earth mat shall be buried in the ground at a depth of 500mm. The earth mat shall extend over the entire switch of 500mm. The earth mat shall extend over the entire switch yard.yard.

All junctions and risers in the earth flat shall be properly All junctions and risers in the earth flat shall be properly welded by providing additional flat pieces for contact between welded by providing additional flat pieces for contact between two flatstwo flats

Provision shall be made for thermal expansion of steel flats by Provision shall be made for thermal expansion of steel flats by giving smooth circular bends Bending shall not cause any giving smooth circular bends Bending shall not cause any fatigue in the material.fatigue in the material.

After welding, the joints and tap offs shall be given two coats After welding, the joints and tap offs shall be given two coats of Bitumen paintof Bitumen paint

Back filling of earth mat trench to be done with good earth, Back filling of earth mat trench to be done with good earth, free of stones and other harmful mixtures. Back fill shall be free of stones and other harmful mixtures. Back fill shall be placed in layer of 150mm, uniformly spread along the ditch, placed in layer of 150mm, uniformly spread along the ditch, and tampered by approved means and tampered by approved means

EARTH ELECTRODESEARTH ELECTRODESEarth electrodes shall be of CI pipe 100mm (inner dia) 2.75 meters Earth electrodes shall be of CI pipe 100mm (inner dia) 2.75 meters long with a flange at the top and earth flat already indicated and long with a flange at the top and earth flat already indicated and shall be connected to earth grid in the Sub Station. All earth pits shall be connected to earth grid in the Sub Station. All earth pits are to excavated and the preferred backfill is a mixture of coke and are to excavated and the preferred backfill is a mixture of coke and salt in alternate layers. A suitable size cement collar may be salt in alternate layers. A suitable size cement collar may be provided to each earth electrode. All bolted earth mat connections provided to each earth electrode. All bolted earth mat connections and strip connections to plant and equipment panel will be subject and strip connections to plant and equipment panel will be subject to strict scrutiny. Transformer Neutrals shall be connected directly to strict scrutiny. Transformer Neutrals shall be connected directly to the earth electrode by two independent MS strips of 75 X 8mm. to the earth electrode by two independent MS strips of 75 X 8mm. The transformer body earthing shall be done with 75 X 8mm flat. The transformer body earthing shall be done with 75 X 8mm flat. The independent connections of MS strips with earth mat shall be The independent connections of MS strips with earth mat shall be given on either side of the Transformer. All contact surface must given on either side of the Transformer. All contact surface must be filled or ground flat ensures good electrical connection, and the be filled or ground flat ensures good electrical connection, and the contact surface shall be protected with a contact lubricant. contact surface shall be protected with a contact lubricant. Following this all connections shall be painted with heavy coats of Following this all connections shall be painted with heavy coats of bituminous black paint so as to exclude moisture.bituminous black paint so as to exclude moisture.

EARTH GRID EARTH GRID –– WORK DETAILS WORK DETAILS

Neutral connection earth pipe shall never be used for the equipment Neutral connection earth pipe shall never be used for the equipment earthing.earthing.

A separate earth electrode shall be provided adjacent to the structures A separate earth electrode shall be provided adjacent to the structures supporting Lightning Arrestors. Earth connection shall be as short and supporting Lightning Arrestors. Earth connection shall be as short and as straight as practicable. For arrestors mounted near for protecting as straight as practicable. For arrestors mounted near for protecting transformers earth conductors shall be connected directly to the tank.transformers earth conductors shall be connected directly to the tank.

An Earthing pad shall be provided under each operating handle of the An Earthing pad shall be provided under each operating handle of the isolator and operating mechanism of the circuit breakers. Operating isolator and operating mechanism of the circuit breakers. Operating handle of the isolator and supporting structures shall be bonded together handle of the isolator and supporting structures shall be bonded together by a flexible connection and connected to the earthing grid.by a flexible connection and connected to the earthing grid.

All equipment and switchgear etc., erected shall be earthed as per I.E All equipment and switchgear etc., erected shall be earthed as per I.E Rules 1956.Rules 1956.