2 Earthing System - Maintenance

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s.ganesh babu. 1

Earthing System -

Maintenance

S.Ganesh Babu


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The primary objectives of a grounding system

are to:

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Protects personnel and sensitive equipment.

Dissipateslightningstrikes.

Discharges short circuit currents.

Prevents damage from power and switching

surges.

Provides stable reference.

Safety to human life, appliances, machines,

equipments etc.)
http://en.wikipedia.org/wiki/Surge_protectionhttp://en.wikipedia.org/wiki/Surge_protectionhttp://en.wikipedia.org/wiki/Surge_protection


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The primary objectives of a grounding system are to:

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Control Radio Frequency (RF) Emissions& Electromagnetic Interferences.

Provide a stable reference potential for

instrument accuracy.

Provide a discharge path for short circuits

and lightning strikes (fault current /surge

protection
http://en.wikipedia.org/wiki/Radio_Frequencyhttp://en.wikipedia.org/wiki/Emissionshttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Surge_protectionhttp://en.wikipedia.org/wiki/Surge_protectionhttp://en.wikipedia.org/wiki/Surge_protectionhttp://en.wikipedia.org/wiki/Surge_protectionhttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Emissionshttp://en.wikipedia.org/wiki/Radio_Frequency


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What are the risks?

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20+ fires a day are caused by faulty electricalinstallations

Fires are 12 times more likely to be caused by

faulty electrics than, say, by gas

2000+ people are injured through electrocution

every year

As per National Crime Records Bureau (NCRB)of India, around 15 people die every day due to electrical

accidents, which account for nearly 3% of total

accidental deaths.


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What are the risks?

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The majority of deaths, injuries and damage

would have been prevented by an installation

inspection.

Older properties (10+ years old) are mostat risk.

The danger is increased, as it is usually

out-of-sight, under floorboards, behindwalls.

The function of the earthing system is two-

fold.


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Purpose of protective earthing

To ensure the safety of the people and

property within the zone served by it.

It requires a high current capacity path withrelatively low impedence at the fundamental

frequency.

To ensure that the voltages developed under

high fault conditions are not hazardous.


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Effectively earthed system is one in

which:

--The ratio of zero sequence reactance

to that of positive sequence reactanceis less than 3.

-- The ratio of the zero sequence

resistance to that of positive

sequence resistance is less than 1.


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NEED FOR A RELIABLE LOW IMPEDENCE

CONNECTION.

A GOOD LOW IMPEDENCE CONNECTION

TO GROUND CONSISTS OF:

A high conductivity.

Corrosion resistant conductor (copper is a good

choice).

To be buried at a depth in the earth that it will not

freeze nor dry out.

Large enough to contact a suitably a large volume of

earth covering a large area.

Located in such a position that it is not influenced by

other earthing system.


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Effect of large earth volume and area.

A large volume of earth reduces the current density in

the soil and so the resistance to the earth.

A large area connection allows the shaping of the

electric field to be accomplished, reducing the touch and

step voltage.

It is hence, called a clean earth, atleast as clean as it gets.

The traditional thought of providing a low impedence

path to earth is now dispensed with.

Modern practice requires shaping of the field in the

ground to control voltage gradients around the eartrh

electrode.


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EARTH AS A SPHERE WITH DIFFERENT

LAYERS OF EARTH RESISTIVITY

CORE


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VARIATION OF SOIL RESISTIVITY WITH

TEMPERATURE.

TEMPERATURE

DEG.C

Clay mixed with sand, 15% moisture.

Resistivity in ohm-meter

20 72

10 99

0 (water) 138.

0 (ice) 300minus 5 790

minus 15 3,300


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VARIATION OF SOIL RESISTIVITY WITH

MOISTURE CONTENT.

MOISTURE

CONTENT

%AGE BY WEIGHT.

RESISTIVITY OF

CLAY WITH

SAND.

RESISTIVITY OF SAND

0 10 TO POWER 7 -

2.5 1500 3x 10 to power 6

5 430 50,000

10 185 2100

15 105 650

20 63 270

30 42 -


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REDUCTION OF EARTH RESISTIVITY:

METHODS.

Chemicals traditionally used for changingresistivity are:

-- sodium chloride, NaCl (salt)

-- magnesium sulphate MgSo4,-- Copper Sulphate CuSo4,

-- Sodium Carbonate,NaCo3, (Washing Soda),

-- Calcium Chloride, CaCl. Earth resistivity can be reduced to:

-- 0.2 ohm meter using NaCo3,or

-- 0.1 ohm meter using salt.


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1.2 grams per liter of salt in dissolved water has aresistivity of 5.0 ohm meter, while 6 grams per

liter of salt in dissolved water has a resistivity of10 ohm meter.

GYPSUM:

has water retention property.

Low solubility.

Resistivity of 510 ohm meter.

Neither acidic nor alkaline with Ph value between 6.2 to6.9.

BLAST FURNACE SLAG: Use of blast furnace slag on the granulated form is on an

experimental stage.


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BENTONITE:

-- Mixed in the ratio of 1 : 6 with blackcotton soil.

-- Is a volcanic product.

-- Is acidic by nature, with Ph value of 10.5.-- Absorbs 5 times of water.

-- Swells upto 13 times its dry volume.

-- Non corrosive.-- resistivity is 5.0 ohm meter.


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MARCONITE:

-- Developed by Marconi communications Ltd in 1962.

-- Contains crystalline form of carbon with low sulphurand Chloride content.

-- Used in a concrete form with carbonceous aggregaatereplacing HBG metal.

-- Resistivity of 2.0 ohm meter reduces to 0.1 ohm meterwhen mixed with concrete.

-- Retains moisture even in dry / hot climates.

-- Used for anti static flooring and electro magnetticscreening.

-- In slurry state, causes corrosion on metals.-- Alumunium, tin coated, or galvanised steel not to be

used in marconite.

-- Used in Rocky locations.


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The three demands of an Earth system

LIGHTNING AND SHORT CIRCUIT:

The earthing system should protect:

-- occupants.

-- damages such as fire, explosions, and flash over due to

lightning.-- over heating due to short circuit.

SAFETY:

-- must conduct lightning and short circuit currentswithout introducing intolerable step and touchpotentials.


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EQUIPMENT PROTECTION AND

FUNCTIONALITY:

by providing a low impedance path to the

unbalanced and neutral currents that are generated in

an electrical network.

Should provide proper protective coverage to all the

connected equipments and cables, through

protectinve sheilding and zoning to lightning system.


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Electrical earthing is designed primarily to render

electrical installation safe. The purpose of earthing

are :

1. Protection to the plant

2. Protection to the personnel and

3. Improvement in service reliability

Non- 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, lightningarresters 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.


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Connecting lead should have sufficient currentcarrying capacity.

L A s should have independent earth electrodewhich should be inter connected to the stationgrounding system.

All paints, enamel, seals should be removedfrom 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 Ohmis maintained.

Suitable grounding mat should be provided inthe sub station yard.


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I n a Sub Station the following shall be earthed.

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

Apparatus, frame work and other non-current carryingmetal work associated with each system, for exampletransformer tanks, switch gear frame work etc.,

Extraneous metal frame work not associated with thepower systems, for example, boundary, fence, steelstructures etc.,

The earthing Means connecting of Electricalequipment, machinery or an electrical system with thegeneral mass of earth is termed as earthing orgrounding


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The earthing system must provide an environmentwhich is free from the possibility of fatal electricshock.

The earthing system must provide a lowimpedance path for fault and earth leakage currentsto pass to earth.

The earthing conductors must possess sufficientthermal capacity to pass the highest fault current

for the required time The earthing conductors must have sufficient

mechanical strength and corrosion resistance.

A Sub Station earthing system has to satisfy four

requirements:


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Earthing can be broadly divided as :

System Grounding ( System

Earthing)

Equipment Grounding (Safety

Grounding).

Discharge grounding.


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System Grounding:

It is a connection to the ground of a part of the plant formingpart of the operating circuits for example the star point of thetransformer or the neutral conductor. The grounding of thelighting, arrestors also comes under the head of systemgrounding. The provision of system ground reduces to

considerable extent the magnitude of the transient overvoltages and there by increases the life of electrical equipment

besides minimizing the services interruptions.

Thus the fundamental purpose of system ground is the

protection of installation and improvement in quality ofservice. The system ground also will ensure the safety of the

personnel to some extent, as it helps to clear the fault speedily.


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Safety Grounds (Equipment Grounding)

It is a connection to the ground of non-current carrying parts of the equipments like

Motors, Transformer Tanks, Switchgearenclosures, Metallic enclosures of allelectrically operated equipments and also theinstallations used to carry/ Support electrical

equipments.

By connecting the frames to a low resistanceground system, a sufficiently high currentwill flow into the ground when accidentallythe live parts of the equipment / Machinerytouch the frames, and consequently saves theoperating personnel from fatal accidents.


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Safety Grounds (Equipment Grounding)

Thus the equipment grounding isbasically intended to safeguard to a

great extent from the hazards of touch

voltages. The safety ground is so

designed that the potential differenceappearing between the frames and the

neighboring ground is kept within safe

limits.

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Separation of system and safety grounds:

During ground fault conditions, the fault currentflows via the system ground. When the system andsafety grounds are inter connected, the fault currentflowing (via) the system ground rises the potential ofthe safety ground. Also the flow of current to safety

ground results in hazardous potential gradient in andaround sub station. In view of the above it is sometimes suggested that separate system and safetygrounds will avoid the danger arising due to potentialgradients. The idea is that by connecting the systemground to a separate earthing system situated in anaccessible spot, the ground fault current does not flowthrough the safety ground.


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However, this separate system of grounds

has many disadvantages and can be more

hazardous as mentioned below With separate grounds we can avoid

danger due to potentials only for faults

outside the stations. Short circuit currents will be more if the

fault occurs in the sub stations.

The resistance may be more and in somecases sufficient currents may not flow to

operate the relays.


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For effective separation of the earthing systems, thesystem ground shall be installed at a distance of at

least twice the diagonal length of the sub stationwhich is covered by safety grounding. The neutral ofthe transformer has to be connected to this remoteearthing by means of insulated leads. Even with this

arrangement one cannot always be sure about thecomplete isolation of the two systems and there isalways a chance of inadequate electrical connectionthrough buried neutral pipes etc., Hence, this is

impracticable, complicated and costly. It is thereforea common practice to install a common groundingsystem and design the same for effective earthing andsafer potential gradients.


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System Earthing

System earthing is governed by provisions of Rule -Of I.E Rules, 1956. Unearthed systems have beentried and due to the phenomenon of Arcing Groundsassociated with them, theses have been abandoned,excepting in a few cases of power station auxiliaries

supply systems where other arrangements are madefor indicating earth faults. In an ungrounded systemthe insulation of all the equipments, lines etc, willhave to be much higher values as compared to those

of equipments and lines of a grounded system. Thisaspect greatly reduces the costs and ensures moresafety.


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Types of System Earthing:

Earthing through a resistance.

Earthing through a reactance.

Earthing through a Peterson coil

Earthing directly or solidearthing.


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Sub Station EarthingBecause of the difficulties and disadvantages involved inmarinating the system grounding and safety grounding

separately it is the common practice now to have a combinedgrounding system at the sub stations. The impedance toground should be as low as possible. The impedance of theearth system shall not exceed the following limits in the substations

Power Stations 0.5 OhmsMajor Sub stations above 110 KV 1.0 Ohms

Minor Sub Stations below 110

KV

2.0 Ohms

Distribution Transformer Station 5.0 Ohms

Transmission line supports 10.0 Ohms


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Touch Potential

Touch potential is the potential differencebetween the ground surface potential wherea person is standing and the potential of hisoutstretched hand (s) which are in contactwith an earthed structure. It is normallyassumed that a personsmaximum reach is

1.0 meter.


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Step Potential

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Step Potential is the potential

difference between outstretchedfeet, at a spacing of 1.0 meterwithout the person touching anyearthed structure


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Mesh Potential

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The maximum potential differencebetween the centre of a mesh in an

earth grid, and an earthed structureconnected to the buried gridconductors. It is worst casescenario of a touch potential.


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Transferred potential

The transferred potential is a touch potential

which is transferred some distance by an earth

referenced metallic conductor. For example,

consider a screened cable connecting two sub

stations which are some distance apart. If aperson disconnects the earthed termination at

one end of a screened cable he may be subjected

to the full ground potential rise occurring due toan earth fault. This can be a very high touch

potential.


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Earthing in a sub station must conform

to the requirements of the Indian

Electricity Rules and follow the

directives laid down in section I and III

of IS : 3043-1966. the earthing systemhas to be designed to have a low overall

impedance, and a current carrying

capacity consistent with fault current.


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The factors which influence the design

are:

Duration of fault.

Magnitude of the fault current.

Resistivity of the underlying strata.

Resistivity of the surface material

Material of the earth electrode.

Material of earthing mat conductor.

Shock duration.

Earth mat geometry.


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Steps to be taken for design purpose:

Finalize lay out plan of the substation.

This gives the area to be covered by the earth mat.

Obtain the earth resistivity of the location where

substation is to be located. Determine the fault current likely to develop at the SS

through system studies. A correction factor of 1.2 to1.5 may be used for the determined value.

Assumption of duration of fault is taken as 1.0 sec For calculating safe step and mesh potentials, a

duration of 0.5 sec may be assumed.


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A uniform corrosion allowance of 0.12 mm per year

is considered for steel as ground conductor.

Life of a substation is taken as 40 years. Steel corrodes 6 times faster than copper.

The electrodes spacing shall not be greater than twice

that of the length of the electrode. The spacing of the mesh earth conductors shall be

between 3 to 5 meters.

Various specifications such as the area of the earth

mat, Number of electrodes, size of the earthconductors shall calculated based on the

mathematical formulae and conductor constants.

Th b f l d i i b h f ll i h b


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The number of electrodes is given by the following thumbrule:

N = I (fault) / 250,

for a earth resistivity of 500 ohm meters.N = I (fault) / 500,

for a earth resistivity of 5000 ohm meter.

The current density of the unbalanced current in a normalsystem shall not exceed 40A / Mtr.sq.

Short time over load under fault condition is given by

I = (7.57 *10cube) / root of rho * t, where t is theduration of fault in secs.

Safe step potential : (116 + 0.7 rho) / root t

Safe touch potential : ( 116 + 0.17 rho ) / root t.

where rho refers to surface earth resistivity and t , theduration of fault current in secs.


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The size of the earth bus and earth conductor

are given by

A = 0.0054 I x root t,

for sweated and rivetted joints.(250deg)

A = 0.0044 I x root t,for brazed joints.(450 deg.)

These values are applicable for copper only

and higher values are to be taken for steel.


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100 X 16 mm and 75 X 8mm size MS steel flats arebeing ordered for forming the earthing system for

EHT Sub station and 33/11 KV Sub Stationsrespectively

Earth mat shall be formed with the steel flats buriedin the ground at a depth of 500mm.

The earth mat shall extend over the entire switchgearyard and beyond the security fencing of structuralyard by at least one meter.

The outer most peripheral earthing conductorsurrounding the earth mat shall be of 100 x 16 mmsize MS flat.


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The intermediate earthing conductors forming theearth mat shall be of 75 x 8 mm size flat.

All the risers used for connecting the equipment

steel structures etc., to earth mat shall be of 50 x6mm size excepting for earthing of L A s andtransformer neutrals for which 100 x 16 mm or 75x 8 mm size shall be used.

All Junctions (crossing of the steel flats while

forming the earth mat and taking risers from theearthmat for giving earth connections toequipments, steel structural conducts, cableshearths shall be propersly welded.

Proper earthing lugs shall be used for connectingthe earth terminals of equipments to the earthingsteel flat.


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Provisions shall be made for thermal expansion of thesteel flats by giving suitable bends.

The earth mat shall be formed by placing 75 x8mm MSflat at a distance 5 meters along the length & breadth ofthe sub station duly welding at crossing.

All the equipments, steel structural, conduits, cablesheaths shall be solidly grounded by connecting to theearthing mat at least two places for each.

The ground mat of the switchyard shall be properlyconnected to the earth mat of the control house at leastat two points.

welding is done shall be given a coat of blackasphalitic varnish and then covered with hessain tape toavoid rusting.


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All paints, enamel and scale shall be removed frompoint of contact in metal surfaces before applyingground connections.

The risers taken along the main switchyard structuresand equipment structures up to their top) shall beclamped to the structure at an interval of not more thanone meter with ground connectors.

75 X 8 mm ground conductor shall run in cable trenchesand shall be connected to the ground amt at an intervalof 5 meters.

Grounding electrodes 2.75 Mtrs length 100 mm dia 9mm thickness CI Pipes shall be provided at all their

peripheral corners of the earthiong mat and also atDistance of 10 Mtrs along length & width of switchgearand in the entire switch yard.


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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 conductor.

The switchyard surface area shall be covered by a

layer of crushed rock of size 25 x 40 mm to a depth

of 100mm

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 mat.


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Power Transformers neutral shall be providedwith double earthing. Neutral earthing and

body earthing of power transformers shall be

connected to separate earth electrode.

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

The joints and tap-offs where welding is done

shall be given a coat of black asphaliticvarnish and then covered with hessain tape toavoid rusting.


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THE PERMISSIBLE LIMITS OF STEP POTENTIAL AND TOUCH

POTENTIAL SHALL BE

Maximum Acceptable step Voltage

Fault clearance times

Fault clearance times 0.2 Seconds 0.35 Seconds 0.7 Seconds

On soil 1050 V 600 V 195 V

On chippings 150mm) 1400 V 800 V 250 V

Maximum Acceptable Touch Voltage

Fault clearance times

Fault clearance times 0.2 Seconds 0.35 Seconds 0.7 Seconds

On soil 3200 V 1800 V 535 V

On chippings 150mm) 4600 V 2600 V 815 V


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

S.

No

Item Material to be used

1 Grounding Electrodes CI pipe 100 mm (inner dia)

Meters long with a flange at

the top2 Earth mat 75 X 8mm MS Flat

3 Connection to between

electrodes and earthmat

75 X 8mm MS Flat

4 Connection to betweenearth mat and equipment

(Top Connections)

50 x 6mm MS Flat


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The size of trench for burying earth mat shall be 300mm X500mm. The earth mat shall be buried in the ground at a depthof 500mm. The earth mat shall extend over the entire switch

yard. All junctions and risers in the earth flat shall be properly

welded by providing additional flat pieces for contact betweentwo flats

Provision shall be made for thermal expansion of steel flats by

giving smooth circular bends Bending shall not cause anyfatigue in the material.

After welding, the joints and tap offs shall be given two coatsof Bitumen paint

Back filling of earth mat trench to be done with good earth,

free of stones and other harmful mixtures. Back fill shall beplaced in layer of 150mm, uniformly spread along the ditch,and tampered by approved means

EARTH ELECTRODES


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EARTH ELECTRODES

Earth electrodes shall be of CI pipe 100mm (inner dia) 2.75 meterslong with a flange at the top and earth flat already indicated andshall be connected to earth grid in the Sub Station. All earth pitsare to excavated and the preferred backfill is a mixture of coke andsalt in alternate layers. A suitable size cement collar may be

provided to each earth electrode. All bolted earth mat connectionsand strip connections to plant and equipment panel will be subject

to strict scrutiny. Transformer Neutrals shall be connected directlyto 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 independent connections of MS strips with earth mat shall begiven on either side of the Transformer. All contact surface must

be filled or ground flat ensures good electrical connection, and thecontact surface shall be protected with a contact lubricant.Following this all connections shall be painted with heavy coats of

bituminous black paint so as to exclude moisture.

EARTH GRID WORK DETAILS


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EARTH GRIDWORK DETAILS

Neutral connection earth pipe shall never be used for theequipment earthing.

A separate earth electrode shall be provided adjacent to thestructures supporting Lightning Arrestors. Earth connectionshall be as short and as straight as practicable. For arrestorsmounted near for protecting transformers earth conductors

shall be connected directly to the tank.

An Earthing pad shall be provided under each operatinghandle of the isolator and operating mechanism of the circuit

breakers. Operating handle of the isolator and supporting

structures shall be bonded together by a flexible connectionand connected to the earthing grid.

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

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2 Earthing System - Maintenance