Bonding & Earthing (Final)

8/3/2019 Bonding & Earthing (Final)

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BONDINGBond means an electrical

connection between two or

more conductors or non currentcarrying metallic parts oftraction masts or structures or

supports and rails.


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Transverse Bond It means a bond

between two rails of a track or tworails of adjacent tracks.

Longitudinal BondIt is an electricalconnection across a rail jointbetween consecutive lengths of rails.

TYPES OF BOND


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Impedance Bond It is a bond

installed by signal and telecomdepartment, which provides a low

impedance path for the tractionreturn current and relatively highimpedance path for track circuit

current.


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Fig.1. Impedance Bond


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Structure Bond Bondconnecting the non-current

carrying metallic parts oftraction mast or structure or

support to the traction rail.


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Fig: Structure Bond


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Signal bond It is an electricalconnection across a rail joint,provided by the Signalling &

Telecommunication Department,to facilitate flow of track circuit

current.


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Terminal Bond It allows delimiting

the track circuit at a boundary with aninsulated Rail joints.

Cross Bond It means a bond

between two rails of a track or two

rails of adjacent tracks. It is also called

a transverse bond.


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INTEGRAL TRANSVERSAL LINK ( ITL )

It is interconnection of Dn line OPC,

BEC and Up line OPC, BEC and tracks

direct or through impedance bond.

This shall be provided at an

interval of 1 Km or less than one km.


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TYPICAL ARRANGEMENT

INTEGRAL TRANSVERSE LINK (ITL)

ABB LIMITEDALTERNATE ARRANGEMENT

INTEGRAL TRANSVERSE LINK ( TYPICAL LOCATION AT SUBHASH NAGAR FP)

AT SUBHASH NAGAR FP


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OVERHEAD PROTECTION CABLE (OPC)

It is an ACSR conductor run on traction masts or

structures or supports and clamped to their

metallic parts/supports and connected to earth

through ITL.

ACSR conductor consists of 7 steel wires and 12

aluminium wires each 2.5 mm dia .

Its cross sectional area is 93.3 sq mm andoverall diameter is 12.5mm.

Its tension should be 400kgf.


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BURIED EARTH CABLE (BEC)It is a flexible Copper Conductors having 7 strands

each of 2.5 mm ,35 sq mm x-sectional area and overall dia of 7.5 mm.

It is run along the viaduct with which all themetallic reinforcement steel bars of via duct, piers

parapet and equipments at viaduct are connected

to maintain proper earthing.

It is connected to earth to earth grid of Auxiliarysubstation through ITL


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PLINTH OR CONTINUITY JUMPER

An insulated stranded flexible copper

conductors of approx.35 sq mm is used forinter connecting two track plinth to maintaincontinuity.

The each end of deck is connected with theBEC for proper earthing.


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EARTHING

OF

ELECTRICL

SYSTEM


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WHAT IS EARTHING?Earthing is an essential requirement to drain the potential

deposition on any machine frame, structure, support,electrical installation etc due to poor insulation for

achieving a safe working upon.

E E

FAULT CURRENT THROUGH EARTH

LINK

FUSE

R

YB

N

230VFAULT TO

EARTH

IN WINDING


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WHY EARTHING?

Earthing is designed primarily to preserve the securityof the system by ensuring that the potential on each

conductor is restricted to such a value as is consistent

with the level of insulation applied.

Earthing shall generally be carried out in accordance

with the requirement of Indian Electricity rules1956(I.E.Rule 2003) & IS 30431987.


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OBJECTIVE OF EARTHING

It should stabilise circuit potential with respect toground potential and limit the potential rise.

It should protect men & materials from injury or

damage due to over voltage or touching.

It should provide a low impedance path to fault

currents to ensure prompt & consistent operation of

protective devices.


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It should keep the maximum voltage gradient

along the surface inside & around the substation

within safe limits during ground fault.

It should protect underground cables fromoverall ground potential rise & voltage gradient

during ground fault in the system.


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IMPORTANT I.E. RULE

RELATED TO EARTHING:

RULE No: 33.Earth terminal on consumers premises.

RULE No: 61.

(A) MAX: PERMISSIBLE RESISTANCE OFEARTHING SYSTEM.

Large power station: - 0.5 ohms.

Major sub-station: - 1.0 ohms.

Small sub-station: - 2.0 ohms.In all other cases : - 8.0 ohms.

The earth continuity: - 1.0 ohms.

inside an installation


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(B) CONNECTION WITH EARTH

Earthing of neutral conductor of a 3-phase, 4-wire

system.

Earthing of all metal casing / covering of electric supply

lines or apparatus.

Testing of such earth resistance not less than once in

every two years during a dry day of a dry season shall

be conducted and recorded.

Test results should be recorded and shall be made

available to the EIG or Assisting officer to EIG, when

required.


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RULE No: 67.

CONNECTION TO EARTH

All equipments associated with HV /EHV installation,

shall be earthed by not less than two distinct and

separate connection with the earth having its own

electrode, except an earth mat

Testing of such earth resistance not less than once inevery year during a dry day of a dry season shall be

conducted & recorded.


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RULE No: 90. EARTHING

In distribution system, all metal supports and allreinforced/pre-stressed cement concrete supports of

overhead line and and metallic fittings attached shall be

permanently and effectively earthed.

Each stay wire shall be similarly earthed, unlessinsulators have been provided in it at a height not less

than three mtrs from the ground.

Every 5th pole as a minimum shall be grounded, if the

foundations are not cements concrete blocks.

RULE N 91


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RULE No: 91.

SAFETY AND PROTECTIVE

DEVICE

Every overhead line erected over any part of street or

public place shall be protected with a device, approved

by the EIG, for rendering the line electrically harmless

in case it brakes.

The owner of every high and extra high overhead line,

shall be protected to the satisfaction of the EIG, to

prevent unauthorised persons from ascending any of

the supports of such overhead lines.


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RULE No: 92.

PROTECTION AGAINST LIGHTENING

The owner of every overhead line which is so exposed

, as may be liable to injury from lightening, shall

adopted efficient means for diverting to earth, any

electrical surge during lightening.

The earthing lead for any lightening arrester shall not

pass through any iron or steel pipe but shall be taken

as directed as possible from the lightening arrester to aseparate earthing electrode / mat.


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DISTINCTION BETWEEN

GROUNDING AND EARTHING

Grounding: Grounding implies connection of currentcarrying parts to ground.It is mostly either generator ortransformer neutral. Hence it is popularly called neutralgrounding.

Grounding is for equipment safety.

In case of resistance grounding system, it limits the core

damage in stator of rotating machines. In case of solidlygrounded system, substantial ground fault current flowsenabling sensitive fault detection and fast clearance.


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Earthing: Earthing implies connection of non currentcarrying parts to ground like metallic enclosures.

Earthing is for human safety.

Under balanced operating condition of power systems,earthing system does not play any role. But under anyground fault condition, it enables the ground faultcurrent to return back to the source without endengeringhuman safety.


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Generator Transformer

NG NG

Earthing

Neutral Grounding and Earthling


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Earth As Conductor:

Resistivity() of earth is typically 100-M.

Resistivity() of copper is 1.7x 10 -8-M.

Resistivity() of G. I. is 1.7x 10-7

-M.Take as reference, 25x4mm copper strip.

To obtain the same resistance, the size of G.I. Will be65x10mm.

The corresponding figure for earth is 800x800 meters (158

acres.)Hence, it shows metallic conductor is a preferred alternative

conductor to earth to bring the fault current back to source.

EARTH AS CONDUCTOR


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Generator Transformer

NG NG

Earthing


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ELECTROD RESISTANCE TO EARTH

Conventional practice by Electrical engineer to measure

the earth resistance is by using ohms law.

This is similar to CT, where the flow of primary current

results in voltage appearing across CT secondary. Which

drives the current through the connected relay(burden).

For electrode resistance to earth, current is injected to earth

by electrode and electric field travels through the earth.The

voltage appears at certain distance from electrode and the

resulting impedance is electrode resistance to earth.


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IF

CT RV

x

v1

Resistance area of driven earth rod


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HEMISPHERICAL ELECTORD

Consider a hemispherical electrode used for injection of

current.Current flows through a series of hemisphericalshells of earth of continuously increasing cross section.

The resistance offered by earth to spread of electrical field

is given by:

RX = dX / 2 X2.

The resistance as a function of distance from electrode.

The most interesting aspect is that almost 95% of final

resistance is contributed by soil within 5meters of the

electrode.


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dx

1

x

Spherical shells

Resistance to earth of hemispherical electrode


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Now consider two location A & B 100 Km apart from each

other with respect to earth grids.Assume current is

discharged at A. Only the soil within first 5 to 10 metersfrom A offers substantial resistance.

The resistance offered by earth subsequently to reach B is

very minimal.

This the reason of our practice to treat earth pits with lime ,

charcoal, & watering the pits. This reduces the resistance ofthe soil locally around the earth electrode. As the resistance

away from the earth electrode is minimal.


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100

90

80

70

60

50

40

30

20

10

00 5 10987621 3 4

4(-100MM)RadiusHemi-sphere

Distance in Met.

Resistancein%

RESISTANCE TO EARTH OF HEMISPHERICAL ELECTRODERESISTANCE TO EARTH OF HEMISPHERICAL ELECTRODE

INFLUENCING FACTORS FOR ELECTRODE


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INFLUENCING FACTORS FOR ELECTRODE

RESISTANCE

The major factor is the length.

Diameter or width(Cross section) has very minor influence.

The resistance of pipe electrode is given by:-

R = ( / 2 L) [ LN{ 8L / ( x 2.7183)}].Where, L = Length in Met.(pipe)LN=Nominal length(buried conductor)

= Diameter in MetLet, consider the case of a length = 6 Met.

For = 2.5Cm, R = 16.4 .

For = 10 Cm, R = 15.3.


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A horizontal earth strip of 75x10mm Cu and 45x10mm GI

both of same length will offer almost same electrode

resistance.

Other interesting observation is that the electrode resistance

is not much dependent on type of electrode materials like

Cu,Al or GI.Resistance is the function of physicaldimension, mainly length.

So, it is observed that 300% increase in diameter, resistance

decreases by app: 7% only.


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PLATE ELECTRODE

In early days only plate electrode were used.

It was presumed that to get low electrode resistance toearth,surface area should be large as per conventional ohms

law resistance.

In some cases efforts were made to cover the entire site

with plate electrode.

One solid plate & another annular ring both with same

radius of 50cm are taken for using as earth electrode.

Calculation shows that resistance to earth in both the cases

is 29.2 .


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Resistance for strip or horizontal wire electrode is

measured by RYDERs formula:-

R = ( / 2 L) [ LN(8L/T)+LN(L/h)- 2

+(2h/L)-(h2/L2)].

Where,

L = Length in Met.(electrode)

LN=Nominal length(buried conductor)

h = Depth in Met.

T = Width in Met.(for strip).

= 2 x diameter in Met.(for wire)


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1 MET

5 CM

50 CM50 CM

RESISTANCE TO EARTH OF PLATE ELECTRODE


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Similarly, one plate electrode & one strip electrode of

same volume are taken. But resistance to earth of plate

electrode is almost three times that of strip electrode.

The linear dimension of plate electrode is perimeter,

i.e,4M., where as for strip electrode is 13M.

Hence, hypothesis is that electrode resistance is

dominated by length of the electrode buried.

Thus, it is concluded that plate electrode is inefficient.


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1 M

75 mmX 8 mm X 13.3M

0.008M3

REL= 9. 5

1 M

P =100 -M

1 M X 1M X 8 mm

0.008M3

REL = 26..2


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PARALLEL ELECTRODES

To obtain low effective earth grid resistance, electrodes are

connected in parallel. So that the total resistance will be halfof individual resistance.

This is again due to our extrapolation of conventional ohms

law concept. But it is true only when the separationdistance between electrodes are adequate.

For discharging the field effectively, each electrode needs

exclusive soil below it. If the rods are too close, resistance

area of one electrode will interfere with that of other and

expected gain is not realised. As a thumb rule if the rod

length is L, separation distance shall be 2L.


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I

Over lapping resistance areas of two earth rods


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L

Separation distance

2L

I

SEQUENCE IMPEDANCE TRANSMISSION LINE


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SEQUENCE IMPEDANCE TRANSMISSION LINE

The values can be obtained from any line parameter

evaluation program considering one circuit or bothcircuit. S/C D/C

ZPOS(ohm/Km) 0.15 + j 0.41 0.08 + j 0.22

ZZERO(ohm/Km)0.37 + j 1.29 0.29 + j 1.04Positive sequence impedance of D/C line is almost 0.5

times of S/C as expected. But Zero sequence impedance

of D/C line is only about 0.8 times of S/C line.This is

because positive sequence does not involve earth returnbut zero sequence involves earth return.Only if the separation distance is large enough, they will

behave like two single circuit line. This is seldom

achieved in practice.

RESISTANCE OF EARTH GRID


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RESISTANCE OF EARTH GRID

In EHV switchyard, earthing grid is formed by a mesh

of horizontal strip electrode & vertical rod electrode.

The resistance to earth of the entire grid is calculated

by SVERAK formula :

RG = C1 +C2 1+(1/C3) .C1 = 1/L; C2 = 1/ 20A ; C3 = 1+ h 20/A.A = Area of earthing grid.

L = total length of buried conductor includingrod electrode in Meter.

= resisvity in ohm-meter.


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h

Switchyard earthing grid

METHODS TO REDUCE ELECTRODE


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METHODS TO REDUCE ELECTRODE

RESISTANCE TO EARTH

1.To reduce soil resistivity() to a low value.Typical values for different type of soil:

Soil type Wet Moist Dry Bed rock

( in -M) 10 100 1000 10,000Treatment of soil--- such as watering, adding cock,

wood charcoal, bentonite clay, common salt.

After treatment there is a gradual decrease in soil

resistivity, However there will be gradual increase in soil

resistivity with passage of time as salt is washed away by

continual water seepage.


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200

160

120

80

40

0

025 40353010

5 15 20

Resistance,o

hm-Met

Months

Before Treatment

EFFECT OF ARTIFICIAL TREATMENTEFFECT OF ARTIFICIAL TREATMENT

After Treatment

After

Retreatment


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Second method is to increase the length of

buried conductor to the maximum extent

possible, the increase in cost has to be borne at

the beginning budget.

If parallel electrodes are considered for

individual earth pits to get low resistance, it

must be emphasized that unless sufficient

spacing exists between electrodes,desired

reduction in earth resistance is not realized in

practice.

ELECTRODE SIZING


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ELECTRODE SIZING

The choices for materials & size are only with respect to the

amount of fault current to be discharged to earth.

The current density(A/mm2)as per IS-3043.

Materials Cu Al GI

0.5 sec rating 290 178 113

1 sec rating 205 126 80

Earthing grid for EHV switchyards are designed for 0.5 sec duty

& for others 1sec duty is selected.

HUMAN ELEMENT & ELECTRIC SHOCK


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HUMAN ELEMENT & ELECTRIC SHOCK

Electric shock is possible only when the human bodybridges two points of unequal potential.

This is the reason why a bird can sit comfortably on a220kv

line conductor without getting electrocuted as the voltagebetween its leg (IR drop) is insignificant.

Max: tolerable current for human body is 160mA for one

second.If this limit exceeds, it will result in death due to

ventricular fibrillation(heart attack)

Allowable body current IB(Amperes) for two body weights,


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as per IEEE Std:-80. is given:

IB = 0.116/TS for body weights of 50kg.= 0.157 /TS for body weight of 70kg.

TS =duration of current exposure (fault clearance time).

TS IB ( 50kg) IB (70kg).0.2sec 259mA 351mA.

0.5sec 164mA 222mA.

1.0sec 116mA 157mA.

Average value for human body resistance under dry

condition is 8 to 9k-ohms. But for design purpose, assumed

as 1k-ohm.


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GROUND POTENTIAL RISE

Ground potential rise(GPR) is the voltage to which theearth mat is going to rise when it discharges the

current.

If IG is the current discharge to earth & RG is the

earth grid resistance,

GPR = IG X RG,

IG = K x IF ( IF is the fault current).

IG is always not equal to fault current.

FAULT IS WITHIN THE SWITCHYARD & TRANSFORMER


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CONNECTION IS STAR-DELTA.

Here, entire fault current is discharged to earth to return to

source 2. In this case K = 1.

Source 2Source 1 Switchyard

IF

IF

IG = IF

Fault within sub-station (star-delta)Fault within sub-station (star-delta)

FAULT IS WITHIN THE SWITCHYARD & TRANSFORMER IS


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CONNECTED IN DELTA- STAR.

Part of the fault current (IF1) returns to local transformer via metallic

conductor ( earth mat) and does not contribute to GPR. The other part

(IF2) is discharged to earth to return to source2 and contributes to GPR.

In this case, K


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CONNECTION IS DELTA-STAR.

Part of fault current (IF1) returns to transformer at source 1 via earth & contributes

to GPR. The other part IF2 returns to source2 via earth & contributes to GPR at the

other switch yard. Where K


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STEP & TOUCH POTENTIAL

Step & touch potential refer to the potential experienced by a

person standing on a surface when earth mat buried, say

750mm, below surface has risen to GPR.

Step potential is the difference in surface potential experiencedby a person bridging a distance of 1 meter with his feet

without contacting any other grounded object.

Touch potential is the difference between GPR & the surface

potential at the point where person is standing, while his hand

is in contact with grounded structure.


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Touch and step potentials

IG

RGPF = 2R

RGPF= R/2

RR RR Gravel

VTOUCH

VTSTEP

Soil


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If resistance offered by each foot is R,

Then for step potential the resistance is 2Rwhile for touch potential is R/2

Touch potential is the difference between

GPR & surface potential while step

potential is the difference between twosurface potential


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EFFECT OF THIN LAYER OF

CRUSHED ROCK

In outdoor switchyard,a thin layer of crushed

rock is spread on the surface.

The resistivity of gravel ( ) is 2000 -M whilethat of soil is 100 -M. Since of gravel is high ,only a high voltage can force the current throughthe body to cause injuries. The gravel act like

insulator & throws the electric field generated

by GPR back to soil.


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Bonding & Earthing (Final)

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