Concepts and Practices in

Earthing in in

Electrical InstallationBy

Mr. A K Bhattacharyya

What is Earthing?

Earthing means making an electrical connection to a general Mass of Earth.

Earthing in the substation must conform to the requirements of Indian Electricity Rules 1956 and

follow the directives laid down in IS:3043 of 1987 and its revisions.

Earthing is a connection done through a metal link between the body of any electrical appliance, or neutral point, as the case may be,

What is Earthing?(contd)

neutral point, as the case may be, to the deeper ground soil. The metal link is normally of MS flat, CI flat, GI wire which should be penetrated to the ground earth grid.

Why Earthing? Safety of People

Safety of Equipment

Avoid Fire Hazards

CEA Regulations,2010 CEA Regulations,2010

Earthing also increases the reliability of supply as it helps to provide stability of voltage conditions.

Prevents excess peak voltage disturbances and also as a means of providing a measure of protection against lightning.

Qualities of a Good Earthing

v Must be of low electrical resistance,

v Keep touch voltage and Step potential with in limits,

v Must be of good corrosion resistant v Must be of good corrosion resistant

v Must be able to dissipate heat due to high fault current repeatedly

Classification of Earthing The earthing is broadly divided as

a) System earthing (Connection between part of plant in an operating system like LV neutral of a power Transformer winding and earth).

b) Equipment earthing (Safety grounding) b) Equipment earthing (Safety grounding) Connecting body of equipments (like motor body, Transformer tank, Switch gear box, operating rods of air break switches, LV breaker body, HV breaker body,

Feeder breaker bodies etc) to earth.

Connection with earth-(A) All non-current carrying metal parts associated

with HV/EHV installation shall be effectively earthed to a grounding system or mat which will: -

(a) Limit the touch and step potential to tolerable values;

(b) Limit the ground potential rise to tolerable values (b) Limit the ground potential rise to tolerable values so as to prevent danger due to transfer of potential through ground, earth wires, cable sheath, fences, pipe lines, etc.;

(c) Maintain the resistance of the earth connection to such a value as to make operation of the protective device effective.

Connection with earth-(B). In the case of star-connected system with

earthed neutrals or delta connected system with earthed artificial neutral point: -

(a)The neutral point of every generator and transformer shall be earthed by connecting it to the earthing system as defined in rule 61(4) and the earthing system as defined in rule 61(4) and herein above by not less than two separate and distinct connections;

(b)Provided that the neutral point of a generator may be connected to the earthing system through an impedance to limit the fault current to the earth;

What to earth? Neutral of Transformer

Lightning Arrester

Body of all equipments

Cable sheathe Earthing Cable sheathe Earthing

Structures in substations, Pole Earthing

Control Panel Earthing, Battery mid-point

Tertiary winding Earthing

Fencing of Substation

Earth is a poor Conductor of Electricity, Typical Resistivity () of soil is 100 ohm-metre,

and for copper is 1700 micro ohm-metre, Two main constituents of soil are silicon oxide and

Aluminium oxide which are insulators, Soil becomes conductive due to salts and

Soil Properties

Soil becomes conductive due to salts and moisture embedded in between them,

Surface of soil layers-clay and moisture with decayed vegetable material. When dry this does not conduct. With moisture contain, it conducts.

Soil under the surface of earth is non-homogenous, hence resistivity values in wide range between 1 ohm metre to 1,00,000 ohm metres. Depending on type, nature of soil & physical and Chemical Properties.

Sandy soil drains faster, solid rock does not retain

Soil Properties(contd)

Sandy soil drains faster, solid rock does not retain water and have high ,

Black cotton soil or soil with organic contains moisture and have lower ,

Soil resistivity measurement is important for design of earthing system,

Moisture in the soil is the most important element determining its conductivity / resistivity. Conditions which increase / decrease distribution of moisture content in the soil result corresponding changes,

Resistivity goes seasonal changes as per moisture in the soil, due to climate conditions,

Variation of soil resistivity

in the soil, due to climate conditions, Values of resistivity are minimum in rainy season

and maximum in summer / dry season, For safe design of earth mat, measurements in dry

season are adopted.

Effect of Moisture on Soil resistivity

Effect of salt on Soil resistivity

Effect of Salt on Soil resistivity

Salt percentage, by weight of moisture

(moisture in the soil is 15%)

Resistivity Ohm-m

Resistivity as a % of original

value

0 107 100

0.1 18.0 16.8

1.00 4.6 4.3

5.00 1.9 1.8

10.00 1.30 1.2

20.00 1.0 0.95

Effect of Temperature on Soil resistivity

Conventional: Salt, Charcoal, Water

Disadvantage: electrode corrosion.

Material to reduce soil resistivity

Bentonite: High moisture, Swelling to High volume, Moisture retains to long time, No maintenance required.

Earthing Electrodes

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What is Skin effect?

Contact between soil and electrode!

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Pipe ElectrodeRod ElectrodeEarth Electrodes

ohmmeter in soilofy resistivit whereohms

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=4

logL2

100R

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Rod / Pipe Electrodes

Calculation of Resistance for rod / pipe electrode

ohms in resistance R

cm in rod of dia d

cm in rod of length L

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Plate Electrodes

Calculation of Resistance for Plate electrode

R= [(/A)]/4Where R = Resistance in ohms

= resistivity of soil in ohm-metre = resistivity of soil in ohm-metreA = Area on both sides of plate in M2.

Plate Electrode

Electric Shock

Charles Dalziel (1904-1986) studied the effects of electricity on animals and humans. He wrote The Effects of Electric Shock on Man, a book in which he explains the effects of different amounts of electricity on human subjects. He is also the inventor of theground-fault circuit interrupter or GFCI which he invented in 1961. The GFCI is commonly found in home bathrooms or kitchens. The outlet operates normally until 5 milliamps passes from the appliance to the ground. Charles Dalziel was a pioneer in understanding electric shock in humans.

Minimum voltage placed across the arms that would produce a current that could be felt by a person.

The damage caused by electric shock depends on the current flowing through the body; 1 mA can be felt and 5 mA is painful. Above 15 mA, a person loses muscle control, and 70mA can be fatal. A person with dry skin has a resistance from one arm to the other of about 50000 ohms. When skin is wet, the resistance drops to about 5900 ohms.

dry skin resistance = 50 k, and V = A x R, { Volts = amps x resistance }with dry skin, the least voltage that can be felt = 50 k x 1 mA = 50 Volts.wet skin resistance = 5.9 k, (ohmns)with wet skin, the least voltage that can be felt = 5.9 k x 1 mA = 5.9 VoltsAt the potential of 50 volts on wet skin the current felt would be =

= 50 / 5900 = 8.47 mA , which is considered painful on wet skin.

Tolerable current for human body

As per studies by Dalziel, 99.5% of all persons can safely withstand without ventricular fibrillation, the passage of current (IB) for duration ranging from 0.03 to 3.0 sec and is related to energy absorbed by the body as per formula: SB = (IB)2 xRx tsValue of SB = 0.0135 for person weighing 50 kg

i.e. IB = 116 mA for 1 sec.&BSB = 0.0246 for person weighing 70 kg

i.e. IB = 157 mA for 1 sec.

ts IB (50 kg) IB (70 kg)

0.2 sec 259 mA 351 mA

0.5 sec 164 mA 222 mA

1.0 sec 116 mA 157 mA

As per the Indian Electricity Rule no. 67 (1) in everyE.H.V./ H.V. installations :

(a) Touch voltage and step voltage shall be keptwithin limits.

To Provide high resistivity Layer

within limits.

(b) The ground potential shall be limited to atolerable value.

Understanding Important definitions

Touch potential

Step potential

Equipotential

Touch and Step Potential

Touch & Step Potential

Basic Shock Situations in Substations

(a)Touch potential : Touch potential is the differencein voltage between the object touched and the groundpoint just below the person touching the object whenground currents are flowing.

Definitions

As per IE rules one has to keep touch potential less than 523 volts

(b) Step Potential : Step Potential is the difference involtage between two feet, which are one metre apartalong the earth when ground currents are flowing.

Continue.

(c) Equipotential: Two separate points (at samepotential)

(d) Mesh Voltage: It is the maximum touch voltage tobe found within a mesh of ground grid.

Definitions

be found within a mesh of ground grid.

(e) Transferred voltage: It is a special case of touchvoltage where voltage is transferred into or out of thesubstation.

Arc Zone around earthing electrode when a large current flows away current flows away from it to ground

Earth surface potential around ground rod

during current flow

To avoid formation of pools of oil in case of leakages from Transformers and Circuit Breakers

to eliminate spreading of fire to keep reptiles away

To provide high resistivity layer

Continue.

to keep reptiles away to control the growth of grass and weeds to maintain moisture in the soil to discourages persons running in the switch-yard

and saves them of the risk of being subjected to possible high step voltage

The value of tolerable touch voltage inrespect of human body is less than thevalue of tolerable step voltage. Also, aperson in the switchyard may be exposed

To Provide high resistivity Layer(contd)

person in the switchyard may be exposedto touch voltage most often than to thestep voltage. The touch voltage, beingpredominant must be considered for thepurpose of analysis than the step voltage.

Continue ,.

Following is the formula of permissibletouch voltage.E Touch = (116+0.174 )

t

To Provide high resistivity Layer

where, = The soil resistivity where theperson is standing.

t = Fault clearing time.

Continue ,..

The above formula clearly indicates that it isessential to provide high resistivity layerunder the feet of the person standing in theswitch-yard so as to keep the value of thetouch potential within permissible limits.touch potential within permissible limits.

In practice: the touch voltage should be lessthan 523 volts, the step voltage should beless than 1510 volts.

Crushed Stone

The Crushed Stone serves the purpose. The resistivity of the Crushed Stone is

taken as 3000 Ohm-m for calculation of the tolerable touch voltages in most of the tolerable touch voltages in most of the

designs of earth mat of sub-station. Crushed stone, of the size of 30 to 40 mm for a layer of 100 mm is recommended by the CBIP.

Granite, Gneiss - 25000 Ohm-metreBolder Gravel - 15000 Ohm-metreLime Stone - 5000 Ohm-metreMoran Gravel - 3000 Ohm-metreBase Rock Hard - 1190 Ohm-metreRock, Hard - 1150 Ohm-metre

The values of resistivity of the different types of rocks are given below:

Rock, Hard - 1150 Ohm-metreBoulders - 477 Ohm-metre

The range of the values of the resistivity is wide. It is,therefore, essential to know the source of the rock fromwhich the black metal is obtained so that the idea of theresistivity of the black metal can be had prior to laying ofthe metal.

MEASUREMENT OF EARTH RESISTANCEThe measurement of earth resistance is done using threeterminal earth meggars or four terminal earth meggars.

Four Terminal: Four spikes are driven in straight line intothe ground at equal intervals. The two outer spikes areconnected to current terminals of earth meggar and the twoinner spikes to potential terminals of the meggar. Then theinner spikes to potential terminals of the meggar. Then theearth resistance is measured by rotating the meggar till asteady value is obtained.

Three Terminal: Two temporary electrodes are spikes aredriven in straight line one for current and the other voltage ata distance of 150 feet and 75 feet from the earth electrodeunder test and ohmic values of earth electrode is read in themeggar.

Measurement of Earth Resistivity

Combined earth resistance shall be the same at everyearth pit unless it gets disconnected from the earth mat

Permissible values of earth resistancea) Power stations - 0.5 ohms

b) EHT Stations - 1.0 ohms

c) 33KV SS - 2 ohms

d) DTR Structures - 5 ohms d) DTR Structures - 5 ohms

e) Tower foot resistance - 10 ohms

a) Soil is a poor conductor

b) Pure water is a poor Conductor,

c) Add (NaCl) salt and Moisture to reduce earth conductivity,

d) Excess water will not help in reducing earth resisitivity,

e) Charcoal help in holding moisture,

SUMMARY

f) Pipe earthing will help in distribution of current in all directions

g) Transformer neutral requires two earthing electrodes,

h) Lightning arresters should have independent earth pit, which should in turnbe connected to station ground mat.

i) Body (metal) earthing,

j) All extraneous metallic frameworks not associated with equipment (towers,structures, sub-station gantries, building etc.) are connected to Earth mat.

h) Earthing of sub-station fence is also equally important fromviewpoint of touch and step potentials in the area outside the fence.Normally earth mat has to be extended up to 2m beyond the fence soas to ensure that the area in the vicinity of sub-station fence is safe, if

SUMMARY(contd)

somebody happens to come in contact with it under fault conditions.

i) Earthing in the sub-station must conform to the requirements ofIndian Electricity Rules 1956 and follow the directives laid down inI.S:3043 of 1987 and its revisions.

Questions and Answers