Earthing Practices
Introduction Earthing practices adopted at Generating Stations, Substations, Distribution structures and lines are of great importance. It is however observed that this item is most often neglected. The codes of practice, Technical Reference books, andbooks contain a chapter on this sub!ect but the" are often skipped considering them as too elementar" or even as unimportant. #an" reference books on this sub!ect are referred to and such of those points which are most important are compiled in the following paragraphs. These are of importance of ever" practicing Engineer in charge of Substations.
OBJECTIVE OF EARTHING $rime %b!ective of Earthing is to provide a &ero potential surface in and around and under the area where the electrical e'uipment is installed or erected. To achieve this ob!ective the non(current carr"ing parts of the electrical e'uipment is conn connec ecte ted d to the the gene genera rall mass mass of the the eart earth h whic which h prev preven ents ts the the appe appear aran ance ce of dangerous voltage on the enclosures and helps to provide safet" to working staff and public.
Importance o Earthing ! Practices •
The earthing is provided for a) Safet Safet" " of of $ers $erson onne nell b) $revent $revent or atleast atleast minimise minimise damage damage to e'uipment e'uipment as a result result of flow of heav" fault currents. c) Improve Improve reliabi reliabilit" lit" of $owe $owerr suppl" suppl"
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The earthing is broadl" divided as a) S"stem S"stem earthing earthing *+onnec *+onnection tion between between part of plant in an operati operating ng s"stem s"stem like - neutral of a $ower Transformer winding and earth). b) E'uipme E'uipment nt earthin earthing g *Safet" *Safet" groudin grouding) g) +onnecting frames of e'uipment *like motor bod", Transformer tank, Switch gear bo, %perating rods of /ir break switches, etc) to earth.
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The s"stem s"stem earthin earthing g and safet" safet" earthing earthing are interco interconne nnecte cted d and therefo therefore re fault fault current flowing through s"stem ground raises the potential of the safet" ground and also causes steep potential gradient in and around the Substation. 0ut separating
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the two earthing s"stems have disadvantages like higher short circuit current, low current flows through rela"s and long distance to be covered to separate the two earths. /fter weighing the merits and demerits in each case, the common practice of common and solid *direct) grounding s"stem designed for effective earthing and safe potential gradients is being adopted. •
2actors that change the re'uirement of earth electrode a) If an electrical facilit" can epand in s"stem, it creates different routes in the electrode. 3hat was formerl" a suitable low earth resistance can become obsolete standard. b) #ore number of metallic pipes, which were buried underground become less and less dependable as effective low resistance ground connection. c) #ost of the location, the water table graduall" falling. In a "ear or two, area end up with dr" earth of high resistance. d) These factors emphasi4e the importance of a continuous, periodic program of earth resistance testing.
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The earth resistance shall be as low as possible and shall not eceed the following limits5 $ower Stations ET Substations 889- Stations D;t Structures Tower foot resistance
( ( ( ( (
6.7 %hms 1.6 %hms :.6 %hms 7.6 %hms 16.6 %hms
"tep Potentia# Step $otential is the difference in the voltage between two points which are one metre apart along the earth when ground currents flowing.
Touch Potentia# Touch $otential is the difference in voltage between the ob!ect touched and the ground point !ust below the person touching the ob!ect when ground currents are flowing.
"peciication o Earthing Depending on soil resistivit", the earth conductor *flats) shall be buried at the following depths. Soil Resistivit" in ohms;metre 1) :)
76 < 166 166 < =66
Economical depth of 0urial in metres 6.7 1.6
:
8)
=66 < 1666
1.7
To keep the earth resistance as low as possible in order to achieve safe step and touch voltages, an earth mat shall be buried at the above depths below ground and the mat shall be provided with grounding rods at suitable points. /ll non(current carr"ing parts at the Substation shall be connected to this grid so as to ensure that under fault conditions, none of these part are at a higher potential than the grounding grid.
P#ate Earths •
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Taking all parameters into consideration, the si4e of plate earths are decided as $ower Stations > ET Station
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Small Stations
(
#ain ( /uiliar" ( A7 @mm
166 1?mm 76 @mm
The complete specifications for providing earth mats at ET > 889- Substations, Distribution transformers > +onsumers premises are reproduced below.
"peciication or Earthing "$stem I%
EHT "u&station Earthing of e'uipmentBs in the sub(stations is taken of as discussed below5
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Po)er transormers* i.
ii) iii)
+(
The transformer bod" or tank is directl" connected to earth grid. In addition, there should be direct connection from the tank to the earth side of the lightning arresters. The transformer track rail should be earthed separatel". The neutral bushing is earthed b" a separate connection to the earth grid.
Potentia# and current transormers * The bases of the +Ts and $ts. are to be earthed. /ll bolted cover plates of the bushing are also to be connected the earth grid.
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-ightning arresters* The bases of the ./s. are to be earthed with conductors as short and straight as $ossible *for reducing impedance). The earth side of the ./s. are to be connected directl" frolC1 the e'uipment to be protected. Each ./. should have individual earth rods, which are in turn connected to earth grid.
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Circuit &rea/ers* The supporting structures, +.T. chambers, $.T. tanks, +able glands etc., are to be connected to earth.
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Other e1uipment2s* /ll e'uipmentBs, structures, and metallic frames of switches and isolators are to be earthed separatel".
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Fences* $roviding separate earth or connecting to the station earth depends upon the distance of the fence the station earth. If the distance is within feet, an inter( connection made to the station earth. If not, the metallic fences are earthed b" means of earth rods spaced at not more than :66 feet. The gates and support pans ma" be earthed through an earth rod. The cable wires passing under metallic fence are to be buried below at a depth 'f :B? or are to bc enclosed in a insulating pipe *$. -.+ or asbestos cement) for a distance of not less than 7 feet on each side of the fence.
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Ground )ires* The ground wires over the station arc connected to the station earth. In order that the station earth potentials during fault condition7 arc not applied to transmission line ground wires and towers, all ground wires coming to the stations shall be broken It an insulated on the fir7t tower eternal to station b" means of strain disc. Insulators.
The followings are the important features in earthing5 1. The earth mat shall be as per the approved la"out. The earth mat shall be formed with the steel flats buried in the ground at a depth of A76mm on edge. :. The earth mat shall etend over the entire switch"ard as per the la"out. 8. /ll the !unctions of the steel flats while forming the earth mat and taking risers from the earth mat for giving earth connections to e'uipment, steel structures, conduits cable sheaths shall be properl" welded. /ll !oints shall be provided with suitable angle pieces for proper contact between flats. =. $rovisions shall be made for thermal epansion of the steel flats b" giving smooth circular bends. 0ending shall not cause an" fatigue in the material at bends. 7. The earth mat shall be formed b" welding 76@ mm steel flat to the 166 1?mm peripheral earth conductor. The grounding grid shall be spaced about 7 meters i.e in longitude and about 7 meters in the transverse directions. /fter the completion of earth mat, the earth resistance shall be measured. In case the earth resistance is more than one ohm the earth mat shall be etended b" installing etra electrodes, so that the earth resistance is less than one ohm.
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?. /ll fence corner posts and gate posts shall be connected to the ground b" providing 8:mm dia #.S rods of 8 meter length near the posts and connected to the main grounding mat. A. /ll paint enamel and scale shall be removed from surface of contact on metal surface before making ground connection. @. The risers taken along the main switch"ard structures and e'uipment structures *upto their top) shall be clamped to the structures at an interval of not more than one meter. F. 76 @mm ground conductor shall be run in cable routes and shall be connected to the ground mat at an interval of 16 meters. 16.Grounding electrodes of 8:mm dia 8mtr. long #S rods shall be provided at the peripheral corners of the earth mat. The grounding rods shall be driven into the ground and their tops shall be welded to clamp and the clamp together with the grounding rods shall be welded to the ground mat. 11. ightening arrestors shall be provided with earth pits near them for earthing. 1:.+ast iron pipes 1:7mm dia and :.A7 meters long and F.7mm thick shall be buried verticall" in the pits and a miture of 0entonite compound with 0lack cotton soil a ratio of 15? is to be filled 866 mm dia and the pipe for the entire depth. 3here it is not possible to go to a depth of :.A7 meters, 1.8 1.8 ##S plates, :7mm thick shall be buried verticall" in pits of : meters depth and surrounded b" 0entonite miture at least : meter awa" from an" building or structure foundation. The plates shall be at least 17 meters apart. These earth pits in turn shall be connected to the earth mat.
II% Earthing at ,,5V "u&stations 1.
$roviding of earth pit and earth matting include the following connected works5 a) b) c) d) e)
Ecavation of earth pits of si4e :1;:ft :1;:ft Fft in all t"pe of soils. $roviding of +I pipe of 8 inch diameter Fft length with flange. /ll connections to +I pipe shall be with GI bolts and nuts. 2illing of earth pit ecavated with 0entonite with 0lack cotton soil *15?) in alternate la"ers. $roviding of cement collar of si4e :ft diameter :ft height 1 inch below the ground level. The top of the +I earth pipe should be at the surface level of the ground.
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$roviding of earth matting with #S flat A7 @mm including the following connected works5 a) Ecavation of trench in all t"pes of soils of si4e : ft depth and 1 ft. width. b) a"ing of #.S flat A7 @mm in the ecavated trench. c) Inter connecting all earth pits and welding properl" at !ointing location and !unctions. d) 0ack filling of earth completel".
III% Earthing at Consumer2s premises The earthing at +onsumerBs premises shall be as per sketch below using a ?mm thick plate
IV% Earthing at 67P "tructure Three electrodes forming an e'uilateral triangle with minimum distance of ?766 mm, so that ade'uate earth buffer is available. Each Electrode shall be H/B grade GI pipe of : inch thick and @ft long and buried verticall" so as to leave about = inch pipe length above ground level to fi a HB shaped clamp.
Note* 1.
The connections to the three earth connection. Electrodes should be as follows.
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*/)
To one of the earth electrodes on either side of double pole structure *J or K) i. ii.
*0)
%ne direct connection from three 119- ightening /rrestors. /nother direct connection from the T lightening /rrestors if provided.
To each of the remaining two earth electrodes.
i. %ne separate connection from neutral *on the medium voltage side) of the Transformer *Two wires) ii. %ne separate connection from the Transformer bod" and the handle of the 119 /.0 switch *Two separate bod" earths to tank) iii. %ne separate connection from the Earthing Terminal of poles. *+)
=mm G.I wire should be used for earth leads.
Joints • There shall be minimum !oints preferabl" no !oints enroute to earth electrodes • 3here Coints are unavoidable, the" shall be bra4ed, rivetted or welded *and painted with red lead and alluminium paints one after the other and finel" coated with bitumen)
To)er -ine Grounding 1) Ground rods are driven at the base of the tower. 3here it is not feasible, an electrode is located within a distance of :66 ft. of the tower and grounding rods are provided at that point and tied to the tower base b" a single buried wire. :) If low resistance is not obtained with :66ft, crowfoot counterpoise with = wires is installed. The counterpoise conductors shall be ? S3G galvanised steel wires taken awa" from the tower at mutuall" right angles and kept at least 76ft apart. Each of these wires is terminated at a rod at the nearest point where low resistance is obtained. If counterpoise wires cannot be terminated within half span from the tower the wire is carried through a continuous counterpoise to the net tower, where the procedure is repeated.
Earth 8at 6esign Earthing S"stem in a Sub Station comprises of Earth #at or Grid, Earth Electrode, Earthing +onductor and Earth +onnectors. Earth Mat or Grid
A
$rimar" re'uirement of Earthing is to have a low earth resistance. Substation involves man" Earthings throB individual Electrodes, which will have fairl" high resistance. 0ut if these individual electrodes are inter linked inside the soil, it increases the area in contact with soil and creates number of parallel paths. ence the value of the earth resistance in the inter linked state which is called combined earth value which will be much lower than the individual value. The inter link is made thro flat or rod conductor which is called as Earth #at or Grid. It keeps the surface of substation e'uipment as nearl" as absolute earth potential as possible. To achieve the primar" re'uirement of Earthing s"stem, the Earth #at should be design properl" b" considering the safe limit of Step $otential, Touch $otential and Transfer $otential. Step Potential It is the potential difference available between the legs while standing on the ground. Touch Potential
It is the potential difference between the leg and the hand touching the e'uipment in operation. The factors which influence the Earth Mat design are:
a. b. c. d. e. f. g.
#agnitude of 2ault +urrent Duration of 2ault Soil Resistivit" Resistiviti" of Surface #aterial Shock Duration #aterial of Earth #at +onductor Earthing #at Geometr"
The design parameters for the following can be worked out as given in the annexure I
i. ii. iii. iv. v.
Si4e of Earth Grid +onductor Safe Step and Touch $otential #esh $otential *Emesh) Grid configuration for Safe %peration Lumber of Electrodes re'uired
8easurement o Earth Resistance
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The measurement of earth resistance is done using three terminal earth meggars or four terminal earth meggars.
Three Termina#5 Two temporar" electrodes or spikes are driven one for current and the other voltage at a distance of 176 feet and A7 feet from the earth electrode under test and ohmic values of earth electrode is read in the meggar.
Four Termina#5 2our spikes are driven into the ground at e'ual intervals. The two outer spikes are connected to current terminals of earth meggar and the two inner spikes to potential terminals of the meggar till a stead" value is obtained.
Marking *i)
or ! Terminal Meter Current Terminal – C or H Potential Terminal – P or U Earth Terminal – E
"ii#
or $ Terminal Meter Current Terminal C1, C2 Potential Terminal P1, P2
8aintenance o Earthing "$stem %hecking and Testing The Earthing s"stems are to be inspected regularl". Regular checking or !oints and broken connections, if an" and rectif"ing the same will prove to be of immense help in maintenance of earth grid and e'uipmentBs. The condition of the electrodes, !oints are also to be checked. If the electrodes areB corroded immediate steps for replacement are to be taken. The earth resistance is to be measured periodicall". The megger, or testers are used for this purpose.
/s discussed earlier, low earth resistance $ath is a must for clearing the fault current instantaneousl". 2or achieving (low earth values, the following wa"s are followed5 i) / number of electrodes are connected in parallel thereb" providing a low resistance. The ground surrounding the electrodes is treated with common salt which ii) reduces the resistance b" @6M. +alcium chloride and magnesium
F
sulphate ma" also be used. In general practice. is used.
But no) the &entonite
The following #aintenance schedule in mandator" at each of the Substations S&'o
1. :. 8. =.
Item
3atering of Earth $its #easurement of earth resistance of individual earth pits #easurement of combined earth resistance at all the pits +hecking of inter connections between earth pits and tightness of bolts and nuts
Periodicit(
Dail" alf "earl" N alf "earl"... Ouarterl"
N Earth resistance of individual earth pits can be measured b" disconnecting the earth connections to the electrode. This is possible if the connections are made to a common clamp which is in turn is fied round the pipe. ... +ombined earth resistance shall be the same at ever" earth pit unless it gets disconnected from the earth mat
6einitions o Genera# Earthing Terms "oi# Resisti9it$5 This is the resistivit" of a t"pical sample of soil Earth "urace Vo#tage* The voltage between a specified point on the ground around the rod and reference earth.
Earth E#ectrode These are conductors, which are in direct contact with the soil and provide the conductive part in electrical contact with earth. The" can include rods, tape, steel reinforcing bars.
6einitions o Terms associated )ith Po)er "$stems Neutra# Point The common point of a star connected pol" phase s"stem or the earthed mid(point of a single phase s"stem.
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Independent Earth E#ectrode /n earth electrode located at such a distance from other electrodes that its electrical potential is not significantl" affected b" electric currents between Earth and other electrodes.
E:posed Conducti9e Part +onductive part of e'uipment and which is not normall" live, but which can become live when basic insulation fails.
Points o Earthing 1. Earth mat of A7 @ #S flat should be laid as outer of the switch"ard compulsoril" and see that the pole structures are enclosed in the outer mat. :. #ake vertical and hori4ontal sections for the outer mat as shown in the fig. The internal vertical and hori4ontal sections ma" be A7 @ or 76 ? #S flat. 8. The Earth mat should be laid minimum ?66##, below the ground level under the Earth mat and 0entonite powder is to be laid upto :7mm and over the earth mat. The same 0entonite compound with 0lack cotton soil a miture of 15? ratio is to be placed upto 166 mm and the remaining earth trench is to be back filled with the soil. =. See that each and ever" pole structure is earthed with 76 ? #S flat to the Earth #at. 7. 2or ever" breaker there will be fine earth connections to the earth mat with 76 ? #S flat *a) 0reaker bod" *b) Rela" $anel *c) +TBs of the 0reaker *d) and two sides of the breaker structure. ?. ightening arrestor is to be connected one end directl" to the earth mat and the other end is to the nearer earth pit or to the earth mat. A. ine Isolators are to be connected directl" to the earth mat. @. The $ower transformer bod" is to be connected two sides to the earth mat. F. Twin neutral earthing should be done to $ower Transformer as shown in the fig. one Earth flat of si4e A7 @mm #.S flat is directl" connected to the earth pit and the earth pit is again connected to the Earth mat. The second neutral is directl" connected to the earth mat, 16.$rovide fleible !umpers thoroughl" bra4ed as shown in figure 11. /ll /0 switches operating rods are to be provided with coil earths and the /0 switch support is to be earthed to the earth mat. 1:./ll the eposed earth flat, which is dropping down from the breakers, +TBs structures should be applied with bituminous paint. 18.A7 @ #S flat is to be laid around the control room from main earth and the panels of the breakers, midpoint of the 0atter" and 0atter" stand structures are to be earthed to the earth flat and make a section in front of the control room.
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1=.The distance between an" two earth electrodes should be twice the length of the Electrode. 17.The cable *11 889-) sheaths are to be earthed with :7 8 GI strip to the Earth mat. 1?.The neutral of the station transformer is to be connected to the Earth mat directl" with :7 8 GI strip. The bod" of the Station transformer two sides to be connected to the earth mat with :7 8 GI strip. 1A./ll the welding !oints should be painted with bituminous paint.
V( CONC-;"ION The various practices and standards are to be followed in respect of providing Earthing in our s"stems. If proper Earthing is not done, there is ever" likelihood of e'uipmentB I getting damaged and also have to be personnel (public and staff. ence due attention is to be given in providing and maintaining of earth grid properl" and in good condition.
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Anne:ure I +alculation of design parameters for the earthing is given below5
6ata Needed a. b. c. d. e. f.
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Soil Resistivit" Resistivit" of +rushed Rock +rushed Rock Surface a"er Thickness Switch Kard overall Dimensions Earth 2ault evel Duration consider for earth fault.
CA-C;-ATION OF "I
I
/P
/
Tcap 16 <=
ln 9o Q Tm
tc < α r $v
9o Q Ta
Simplified formula for Steel Grid +onductor /rea / P I √t 9 3here
/ P area of earth conductor in mm :. I P short circuit current in 9/. t P duration of short circuit current in seconds. 9 factor P @6 for steel.
If mat is made of 2lat then / P /rea of 2lat Give allowance of 87 M for corrosion If it is a conductor / P pie r : r P √ /;pie
18
"TAN6AR6 F-AT" i) ii) iii)
16 ? :6 ? 86 ?
iv) v) vi)
=6 ? 76 ? 76 @
vii) viii) i)
?6 ? ?7 16 A7 16
)*+ =6 mm #ild Steel Rod
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6etermination o 8a:imum Permissi&#e "tep and Touch Potentia# E step P
* 1666 Q ?
s
$s ) 6.11?
/t where $s P Resistivit" of +rushed Rock s P Reduction factor for derating the value of surface la"er is a function of reflection factor H9B of crushed rock. 9P
= > ?
Q
s s
=is the Soil resistivit" = s is the resistivit" of +rushed Rock To plot the value of =s where
Refer the graph fig. @ of page =1 of I EEE @6. /t hs P and 9 P +alculate =s Then calculate E step P * 1666 Q ? s $s ) 6.11? √t Similarl" E Touch P
* 1666 Q ?
s
$s ) 6.11? √t
1=
+alculation of #esh $otential Em P 9m 9 i $ IG. where 9m P Geometrical 2actor 9i P +orrection factor for Grid IG P #aimum earth fault current in amps. P Total ength of +onductor in metres. The value of 9m can be calculated from the formula Ln D2 + (D + 2ho) 2 h + K ij ln K m = 1 2Π
1! h"
D"
#"
Kn
Π (2n-1)
9n P √ 1 Q h;ho
3here 3here
h P depth of buried conductor ho P reference depth D P Spacing between the Grid d P diameter of earth conductor n P no. of parallel conductor +alculate 9m P 9i P correction factor for grid geometer 9i P 6.?7? Q 61A: n IG P +urrent Diversion 2actor which is ?6M full load current So
Em
P $ K m $ K i $ I %
=
&olt
L
The #esh $otential alwa"s less than the permissible touch > step potential. Then onl" the design is considered as safe.
Ca#cu#ation o Num&er o E#ectrode R P
166
ln
=l d
ohm
17
#inimum Lo. of parallel electrodes needed P -alue of R Earth Electrodes
"tations a) arger $ower Stations and #a!or sub(stations
P#ate E#ectrodes G.I. $lates of 1:6 1:6 cm and 1:.7 mm thick
Pipe e#ectrodes i)G.I. $ipes of 17:mm dia 8.6=@mts. long and not less than
1:.7. thick. ii) In case of G .I. or steelpipes, not smaller than 8@mm internal diameter. b) Small sub(stations
?6 ?6cm G.. plates of F.= mm thick
G.I. pipes of 76.@ min internal diameter and 8.67@mts., long.
Earth conductors a) #ain and subsidiar" connection 5 +ross Section not less than 1?1s'. cmU+opper) b) 0ranch connections 5 +ross section not less than ?=.7 s'. cm.
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