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IS : 3043 - 1987(Reaffirmed 2001 )

Indian StandardCODE OF PRACTICE FOR EARTHING(First Revision)

Second Repriat FEBRUARY1998

UDC 6 2 1 3 1 6 .9 9 : 0 0 6 .7 6

Copyright 1988B U R E A U O F I N D I A N S T A N D A R D SMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARGNEW DEL HI 110002

Price Rs 225.00 September 1988

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I S 1 3 0 4 3 - 1 9 8 7Indian Standard

CODE OF PRACTICE FOR EART HING(First Revision)

Electrical Installations Sect ional Commit tee , ETDG 20Chmrmmn

S H B I M. L. D O N Q K EM-3 Saty am, 88 Sion Circ le , Bombay 400022RepresentingEngineer- in-Chief 's Branch, Army Headquarters ( Ministry ofD e f e n c e ), N e w D e l h iMembers

S H B I P. A M A N T H A B A M A NS H B I S. K. S H A N O A B I ( Alternate)

S H B I P. D. B A O A D ES H B I R. K. K A U L ( Alternate)

S H B I V. S. B H A T I AS H B I M. M. S H K T H M A ( Alternate)

S H B I K. V. C H A U B A LS H B I K. S. J O S H I ( Altirnatt)

S H B I R. R. C H O C D H U B IS H B I N. B A L A S U B R A M A N I A N ( Alternate )C H I E F E L E C T R I C A L E N G I N E E R

D E P U T Y D I B E C T O B S T A N D A R D S ( E L E O )-DI,R D S O ( Alternate )C H I E F E L E C T R IC A L IM S F K OT O B T O G O V E B N B N T

OF T A M I L N A D T JE L E C T B I C A L I N S P E C T O R ( T E C H N I C A L ) TO

G O V E R NM E N T O F T A M I L N A D U ( Alternate)C H I E F E N G I N E E R ( E L E C )-I

S U P E R I N T E N D E N T S U B V K Y O B or W O R K S( ELEC )-I ( Alternate )S H B I D E V E N D K B N A T H

S H B I T. P. R. S A B M A ( Alternate )S H B I K. W. D H A B M A D H I K A B I

D B V. N. M A L L B B ( Alternate )S H B I G. L. D C A

S H B I S. K. S E T H I ( Alternate )S H B I R. G K H A N N A

S H B I P. S. S A W H K K Y (Alternate )M E M B E B ( H Y D R O - E L E C T B I C )

DIBECTOB ( H E D )-I ( Alternate)E R S. P A N E E R S E L V A M

S H B I V. J A N A B D E A N A H (Alternate )S H R I K. P. R. P I L L A I

S H B I C. R. R. M E N O N ( Alternate )S H B I V. R A D B A K R I SH N A NS H B I H. S. R A OP B O F G. R A V E E N D R A N N A I BS H B I S. R. S A R D AS H B I R. S A T H I Y A B A L

S H B I K. K. M O N D A L ( Alternate )S H B I H. K. S I T A B A M

S H B I S. K. P A L I T ( Alternate )S H B I P. S B I N I V A S A P O T I

S H B I J O S E P H P H I L O M B N Y ( Alternate )S H B I D, S. T A W A B I

S H B I S. J. H A B I D A S ( Alternate )S H B I G. N. T H A D A N I

S H B I S. K, G H O S H (Alternate )S H B I G. S. T H A K U BS H B I V. T. W A B A N G

S H B I R. P. P A T K L ( Alternate )S H B I S. P. S A C H D E V ,Director ( Elec tech )

Tata Consult ing Engineers , BombaySiemens India Ltd, BombayFederat ion of Electr ic i ty Undertaking of India , BombayLarsen & Toubro ( Construction Group ), Madr asRai lway Board ( Ministry of Rai l ways ), Ne w De l h iChief Electr ical Inspector to Governmen t of Tam il Nad u, M adras

Central Publ ic Works Depa rtmen t , New Delhi

Larsen & Toubro Ltd, BombayJyot i Ltd, VadodaraRural Electr i f icat ion Corporat ion Ltd, New DelhiDe l h i E l e c t r i c Supp l y Unde r t ak i ng , Ne w De l h iCentral Electr ic i ty Authori ty , New DelhiTamil Nadu Electr ic i ty Board, MadrasFact Engineering and Design Organizat ion, U dhyog ama ndalBharat Heavy Electr icals Ltd, HyderabadCrompton Greaves Ltd, BombayChief Electr ical Inspector to the Governm ent ofKerala , Trivandrum.Maharashtra State Electricity Board, BombayTarif f Advisory Committee ( General Insurance ), Bombay

Calcutta Electric Supply Corporation Ltd, CalcuttaKarnataka Electr ic i ty Board, BangaloreElectr ical Engineer to Government ofMaharashtra, BombayEngineers India Ltd, New DelhiChief Electrical Inspector, Government of Madhya Pradesh, BhopalBombay Electr ic Supply and Transport Undertaking, BombayDirector General , B IS ( Ex-qfficio Member )

SecretaryS H B I K. G A N E S HDeputy Director ( Elec tech ), BIS ( Continued on page 2 )

Copyright 1988B U R E A U O F I N D I A N S T A N D A R D SThis publication is protected under the Indian Copyright Act (XIV 1957 ) and product ion in whole or in part byany means except with writ ten permiss ion of the publisher shall be de e me d to be an infr ingment of copyrightunde r t he s a i d Ac t .

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IS > 3643 1987( Continuedfrompage I )

Panel for the Revision of IS : 3043, E T DC 20 : P38Convener

S U B i N . B A L A S O B B A M A N I A NMembers

P B O F G . R A V E E N D B A N N A I RS H B I V . S A T H T A N A T H A NS H B I G . S . T H A K T J BS H B I R . S A T H I Y A B A LS H B I K . P . R . P I L L A I

RepresentingLarsen & Toubro ( Construction Group ) , MadrasChief Electrical Inspector to the Government of Kerala, TrivandrumTam il Nad u Electricity Board, MadrasChi ef Electrical Inspector, Governm ent of Madh ya Pradesh, BhopaiTariff Advisory Com mitte e, M adrasFact Engineering and Design Organization, Udyogamandal

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I S J

CONTENTSPage

0 . F O R E W O R D . . . 51 . S C O P E . . . 6

S E C T I O N 1 G E N ER A L G U I D E L I N E S2 . T E R M I N O L O G Y . . . ?3 . E X C H A N G E O F I N F O R M A T I O N . . . 84 . S T A T U T O R Y P R O V I S IO N S F O R E A R T H I N G . . . 95 . F A CT O R S I N F L U E N C I N G T H E C H O I C E O F E A R T H E D A N D U N E A R T H E D

S YS TEMS . . . 106 . S Y S T EM E A R T H I N G . . . 1 17 . E Q U I P M E N T E A R T H I N G . . . 1 5

S E C T IO N 2 C O N N E C T I O N S T O E A R T H8 . R E S I S T A N C E T O E A R T H . . . 1 69 . E A R T H E L E C T R O D E . . . 1 9

1 0. C U R R E N T D E N S I T Y A T T H E S U R F A C E O F A N E A R T H E L E C T R O D E . . . 2 71 1 . V O L T A G E G R A D I E N T A R O U N D E A R T H E L E C T R O D E S . . . 2 71 2 . C O N N E C T I O N S T O E A R T H E L E C T R O D E S E A R T H I N G A N D P R O T E C T I V E

C O N D U C T O R S . . . 2 71 3 . E A R T H I N G A R R A N G E M E N T F O R P R O T E C T I V E P U R P O S E S . . . 3 21 4. E A R T H I N G A R R A N G E M E N T F O R F U N C T I O N A L P U R P O S E S . . . 3 21 5 . E A R T H I N G A R R A N G E M E N T S F O R C O M B I N E D P R O T E C T I V E A N D

F U N C T I O N A L P U R P O S E S . . . 3 21 6. E Q U I P O T E N T I A L B O N D I N G C O N D U C T O R S . . 3 31 7. T Y P I C A L S C H E M A T I C O F E A R T H I N G A N D P R O T E C T I V E C O N D U C T O R S . . . 3 3

S E C T IO N 3 E A R T H F A U L T P R O T E C T I O N O N C O N S U M E R ' SP R E M I S E S1 8. E A R T H F A U L T P R O T E C T I O N I N I N S T A L L A T IO N S . . . 3 41 9. S E L E C T I O N O F D E V I C E S F O R A U T O M A T I C D I S C O N N E C T I O N O F S U P P L Y . . . 3 9

S E C TI O N 4 P O W E R S T A T I O N S , S U B S T A T I O N S A N DO V E R H E A D L I N E S2 0 . E A R T H I N G I N P O W E R S T A T I O N S A N D S U B S T A T I O N S . . . 4 32 1 . E A R T H I N G A S S O C I A T E D *W I T H O V E R H E A D P O W E R L I N E S . . . 5 2

S E C T IO N 5 I N D U S T R I A L P R E M IS E S2 2 . G U I D E L I N E S F O R E A R T H I N G I N I N D U S T R I A L P R E M I S E S . . . 5 3

S E C T I O N 6 S T A N D B Y A N D O T H E R P R I V A T E G E N E R A T IN GP L A N T S2 3 . E A R T H I N G I N S T A N D B Y A N D O T H E R P R I V A T E G E N E R A T I N G P L A N T S

( I N C L U D I N G P O R T A B L E A N D M O B I L E G E N E R A T O R S ) . . . 5 6

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IS t 3049 - 19 f7PageS L C T I O N 7 M E D IC A L E S T A B L I S H M E N T

24. PROTECTIVE MEASURES THROUG H EARTHING IN ME DICAL ESTABLISHMENT . . . 64

25. SUP PLY CHARACTERISTICS AND PARAMETERS .. . 65

S E C T I O N 8 S T A T I C A N D L I G H T N I N G P R O T E C T I O N E A R T H I N G( Un der consider ation. Clauses 26 and 27 reserved for Section 8 )

S E C T I O N 9 M I S C E L L A N E O U S I N S T A L L A T I O N S A N DC O N S I D E R A T I O N S28.2 9 .30.31.32.33.34.

EARTHING IN POTENTIALLY HAZARDOUS AREASTELECOMMUNICATION CIRCUITS AND APPARATUSBUILDING SITESMI N E S A N D Q U A R R I E SSTREET LIG HTIN G AND OTH ER ELECTRICALLYFURNITUREEARTHING OF CONDUCTORS FOR SAFE WORKINGMAINTENANCE OF EARTH ELECTRODES

SUPPLIES STREET

69707171737476

S E C T I O N 10 M E A S U R E M E N T S A N D C A L C U L A T I O N S35. CALCULATION OF EARTH FAULT CURRENTS . . . 7636. MEASUREMENT OF EART H RESISTIVITY . . . 7737. MEASUREMENT OF EART H ELECTRODE RESISTANCE . . . 7938. MEASUREMENT OF EARTH LOOP IMPEDANCE . . . 80

S E C T I O N 11 D A T A P R O C E S S I NG IN S T A L L A T I O N S39. EA RT HIN G REQUIREMENTS FOR INSTALLATIONS OF DA TA PROCESSINGEQUIPMENT #> 8040. EXAMPLE OF U S E OF TRANSFORMERS .. . 83

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IS i 3043 1987Indian Standard

CODE OF PRACTICE FOR EARTHING(First Revision)

0 . F O R E W O R D0.1 This India n S tand ard ( First Revision ) wasadopted by the Bureau of I nd ian Stan dard s on August 1987, after the draft finalized by th eElectrical Installations Sectional Committee, hadbeen approved by the Electrotechnical DivisionCouncil .0.2 The Indian Electricity Rules, together withthe supplem entary regulations of the State Elect r ic i ty Depar tments and Electr ici ty Under takings ,govern the electrical installation w ork in gene rating stations, substations, industrial locations,buildings, etc, in the coun try. To en sure safetyof l ife and app aratu s against ear th faults, i t w asfelt necessary to pre pa re a code of prac tice forearthin g. Th is code of practice is intend ed toserve as a consolidated guide to all those wh oare concerned with the design, installation, inspection and ma inten anc e of electrical systems an dapparatus .0.3 Th e subject of earth ing covers the problem srelating to condu ction of electricity th roug hear th. T he terms ear th and ear thing have beenused in this c ode irrespective of reliance bein gplaced on th e e arth i tself as a low impe dan ceretu rn path of the fault curren t. As a ma tter offact, the earth n ow rarely serves as a pa rt of th ereturn circuit but is being used mainly forfixing the voltage of system neu trals . Th e ea rthconnection improves seivice continu ity an davoids damage to equipme nt an d dang er tohuman life.0.4 The object or an earthing system is to provideas nearly as possible a surface un der an d a rou nda station which shall be at a uniform pote ntialand as nearly zero or absolute ear th poten tial aspossible. T he pu rpose of this is to ensure that, ingene ral, all parts of app ara tus o ther tha n liveparts , shall be at ear th poten tial , as well as toensure tha t operators and atten dan ts shall b e atear th potential at all t imes. Also by providin gsuch an earth surface of uniform poten tial un deran d surroun ding the station, the re can exist nodifference of potential in a short distance bigenough to shock or injure an at ten dan t whe nshort-circuits or other abn orm al occurrences tak eplace. The recommendat ions in this code arem ad e in ord er that these objects ma y be carriedout .

0 .5 Ear thing associated wi th current-carryingcon ducto r is norm ally essential to the security ofthe system and is generally known as systemear thing, whi le ear thing of n on-current car ryingme tal w ork a nd condu ctor is essential-to the safetyof human life, animals and property, and is general ly known as equipment ear thing.0.6 Since the publication of this stan dard in 1966,cons iderable exper ience has been gained throughthe implementation of i ts various stipulations.Mo reove r, several new concepts hav e been introduced the world over, on th e und erstand ing offunctional and protective earthing with a view totake into accou nt a variety of complex problem sencountered in actual practice. In the context ofincreased use of electric power and the associatednee d for safety in the design of insta llation s, itha d b ecome necessary to pre pa re an overallrevision of the earlier version of the Code.0.7 In this Code , the terms 'ear thing ' and'grounding ' are used synonymously. However ,this Code introduces several new terms ( see 2.15,2.17, 2.28, etc ) an d distinguishes earthin g 'conductor ' f rom 'protect ive conductor ' .0.8 This Code includes comprehensive guidelineson choosing the prope r size of th e various components ot the earthing system, particularlyearthing and protective conductors as well asear t h elect rodes . Guid ance included on determ inat ion of relevant 'k' factor depend ing on ( seeSec 2 ) ma terial properties and bou nda ry co nditions, and the associated minimum cross-sectionalarea would assist in a more scientific design of theearthing system under various circumstances.0.9 For the first t ime, the Cod e also includescom prehen sive guidelines on ea rth fault p rotec t ion in consumers ' premises to com men suratewith the provisions of IE RuUs 1956. It includesspecific guide lines on ea rth ing system design toachieve the desired degree of shock haz ard protection from earth leakages. Th e rules given inSection 3 of the Code should be read in conjunction with corresponding regulations given in thewiring code ( see IS : 732 ) .

0.9.1 Protection against shock, both in normalservice ( direc t con tact ) an d in case of fault( indirect co nta ct ) can be achieved by several5

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I S i 3 0 4 3 - 1 9 8 7measures. Details of such protective measures andguidan ce on their choice is the subject matter ofde ba te in th e process of revision of IS : 732*.Earth fault/ leakage protection sought to be achieved throu gh equi potential bond ing and autom aticdisconnection of supply is envisaged to prevent atouch voltage from persisting for such a durationthat would be harmful to hum an beings. G uidance on achieving this protec tion is covered inSec 3 of the Cod e.

0.9.2 While detailed guidelines are covered inspecific portion s of the Co de , the following shallbe noted:a) For solidly g rou nd ed systems, it sh all besufficient to check whether the characteristics of protective device for autom aticdisconnect ion, ear thing ar rangements andrelevant impedances of the circuits areproperly coordina ted to ensure that voltagesappearing between simultaneously accessible, exposed and extraneous conductiveparts are within the magnitudes that wouldnot cause danger ;b) For systems where the earthing is deem edto be adequate, i t shall be checked whetherthe ma in overcurrent protective device iscapable of meeting the requirements in thewiring code; andc) Wh ere the ma in ove rcurrent protectivedevice did not fulfil the requ irem ents orwh ere the earthing is considered in ad equate, then a separate residual currentdevice wou ld b e necessary to be installed,the earth fault loop impedance and the

tripping characteristics so chosen that theycomply with safe touch voltage limits.0. |0 T he revision of the Code aims at consolidating in one vo lum e all th e essential guidelinesneed ed for prep aring a good earth ing design inan electrical installation. The revision alsoattemp ts to be mo re elabo rate tha n the earlierversion, especially in areas of specific interestkeeping in view th e n eed and wide experiencegained the wor ld over .

0*11 For convenience of identifying areas of interest by an y specific users of the Co de, th e informatio n contained in this stand ard is divided intodifferent Sections as follows:Section 1 Ge nera l guidelines;Section 2 Conn ections to earth;Section 3 E arth-fault protection in consumer's premises;Section 4 Pow er stations, substations an doverhead lines; vSection 5 Indu strial premises;Section 6 Stand by and other private generat ing plant ;Section 7 Me dical establishments;Section 8 Static an d lightning protectiongrounding ;Section 9 Miscellaneou s installations an dconsiderations;Section 10 M easure me nts and calculations;a n dSection 11 Data processing installations.0.12 In the prep aration of the Code, assistancehas been taken from the following:I EC Pu b 364 ( an d Pa rts ) Electrical installations in buildings. Intern ationa l Electro-technical Commission.BS Docu ment 84/21243 Draft s tandard codeof practice on earthing ( revision of CP 1013:1965 ). British Standards Institution.AN SI/IE EE Std 142-1982 IE EE Recomm ended practice for grounding of industrial and

commercial power systems. AmericanNat ional Standards Ins t i tute ( USA ) .0.13 For the purpose of deciding whether a particula r requirem ent of this stan dard is compliedwith, the final value, observed or calculated,expressing the result of a test or analysis shall beroun ded off in ac cordance witrTIS : 2-1960*. Th enum ber of significant places retained in the roun ded off value should be the same as that of thespecified value in this standard.

Code of practice for electrical wiring installation. 'R ule s for rounding off numerical values ( rnised }.

1 . SCOPE1.1 Th is code of practice gives guida nce on themethods that may be adopted to ear th an elect r i cal system for the purpose of limiting the potential( with respect to the general mass o f th e earth )of curren t carry ing cond uctors forming p art ofthe system, that is , system ea rthing an d non -

current carrying metal work association with,equip me nt, appa ratus and appliance connectedto the system ( that is , equipment earthing ) .1.2 This Code applies only to land-based installations an d it does not apply to ships, aircrafis oroffshore installations.

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IS t 3043 . 19872 . 2 3 P E N C o n d u c t o r A conductor combining the functions of both protective conductor andneutral conductor .2 . 2 4 P o r t a b l e E q u i p m e n t Equipment whichis mov ed while in opera tion or which ca n easilybe moved f rom one place to another whi le connected to the supply.2.25 P o t e n t i a l G r a d i e n t ( A t a P o i n t ) T h epoten tial difference per unit length me asured inthe di rect ion in which i t i s maximum.

N O T * 1 Whe n an e lectr ic force i s due to potential difference, it is equal to the potential grad ient.NO T B 2 Poten tial gradient is expressed in v oltsper unit length.

2 .2 6 P r o t e c t i v e C o n d u c t o r A conductor usedas a mea sure of protection against electric shockand intend ed for connec ting any of the followingparts :a) Exposed conduct ive par ts ,b) Extraneous conductive parts ,c) Main ear thing terminal , andd) Earthed point of the source or an artificialneutral .

2.27 R e i n f o r c e d I n s u l a t i o n Single insulationapplied to l ive parts , which provides a degre e ofprotection against electric shock equivalent todouble insulation un der the co nditions specified inthe relevant s tandard.NOTB T he term 's ingle insulat ion' does not implythat the insulat ion has to be one hom ogeneous pie ce . I tmay comprise several layers that cannot be tested singlyas supplementary or basic insulation.

2.28 R e s i d u a l C u r r e n t D e v i c e A mechan i cal switching device or association of devicesintend ed to cause th e ope ning of the contactswhe n the residual curre nt attain s a given valueunder specified conditions.2 .2 9 R e s i d u a l O p e r a t i n g C u r r e n t Res idua lcurre nt which causes the residual curren t deviceto operate under specified conditions.2 .30 Re s i s tan ce Area ( For an Ea rth Elec t r o d e o n l y ) T he surface are a of groun d( arou nd an earth electrode ) on wh ich a significant voltage gradient may exist .2 .31 Safe ty E xtra Low Vo l tage Se e IS :9409-1980*.2 .3 2 S i m u l t a n e o u s l y A c c e s s i b l e P a r t s C o n ductors or conductive parts which can be touchedsimultaneously by a person or, where app licab le,by livestock.

NOTE 1 Simultaneously access ible parts may be:a) l ive parts,b) exposed conduct ive parts ,c) extraneous cond uct ive parts ,d) pr otect ive conductor s , ande) earth e lectrodes .

Classification of electrical and electronic equipmentwith regard to protection against electric al shock.

NOTB 2 This term appl ies for l ivestock in locat ions speci f ical ly intend ed for these anim als .2.33 S u p p l e m e n t a r y I n s u l a t i o n I n d e p e n d ent insulation applied in addition to basic v insulation, in order to provide protection againstelectric shock in the even t of a failure of basicinsulation.2 . 3 4 S w i t c h g e a r An assembly of m ain an dauxiliary switching apparatus for operation, regulation, protection or oth er co ntrol of electricalinstallations.

NOTB A more comprehen sive def ini tion of th eterm 'Switchgear' can be had from IS : 1885 ( Part17 ) -1979.2.35 V o l t a g e , N o m i n a l Voltage by which aninstallation ( or pa rt of a n installation ) is designa ted .2.36 T o u c h V o l t a g e T he p oten tial differencebetween a grounded metal l ic s t ructure and a pointon the earth's surface separated by a distanceequal to the norma l ma xim um hor izontal reach ,approximately one metre ( see Fig. 1 ) .2.37 S t e p V o l t a g e T he potential differencebetween two points on the earth's surface, separated by distance of one pace, that will be assumedto b e one m etre in the di rect ion of max imumpotent ial gradient ( see Fig. 1 ) .2 .3 8 E q u i p o t e n t i a l L i n e o r C o n t o u r T h elocus of points havin g th e same poten tial at agiven t ime.2.39 M u t u a l R e s i s t a n c e o f G r o u n d i n g E l e c tr o d e s Equal to the vol tage change in one ofthem produced by a change of one am pere ofdirect cur rent in the o ther an d is expressed ino h m s .2 .40 Earth Grid A system of grounding electrodes consisting of inter-connected connectorsbur ied in the ear th to provide a common groundfor electrical devices and metallic structures.

NOTEThe term 'earth grid' does not include'earth mat'.2 .41 Earth Mat A grounding system formedby a grid of horizon tally buried conductors andwhich serves to dissipate the earth fault curren t toear th and also as an equ ipotent ial bonding conductor system.3. E X C H A N G E O F I N F O R M A T I O N3.1 Wh en the ear thing of a consumer 's ins tal lation is being planned, prior consultation shall takeplace between the consultant or con tractor an dthe supply auth ority. W he re necessary, con ;ulta-t ions wi th the Posts & Telegraphs Dep ar tmen tshall also be carried out in order to avoid a nyinterference with the telecommunication system.

Electrotechnical vocabulary: Part 17 Switchgear a ndcontrolgear (first revision).

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I S i 3043 - 1987

STEP| R I Q R F RX

I R 0U R1 "*T7POTENTIAL RISE ABOVE REMOTEEARTH DURING SHORT CIRCUIT

STEP VOLTAGE AT A GROUNDED STRUCTURE j

E TOUCH Rl

] R 0 IKPOTENTIAL RISE ABOVE REMOTEEARTH DURING SHORT CIRCUIT

TOUCH VOLTAGE AT A GROUNDED STRUCTUR EF io . 1 S T E P A N D T O U C H V O L T A G E S

4 . S T A T U T O R Y P R O V I S I O N S F O RE A R T H I N G4.1 Earti ng shall gen erally b e carried out inacco rdan ce wi th the requirem ents of Indian Electricity Rides 1956, as ame nde d f rom t ime to t ime an dthe relevant regulations of the Electricity SupplyAuthor i ty concerned.4 .2 Al l medium vol tage equ ipme nt shal l beear thed by two separate and dis t inct connect ionswith ear th . In the case of high land extra highvoltages, th e neu tral points shall be ear thed bynot less tha n two sepa rate an d distinct connections with earth, each having its own electrode atthe gene rating station or substation an d ma y beearth ed at any other p oint provid ed no interference is caused by such earth ing . If necessary, th eneutral may be ear thed through a sui tableimpedance .

4.2.1 In cases where direct earthing may proveharmful rathe r tha n provide safety ( for exam ple,high frequency and mains frequency corelessinduction furnaces ) , relaxation may be obtaine dfrom the competent author i ty .

4.3 Earth electrodes shall be provided at generating stations, substations and consumer premisesin accordance with the requirements of thisCode ..4.4 As far as possible, all ea rth conn ection s shallbe Visible for inspection.4.5 All con nectio ns shall be carefully m ad e; ifthey are poorly m ad e or ina deq uate for the purpose for wh ich they a re inte nde d, loss of life orserious personal injury may result .4.6 Each earth system shall be so devised that thetesting of individua l earth electrode is possible. I ti s recommended th at the value of any ear thsystem resistance shall be such as to conform wit hthe degree of shock protection desired.4 .7 I t i s recommended that a drawing showingthe main ear th connect ion and ear th elect rodes beprepared for each installation.4 .8 No addi t ion to the current-carrying sys tem,ei ther temporary or perm anen t , shall be m adewhich will increase th e ma xim um available earth

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IS : 3043 . 1987fault current or its duration until i t has beenascer ta ined that the exis t ing arrangement of ear thelectrodes, eart h bus-bar, etc, are capable ofcarryin g the ne w value of ear th fault curren twhich may be obta ined by th is addi t ion .4.9 No cut-out, l ink or switch other than a linkedswitch arra nge d to opera te simultaneously on theearthed or earthed neutral conductor and the liveconductors, shall be inserted on any supplysystem. Th is, however, does not include the caseof a switch for use in con trolling a gen era tor ora transformer or a link for test purposes.4.10 All m ater ials, fittings, etc, used in ear thi ngshall conform to Ind ian Stan dard specifications,wherever these exist.5 . F A C T O R S I N F L U E N C I N G T H E C H O I C EO F E A R T H E D O R U N E A R T H E DS Y S T E M5.1 Service Cont inui ty5.1.1 A nu m ber of industr ial plant systems hav ebeen opera ted unear th ed a t one or more vol tagelevels. Th is is basically guided by the thou ght ofgaining an additional degree of service continuityvarying in i ts importance depending on the typeof plant. Ea rthe d systems are in most cases designed so that circuit protective devices will removeth e faulty circuit from th e system reg ardless ofthe type of fault. However, experience has shownthat in a number of systems, greater service continui ty may be obta ined with ear thed-neutra l thanwith unear thed neutra l systems.5.2 M u l t i p l e F a u l t s t o G r o u n d5.2.1 While a ground fault on one phase of anunearthed system generally does not cause aservice interr uptio n, the occurrence of a secondgro un d fault on a different pha se before the firstfault is cleared , does result in an outage; T helonger a ground fault is allowed to remain on anune arth ed system, greater is the likelihood of asecond one occurring in another phase and repairsar e required to res tore serv ice . W ith an unearthed system, an organized m aintenan ce program me is therefore extremely impo rtant so tha tfaults are located and remo ved soon after detect ion .

Experience has shown that mult ip le groundfaults are rarely, if ever, experienced on earthed-neutral systems.5 . 3 A r c i n g F a u l t B u r n d o w n s5.3.1 In typical cases, an arcing fault becomesestablished between two or more phase conductorsin an une arth ed systems or betw een phase an dgro und in a solidly earthed-n eutral system. Thiswould result in sc e r e dam age or destruct ion toequ ipm ent. However, arcing fault curren t levelsm ay be so low tha t phase ov ercurr ent protectivedevices do not operate to remove the fault quickly.Such faults are cha racteristic of open or coveredfuses, partic ularl y in switchgc ar or metal-enclos ed

switching and motor control equipm ent. It isgenerally recognized that protection under suchcircumstances is possible by fast and sensitivedetection of the arcing fault current and interruption with in 10-20 cycles. In solidly eart hed -neu tral systems, this is possible as an arci ng faultwould produce a current in the ground path,thereby providing an easy means of detection andtripp ing against ph ase-to-groun d arcing faultbreakdowns.5 . 4 L o c a t i o n o f F a u l t s

5.4.1 O n an une arth ed system, a ground faultdoes not open the circuit. So me m eans of detecting the presen ce of a grou nd fault requ ires to beinstalled In earthe d system, an accidental groundfault is bo th indi cate d at least partia lly locatedby an autom atic interruption of the accidentallygrounded circuit or piece of equipment.5.5 Safety

5.5.1 W heth er or not a system is grou nded ,protection of personnel an d property from haz ard srequire thorough grounding of equipment andstruct ures. Pro pe r grou ndin g resu lts in less likelihood of accidents to personnel. Ot her haza rds ofshock and fire may result from inadequate grounding of equipmen t in unear thed and ear thedsystems. Ho we ver, relatively high fault curren tsassociated with solidly earth ed system may present a haza rd to workers from exposure to hotarc products and flying molten meta l. This protection is, however, reduc ed because of use ofmetal-enclosed equipment.5 .6 A b n o r m a l V o l t a g e H a z a r d s

5.6.1 T he possible over-voltages on the un earthed system may cause mo re frequent failures ofequipm ent tha n is the system, if earth ed. A faulton one phase of an une arthe d or imped ance-gro und ed system places a sustained increasedvoltage on the insulation of ungrounded phasesin a 3-phase system. This voltage is about 1*73t imes the norm al voltage on the insulation. Thisor other sustained over-voltages on the unearthedsystem may not immediately cause failure ofinsulation but may te nd to reduc e the life of theinsulatio n. Som e of the mo re common sources ofover-v oltage s on a powe r system ar e the following:a) Lightning,b) Switching surges,c) Static,d) Contact with a high voltage system,e) Line-to-ground fault,f) Res onan t conditions, andg) Restriking ground faults.

5.6.2 Surge arresters a re recom men ded forlightning protection. Grounding under such casesare separately discussed in Section 8. Neutral

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IS t 3043 1987grounding is not likely to reduce the total magnitude of over-voltage produced by lightnin g orswitching surges. It can, however, distrib ute thevoltage between phases and reduce the possibilityof excessive voltage stress on the pha se-to -gro undinsulation of a particular phase. A system groundconnection even of relatively hig h resistance caneffectively pre ven t static voltag e bu ild-u p ( se eSec 8 ). Even under conditions of an H V linebreaking an d falling on an LV system, an effectively grounde d LV system will h old th e systemneutral close to the ground potential thus limitingthe over-voltage. An une arth ed system will besubjected to resonan t over-voltages. Field exp erience and theoretical studies have shown the worldover tha t arcing , restriking or v ibrat ing grou ndfaults on unea rthed systems can, under certainconditions, produ ce surge voltages as high as 6t imes the normal voltage. Neutral grounding iseffective in reducing transient build up by r educ ing the neutral displacement from grou nd potential and th e destructiveness of any high frequencyvoltage oscillations following each arc initia tionor restrike.5.7 C o s t

5.7.1 T he cost differential betwe en eart hedand unearthed neutral system will vary, depe nding on the method of grounding the degree ofprotection desired, and wheth er a new or anexisting system is to be earthed.6. SYSTEM EARTHING6.0 Basic Object ives

6.0.1 Ea rthin g of system is designed prim arilyto preserve the security of the system by ensuringthat the potential on each cond uctor is restrictedto such a valu e as is consistent wi th th e level ofinsulation applied. From the point of view ofsafety, it is equally important that earthing shouldensur e efficient an d fast op era tion of pro tect ivegear in the case of earth faults. Most high voltagepublic supply systems are e arth ed. App roval hasbeen given in recent years to unearthed overheadline systems in certa in countries, bu t these hav eonly been small 11 kV systems deriv ed from33 kV mains, whe re the capacity eart h current isless than 4 A and circumstances are such that thesystem will not be appreciably extended.

6.0.2 The l imitat ion of earthing to one pointon each system is designed to prevent the passageof current through the earth under normal conditions, and th us to avoid the accom pany ing risksof electrolysis an d interference with com mun ication circuits. W ith a suitable designed system,properly operated and main tained, earth ing atseveral points may be permitted. Th is m ethod ofearth ing becomes economically essential in systems at 200 kV and upwards.6.0.3 The system earth-resistance should be

such tha t, whe n any fault o ccurs against wh ich

ear thin g is designed to give protection , the pr otective gear will op erat e to ma ke the faulty ma inor plant harmless. I n most cases, such oper ationinvolves isolation of the faulty ma in or pl an t, forexample, by circuit-breakers or fuses.6.0.4 In the case of unde rgroun d systems, thereis no difficulty wh ateve r but , for e xam ple, in the

case of ove rhead -line systems prot ected by fusesor circuit-breakers fitted with overcurrent protection only , the re ma y be difficulty in arr ang ingtha t the value of the system earth-resistance issuch tha t a conductor fal ling and mak ing goodcontact with the ground results in operation ofthe prote ction. A low system-earth resistanceis required even in the cases where an a rc-suppression coil is installed , as its op eration ma ybe frustrated by too high an earth-electro deresistance.6.0.5 Earthing may not give protection againstfaults that are not essentially earth faults. For

example, i f a phase conductor on an overhead spurline breaks , an d the part rem ote from the supplyfalls to the groun d, it is unlikely th at any pro tect ive gear relying on earthing , other tha n currentbalan ce protectio n at the substation, will opera tesince the earth-fault current circuit includes theimpedance of the load that would be high relat iveto the rest of the circuit.6.0.6 For the purposes of this code of practice,it is convenient to consider a system as comprisinga source of energy and an installation; the formerincluding the supply cables to the latter.

6 .1 C l ass i f i ca t i on o f Sy s t e m s Bas ed onT y p e s o f S y s t e m E a r t h i n g6.1.1 Internationally , i t has been agreed toclassify the ear thin g systems as TN System, TTSystem an d IT System. They a re :a ) TN system has one or more points of thesource of energy directly earth ed, and theexposed and extraneous conductive partsof the instal la tion are connected by mean sof protective conductors to the earthedpoint(s) of th e source, th at is, ther e is ametallic pa th for ear th fault curren ts toflow from th e installation to the earth ed

point(s) of th e source . T N systerns ar efurther sub-divided into TN-G, T N - S a n dTN-C-S systems.b) TT system has one or more points of thesource of energy directly earthed and theexposed and extraneous conductive partsof the installation are connected to a localearth electrode or electrodes are electrically independent of the source earth (s).c) IT system has the source ei ther unea rthed or earthed through a high impedanceand the exposed conductive parts of theinstallation are connected to electrically

independent earth electrodes.

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I S : 3043. 19876.1.2 It is also recognized that , in pract ice, asystem may be an admixture of type for the purposes of this code, earthing systems are designatedas follows:a)

c)

TN-S System {for 240 V single phase domestic!commercial supply ) Systems where thereare separate neutral and protective conductors throughout the system. A systemwhere the metallic path between theinstallation and the source of energy is thesheath and armouring of the supply cable( see Fig. 2 ).b) Indian TN-S System {for 415 V three-phasedomestic commercial supply ) An independent earth electrode within the consumer'spremises is necessary ( See Fig. 3 ).

Indian TNC-SystemThe neutral and protective functons are combined in a singleSOURCE OF ENERGY

couductor throughout the system ( forexample earthed concentric wiring (seeFig. 4 ).d) TN-C-S Systtm The neutral and protective functions are combined in a single

conductor but only in part of the system( set Fig 5 ).e) T-TN-S System {for 6-6\ll kV thtee-phasebulk supply ) The consumers installation,a TN-S system receiving power at a captivesubstation through a delta connectedtransformer primary ( see Fig. 6 ) .f) TT System {for 415V three-phase industrialsupply ) Same as 6.1.1 (b) ( see Fig 7. ).g) IT SystemFig. 8 ). Same as 6.1.1 (c) ( see

SOURCE fEARTH {IEQUIPMENT IN \INSTALLATION

-L2-L3-NPE

EXPOSEDCONOUCTIVEPART'CONSUMERINSTALLATION

NOTE The protective conductor ( FE ) is the metallic covering ( armour or load sheath of the cable supplyingthe installation or a separate conductor ).. AH exposed conductive parts of an installation are connected to this protective conductor via main earthingterminal of the installation.Fio . 2 TN- S SYSTEM SEPARATE NEUTRAL AND PROTECTIVE CONDUCTORS THROUGHOUT THE SYSTEM,230V SIMPLE PHASE. DOMESTIC/COMMERCIAL SUPPLY FOR 3~TN-S ( >SM FIO. 3 )

SOURCE OF ENERGY LIL2

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r - < > A A j

415 V Three phase Domestic/Commercial supply having 3

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IS t 3843 1987

SOURCE OF ENERGY

SOURCEART CE I 0" i I t v.^3 ^ C O N S U M E R \EXPOSEO CONDUCTIVE \ \ ^

LI L2 1 3 COMBINEOPE & NCONDUCTORI ADDITIONALI SOURCE EARTHj (MAY BE PR OVIDED)

I

T-> INSTALLATIONP A R T S

All exposed conductive parts are conne cted to the PEN conductor. For 3 / consumer, local earth electrode hasto be provided in addition.Fio. 4 INDIAN TN-C SYSTEM ( NEU TRA L AND PROTECTIVE FUNCTIONS COM BINED INA SINGLE CONDUCTOR THROUGHOUT SYSTEM )

COMBINEDPE &. NC0N0UCTDR

The usual form of a TN-C-S system it at shown, where the supply it TN -C and the arrangement in theinstallations in TN-S.This type of distribution is known also as Protective Multiple Earthing and the PEN conductor it referred to asthe combined neutral and earth ( CNE ) Conductor.Th e supply system PEN conductor is earthed at several points and an earth electrode may be necessary at ornear a consumer's installation.All exposed conductive parts of an installation are connected to the PEN conductor via the main earthingterminal and the neutral terminal, these terminals being linked together.The protective neutral bonding ( PNB ) is a variant of TN-C-S with single point earthing.

FIQ. 5 TN-C-S SYSTEM, NEUTRAL AND PROTECTIVE FUNCTIONS COMBINED IN A SINGLECONDUCTOR IN A PART OF THE SYSTEM

http://exposeo/http://exposeo/http://exposeo/

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IS t 3043 1987SOURCE OF ENERGY

r rONSUMERINSTALLATION

I - ,

->f

100

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.

4

1_ _-E TA= - - = E - E E E E E E E = = = = = = = = = = = =0 5 K) 15 20 25 30 35 40 45 50 55 60 65 70MOISTURE IN SOIL,PERCENT

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IS : 3043 29878.8.2 T h e re duc tion in soil resistivity effectedby salt is shown by the curv e in Fig. 10. T h e saltcontent is expressed in perce nt by we ight of thecontained moisture. I t will be noted tha t thecurve flattens off a t about 5 perc ent salt con tent an da further increase in salt gives but little dec reasein the soil res istivity. T h e effect of salt will bedifferent for different k ind s of soil an d for va riou smoisture contents but the curve will convey anidea of how the soil conductivity can b e imp roved. Decreasing the soil resistivity causes a corre sponding decrease in th e resistance of a drivenearth electrode.

Ula-l555?=?cinfco* ->Iu

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IS i 3043 1987The res is tance R ( in O ) of a 1-2 m x 1*2 mplate is given approxim ately by the formula:

R 275For conventional sizes, the resistance is approxima tely inversely p rop ortio na l to the l inear di

men sions, not the surface are a, that is a 0*9 m x0*9 m plate would have a resistance approximately25 percent higher than a 1 2 x 1 2 m plate. Th ecurre nt loadin g capacity of a 1*2 m x l ' 2 m plateis of the order of 1 600 A for 2 s an d 1 300 A for3 s.Plate electrodes shall be buried such that i ts topedge is at a depth not less than 1 5 m from thesurface of the g roun d. Ho wever , the depth atwhich plates are set should be such as to ensurethat the surrou nding soil i s always dam p. Wh erethe und erlying stratum is solid, for exam ple chalkor sandston e an d ne ar the surface, th e top of the

plate should be approxim ately level wi th the topof the solid stratum.9.2.2 Pipes or Rods T h e resistance of a pipeor rod electrode is given by:

Rwhere

100 p2 T C / lo g 0 T- ohms

/ = length of rod or pipe ( i n cm ) ,d = diam eter of rod or pipe in cm, andp = resistivity of the soil ( in }.m )( assumed uniform ).T h e curves of Fig. 11 are calculated from thisequation for electrodes of 13, 25 and 100 mm diameter respectively in a soil of lOOQ.m respectively.Ch an ge of diam eter has a relatively minor effectan d size of p ipe is gene rally gov erned by resistance to bending or s p l i t t i ng . l t i s apparent thatthe resistance diminishes rapidly with the first fewfeet of driving, but less so at depths greater than2 to 3 m in soil of uniform resistivity.A num ber of rods or pipes may be connectedin parallel and the resistance is then practicallypropor t ional to the reciprocal of the num ber employed so long as each is situated outside the resistan ce area of any other. In practice, this is satisfied by a mu tual separation eq ual to the d rivende pth . Li t t le is to be gained by separat ion bey ondtvvice the driven depth. A substantial gain iseffected even at 2 m separation.Pipes ma y be of cast iron of no t less th an 100m m diam eter , 2-5 to 3 m long and 13 m m thick.Such pipes cann ot be driven satisfactorily an dm ay , therefore, be m ore expensive to ins tal thanplate s for the same effective are a. A lternatively,mild steel water-pipes of 38 to 50 mm diameter are

somet imes employed. These can be dr iven but areless durab le tha n copper rods .

Driven rods generally consist of round copp er,steel-cored cop per or galvan ized steel ( see 9.2.8 )13 , 16 or 19 m m in diam eter from 1 220 to 2 440mm in length.2S0

w 200oz 150UJa. 100

IDu

4 ^ # " ISOLATOR ^ ^O

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IS t 3043 . 1987

SECTION AA .ONLY TOP RINS TO BEWEL DED TO MAIN REINFORCEMENT RODS

F F L

GL Is

EARTHING PAD 100x100/(TO BE CONNECTED TO

M A IN E A R T H G R ID )W E L D

JSUBSTATION BUILDING

' COLUMN

N O T E 1 Top r in g should be ha lf the s ize o f m a in ve r t ica l re in forcem ent rod .N O T E 2 T w o e x t re me c o lu mn s s h o u ld b e e a r th e d l i k e th i s i n e a c h s u b s t a t io n .N O T E 3 T h i s i s a p p l i c a b le to R C G ma s t s a n d e q u ip me n t s u p p o r t s i n O D s w i t c h y a rd .N O T E 4 In s e r t s o th e r t h a n e a r th in g p a d s ma y o r ma y n o t b e w e ld e d to r e in fo rc e m e n t .

F io . 2 3 E A R T H IN G O F F O U N D A T IO N R E IN F O R C E M E N T ( C O N C R E T E E N C A SE DE A R T H I N G E L E C T R O D E )

Th e per imeter fence may need to be ear thedseparately from the m ain station earth electrodesystem ( see 20.6.1 ).T he tertiary win ding of a power transform er

should be connected to the tra nsform er tank by aconnection of sufficient cross-sectional area to carrythe primary short-circuit current.In th e case of pole mo unte d transformers onoverhead line systems, difficulties may arise inareas of high soil resistivity. Here, if the pole carries also isolating switchgear with low level operating handle, up to three separately earthed electrode systems may be required. That for the neutralof the low volta ge system is usually provid ed no tnear er th an one pole span away on the low voltageline. T ha t for the high voltage metalw ork ( trans former tank , switch framework, support m etal

wo rk ), consists of one e art h electro de at or ne arth e pole. Resistances of 5 to 50 ii are sometimes

the mi nim um economically possible. In addition,an earth mat should be provided, near the groundsurface, in the position taken up by a person operating the switch han dle ; this ma t should be connected to the switch hand le. T he m at should beelectrically separated from the main electrode; thisis considered to be achieved by spacing the Jnea rest elem ent of tha t electrode at least 1 m fromthe periphery of the m at and by placing the twoearthing-wires on opposite sides of the pole. Thetops of the main electrodes should be at least 225m m and preferably 750 m m below the groun d,and th e earthin g wire to t he ma in electrode ofoutdoor type rubber or plastics-insulated cable upto a point 2 m above ground level. This cable,between the bottom of the pole and the electrodeshould be laid in a 50-mm diam eter eart hen wa reduct filled solid with bitumen.2 0.3 G e n e r a l E a r t h i n g A r r a n g e m e n t s a tP o w e r S t a t i o n s o f P u b l i c E l e c t r i c i t y S u p p l i e s

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I S i J04 3 - 19872 0 . 3 . 1 Neutral Earthing of Generator Circuits A tm o d er n l a rg e p o w er s t a t i o n s for p u b l i c e l ec t r i c i t ys u p p ly t h e g e n e r a t i o n c i r c u it s g e n e r a l l y c o m p r i s ea s t a r - co n n ec t ed s t a t o r c i r cu i t w i t h an o p e ra t i n gv o l t ag e u p t o ab o u t 2 6 k V , d i r ec t l y co n n ec t edt o a s t e p - u p d e l t a / s t a r t r a n s f o r m e r , t h e h i g h e rv o l t a g e w i n d i n g g e n e r a l l y o p e r a t i n g a t 1 32 V k ,2 7 5 k V o r 4 0 0 k V , w i t h t h e t r an s m i s s i o n s y s t em

n e u t r a l p o i n t d i r e c t l y e a r t h e d .T h e f o l l o w i n g t h r e e m e t h o d s h a v e b e e n u s e df o r e a r t h i n g t h e n e u t r a l o f t h e g e n e r a t o r w i n d i n g s :a ) E a r t h i n g t h r o u g h t h e p r i m a r y w i n d i n g o f am a t c h i n g t r a n s f o r m e r , w i t h r e s is t or c o n n e c t e d a cr o s s t h e s e c o n d a r y w i n d i n g ;b ) e a r t h i n g t h r o u g h a r e s is t o r ; a n dc ) e a r t h i n g t h r o u g h th e p r i m a r y w i n d i n g o fa v o l t a g e t r a n s f o r m e r .Method (a) is cu r re n t p rac t i c e , t h e d es i g n b e i n g s u c h t h a t t h e m a x i m u m s u b s t a i n e d e a r t h f a u lt

cu r re n t i n t h e g e n e r a t o r c i r cu i t i s r e s t r i c t ed t o 1 0t o 15 A , t h u s l i m i t i n g t h e d a m ag e a t t h e p o i n t o ff a u lt . T h e n e u t r a l a n d e a r t h i n g c o n n e c t i o n s , h o w ev er , a r e o f ad eq u a t e cap a c i t y t o w i t h s t a n d fo r3 s t h e ea r t h f au l t cu r re n t t h a t wo u l d f lo w i n t h eev en t o f t h e m a t c h i n g t r an s fo rm e r t e rm i n a l s f la sh i n g o v e r d u r i n g a n ea r t h f au lt . Th e r e s i s t o r u s edfo r t h e a r r an g e m en t is o f t h e m e t a l l i c g r i d n o n -i n d u c t i v e t y p e .Method (b) ca n b e u s ed t o ach i ev e t h e s am ed eg ree o f f au l t - cu r ren t l i m i t a t i o n , b y d es i g n of as u i t ab l e h i g h -cu r re n t r e s i s t o r , b u t i s n o t p re f e r r edon th e gr ou nd s of cos t an d i t s l ess rob us t c ons t ruc

t i o n t h a n t h a t o f t h e e q u i p m e n t u s e d i n m e t h o d( a ) . I t wa s ea r l i e r p ra c t i ce , h o w ev er , t o i n d i v i d u a l l y ea r t h e ach g en e ra t o r a t r J ower s t a t i o n s b yl i q u i d ea r t h i n g r e s i st o r s d es i g n ed t o l i m i t t h e ea r t h -f au l t cu r ren t t o ab o u t 3 0 0 A .Method (c) i s n o w h i s t o ri c , b u t h a d th e a d v a n t a g e t h a t m i n i m a l d a m a g e r e s u l t e d a t a n e a r t hfau l t . I f d es i r ed , t h e g en e ra t o r co u l d r em ai n i nc i r c ui t w h i l e o p e r a t i o n a l a r r a n g e m e n t s w e r e m a d et o p e r m i t i ts w i t h d r a w a l . H o w e v e r , t h i s i m p o s e d ah i g h e r v o l t ag e s t r e s s o n t h e s t a t o r w i n d i n g s an dp l a n t o n t h e u n f a u l t e d p h a s e s , a n d t h e m a c h i n ed es i g n u s u a l ly i m p o s ed l i m i t a t i o n s o n t h i s . Th e

o u t p u t f r o m t h e s e c o n d a r y w i n d i n g o f t h e v o l t a g et r a n s f o r m e r c o u l d b e a r r a n g e d t o a c t i v a t e a na l a rm o r t r i p t h e g e n e ra t o r c i r cu i t a s d es i r ed . Ind e s i g n i n g t h e n e u t r a l a n d e a r t h i n g c o n n e c t i o n s t ot h e v o l t a g e t r a n s f o r m e r , t h e e a r t h - f a u l t c u r r e n tused was th at resu l t ing by f lashover of th e vol ta get r a n s f o r m e r d u r i n g a n e a r t h f a u l t .S o m e o l d p o w e r s t a t io n s h a v e g e n e r a t o r s c o n n ec t ed d i r ec t l y t o d i s t r i b u t i o n s y s t em b u s b a r s ; i ng e n e r a l , th e n e u t r a l t e r m i n a l s o f s u c h g e n e r a t o r sh a v e b een ea r t h e d v i a l i q u i d n eu t r a l ea r t h i n g r e s is t o r s o f s u c h a v a l u e t h a t t h e m a x i m u m s u s t a i n e dea r th fau l t cu rre nt i s of th e or de r of ful l load cur

r en t o f t h e g en e ra t o r . In s t a l l a t i o n s o f n e u t r a l p o i n ts w i t c h b o a r d s w i t h s w i t c h i n g o f n e u t r a l p o i n t s a n d

e a r t h i n g r e s is t o r s h a v e b e e n a b a n d o n e d i n f a vo u ro f i n d i v i d u a l u n s w i t ch ed ea r t h i n g r e s i s to r s .2 0 . 3 . 2 Earthing of Power Station Auxiliary Systems T h e r e a r e , i n c o m m o n u s e , t h r e e m e t h o d s o fea r t h i n g t h e n eu t r a l p o i n t i n p o w er s t a t i o n au x i l i a ry s y s t em s :a ) S o l i d e a r t h i n g ;b ) ea r t h i n g t h r o u g h a v o l t ag e t r an s f o rm er ( o rv o l t ag e r e l ay ) w i t h a s u rg e d i v e r t e r ( b u tn o t a fus e ) s h u n t i n g t h e p r i m a ry wi n d i n g( o r t h e r e l ay ) ;c ) R e s i s t a n c e e a r t h i n g .M e t h o d s ( a ) a n d ( c ) i n v o l v e t h e a u t o m a t i c d i s co n n ec t i o n o f t h e i n d i v i d u a l f au l t c i r c u i t .W i t h m e t h o d ( b ) , a n a l a r m c a n b e a r r a n g e dt o b e o p e r a t e d f r o m t h e s e c o n d a r y o f t h e ' v o l t a g et r an s fo rm e r an d t h e s ch e m e en ab l es a l l au x i l i a r i e st o b e k ep t i n s e rv i ce u n t i l i t i s co n v en i en t t o m ak e

t h e a u x i l i a r y s w i t c h b o a r d d e a d .M et h o d ( a ) is n o r m a l l y u s ed i n p o w er s t a t i o n sw i t h sm a l l e r g e n e r a t i n g s e ts a n d m e t h o d ( c) u s e di n t h e l a rg e r p o w er s t a t i o n s . M et h o d (b ) h as ce r t a i n d i s a d v a n t ag es , s u ch a s t h e co m p l i ca t i o n ina r r an g i n g fo r s p eed y i d en t i f i ca t i o n o f t h e i n d i v i du al fau l ty c i rcu i t an d the poss ib le d i f fi cu lt iesa r i s i n g f ro m fu n c t i o n i n g o f t h e s u rg e d i v e r t e r .

2 0. 4 E q u i p m e n t E a r t h i n g a t P o w e r S t a t i o n s P r a c t i c e i n e q u i p m e n t e a r t h i n g a t p o w e r st a t io n si s i d e n t i ca l t o t h a t fo r l a rg e s u b s t a t i o n s n o t g i v i n gex t e rn a l l o w v o l t ag e s u p p l ie s ( see 20.2 ) . Aco m m o n e a r t h i s u s ed fo r t h e n e u t r a l ea r t h i n g o fg en e ra t o r s an d p o wer s t a t i o n au x i l i a r i e s , an d fo ra ll e q u i p m e n t f r a m e w o r k , c l a d d i n g , p o w e r c a b l e ss h e a t h s a n d e x t r a n e o u s m e t a l w o r k n o t a s s o c i a te dw i t h t h e p o w e r s y s te m s , o t h e r t h a n t h e p e r i m e t e rfence ( see 2 0 . 6 . 1 . ) .2 0. 5 P o w e r S t a t i o n a n d S u b s t a t i o n E a r t hE l e c t r o d e s

2 0 . 5 . 1 General T h e r e q u i r e d c h a r a c t e r i s t i c so f e a r t h e l e c t r o d e s y s t e m a r e :a ) a s u i t ab l y l o w res i s t an ce , u n d e r a l l v a r i a t i o n s

d u e t o c l i m a t i c co n d i t i o n s , for t h e f a u l tc u r r e n t s e n v i s a g e d ;b ) cu r ren t ca r ry i n g cap ab i l i t y fo r a l l cu r ren t sa n d d u r a t i o n s t h a t m a y a r i s e i n n o r m a lo p era t i n g co n d i t i o n s o r d u r i n g f au l t o r s u rg ed i s c h a r g e c o n d i t i o n s , w i t h o u t u n d u e i n c reas e i n r e s i s t an ce ;c) su i tab le locat ion in the v ic in i ty of any l igh t i n g d i s ch a rg e d ev i ces s u ch t h a t ea r t h co n n ec t i o n co n d u c t o r s f ro m s u c h d ev i ces a re a ss h o r t an d s t r a i g h t a s p o s s i b l e t o m i n i m i zes u r ge i m p e d a n c e ; a n dd ) ea r t h e l ec t ro d e i n s t a l l a t i o n s s h o u l d b e d u ra

b l e an d o f s u ch m a t e r i a l an d d es i g n t oa v o i d c o r r o s i o n s .

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IS : 3043 1987For high voltage system earthing, the value ofthe resistance of the earth electrode system, withany adve ntit ious earths du e to the bon ding ofmetalwork, etc, in contact with earth, should besuch th at th e rise in po tential of the electrod esystem above the potential of remote earth is aslow as economically possible. In the absence of

any specific restriction, attempt should be madeto restrict the rise of pote ntial with in safe va lue .At some sites; the rise in earth potential will inevitably exceed these values, an d special pre cautions are necessary.Where the soil of a site is hostile by virtue ofalkalinity or acidity i t ma y be necessary to em bedearth electrodes in rammed neutral soil to avoidcorrosion.Ea rth electrode systems can also representsome haz ard to adjacent und ergro und services orstructural steelwork through electrolytic action

between dissimilar metals ( see 23 ) . W here thisdanger cannot be avoided by selection of compatible metals, the adoption of catho dic protection orother remedical action may be necessary.At power stations and substations the steelinforcement in foundations and piles can be usedto provid e an effective electro de system, w ithou tnecessity to provide further buried electrodes.Where piles are used they should be bonded bywelding and connected to ear th bond ing bars atleast four points.W here no substantial adv entit ious earths exist

or wh ere they are in ade qu ate, i t is necessaryto install electrodes ( see 9.1 , 9-2 a nd 12.1.1 ) .All cladd ing or steel work at a station shouldbe bond ed to the earth ing system a s should allstructural steel work, but attention is drawn toprecautions against undue reliance on the latter asan electrode.20.5.2 Choice and Design Where elect rodesof large surface area are necessary to prov ide therequisite current carrying capacity, earth plates arerecom me nded . Th ese are generally of cast-iron,not less th an 12*5 m m thick , and ar e usually 1'22 m

by T22 m. As an a lternative to plates, cast ironpipes may be installed. These are, for example,about 100 mm in diam eter and 3 m long, but areno t generally as cost-effective as p lates for equivalent surface area.For lower current rat ing requiremen ts , drivenrods are preferred, usually, of die copper-clad steeltype. Th ey are generally driven in group s, preferably with a spacing of not less than their length,although this is not always achieva ble. Closerspacing reduces their effectiveness. T he use of driven rods is adv antag eous wh ere th e de eper stratasof a site have a lower resistivity than the upp er

stratas but they ma y not be suitable if the site isstony or has a rock sub-strata.

At large substation com pou nds, i t is usual to laya mesh of underground earth strips to which systemneutral terminals and the ear th bonding conductors from abov e-groun d structures are co nnec ted.In addi t ion to providing an approximately equi-potential surface over the substation, the earthstrip mesh frequ ently suffices to pro vide an electrod e of suitable resistance and curren t carry ingcapaci ty wi thout augmentat ion.2 0.6 E a r t h i n g C o n d u c t o r s f o r P o w e r S t a t io n sa n d S u b s t a t i o n s

20.6.1 Disposition It is necessary to providepermanent and substant ial connect ions between al lequipment and the earth electrodes so as to afforda low resistance pa th for fault c urren ts bo th toear th and between i tems of equipm ent . In add ition, all oth er me tal p lant in or abo ut the stationshould be connected to th e ma in s tat ion ear thingsystem. The most efficient disposition of earthingcondu ctors requ ired w ill depen d o n the layout ofequ ipm ent and t he following ma y be taken as aguide:a) Indoor Equipment A m ain ear th bar shouldbe provided an d c onnected to the f ramework of eac h item and to the ear th-electrodes. Except for the smallest installations,there should be a connect ion to the ear thelectrodes at each end of the earth bar or,if this is in the form of a ring, at severalpoints on the ring. Th ese Connections ma y,depending on the layout be bur ied cablesof a size ade qu ate for th e sh ort-circuitcurre nt. W he re the structure of a switch

boa rd is extensive or occupies m ore th anone floor, a further parallel main earth barmay be requ ired which should be cross-connected to i t s companion bar at one pointat least in each section of the switchboard.The main ear thbar should be so placedthat cable sheaths can be readi ly connectedto i t. W he n cables are so con nec ted, thebond s should be m ade to the cable glandon which the lead sheath should be plumbed and the armour ing c lamped. Th e mainea rth bar should be accessible for theconnec t ion of any detach able ear thingdevices provided with the switchgear .Branch connections from the main earthba r should be prov ided to all accessoryequ ipm ent, such as control an d relaypanels, constructional steelwork and fire-ext inguishing equipment .

Where busbar protection is effected atswitchboards by f rame leakage, two mainear th bars are required . T he f rame barinterconnecting the framework of the switchuni ts wi ll be connected to the t rue ear thbar through a curren t t ransformer andbolted l inks for test pu rposes. Th e true earthbar should be run separately from the frameea rth ba r in conve nient position for the

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IS t 3043 . 1987connection of cable sheaths and earthingdevices. W here it is mou nted on the switchunits, it should be insulated therefrom byinsulation capab le of with standin g a testvoltage of 4 kV rms alternating currentfor 1 min utes.

W her e insulated cable glands are used,i t is recomm ended that ' is land ' insulationshould be provide d to facilitate testing.b) Outdoor Equipment ( Excluding Pole MountedTransformers ) A m ain earth bar shouldbe provided, so disposed as to allow of theshortest subsidiary connections to all majorequipment, such as transformers or circuitbreakers. Wherever possible, this should bearrang ed to form a ring roun d the s ta tion.Th e main earth bar ( or rin g) should beconn ected wh ere re quire d to earth electrod es. For larger stations, the ring shouldbe reinforced by one or more cross-connections.

From the main earth bar, branch connections should be taken to each item ofapp arat us and wh ere several such items lietogeth er, a , subsidiary ring with shortbran ches is preferable to a num ber oflonger individual branches from the mainbar . T he aim should be to p rovide a meshsystem wh erever this can be contrived withreasonable economy.The operating mechanisms for outdoorairbreak switch disconnectors and earthswitches and circuit breaker control kiosks,etc, not integral with the circuit breakershould be conne cted to the ma in earthgrid by a bra nc h earth connection entirelysepar ate from th at employed for earthin gthe air-break switch-disconnector or earthswitch base, or the circuit-breaker structure . T he further contribution to safetygiven by an insulated insert in the m echanism drive is small com pared with tha tobta ined from such a bran ch eart h connection and, therefore, insulated inserts are not

recommended in operating mechanisms ofapp ara tus installed in substations. W hile sitescovered with hard core and stone chippingswill constitute a surface layer with a relat ively hig h specific resistanc e, in .the int ere stsof safety, a m etal grid can be provid ed atthe operating points to give a level standingarea and an earth connection ma de fromthis grid to the opera ting h andl e .Where i t can be proved that the current carrying capacity of a m ain alum iniumor steel m em ber or welded sections forming

a struc ture are at least equal to tha t of therequired aluminium or copper earth conductor, the s tructure may form part of the

tur e m ad e up of bolte d sections should nbe relied upon to form an efficient e arbond between equipment and the maearth grid, and loops bonding acrostructural joints are required.Con nections to m etal c ladding , ste

structure and metal door frames and wdows or any other metallic panels should made inside buildings.W here the earth wire of an incomiline ends at the term inal supports and not connected to a point on the substatistructure s, a subsidiary ear th connectishould be provided between the substatiearth system an d the base of the suppoIf the latter lies outside the sub-statiofence, the earth connection should be buriwh ere it passes und er the fence and shou

be kept well clear of the latter.Ea rth connections to surge diverteshould be of sample cross-section and direct as possible; they should not pathroug h iron pipes which would increathe im pedan ce to surges of the connectioTh e earth connections of the diverteshould be interconnected w ith the maearthing system since, for the effectivprotection of the substation equipment, definite connectio n of low imp edan cbptween the equipment and the diverters essential.20.6.2 Design

20.6.2.0 General Th e term earthing gapplies only to that part of the grid which is buriein soil. For design calculations of the grid resistancto th e soil, only th e buried par t of the grid is tbe taken into account. That part of the grid whiclies em bedde d in concrete an d also reinforcemenconnected to the groun ding pads do lower thcombined grid resistance but this contributiomay not be taken into account while designing thearthing grid.20.6.2.1 Conductors installed above ground Ea rth ing conductors for power stations and substation s will norm ally be selected from copper oaluminium or steel sections adequately rated in sizto carry the designed ea rth fault or three p hase faucurrent for the appropriate designed maximumduration without exceeding a temperature given iTa ble 6A. Com pliance with this requirem ent wiladditionally ensure satisfactory bonding withouexcessive voltage difference along any conductor.

T he required cross-sectional area of the earth ingcondu ctor is determ ined by the choice of conductor materia l and the maximum duration of thfault current. The generally accepted duration fo

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IS : 3043 198720.6.2.2 Conductors buried as strip electrodes The earthing grid consists of the vertical pipe electrodes or plate electrodes interconnec ted by horizontal conductors which serve as a s tr ip electrode( 9.2.3 ) in add ition to forming a earth ing grid. Itis recom mend ed tha t the duration of earth faultcurrent should be taken as one second for 230 and400 kV substations, and 3 seconds while desi gnin g

earth grids for all other voltage levels.The other factors which shall be taken as theconsideration while designing the earth grid aregiven below:a) F act or of safety for th e ab ility of the ea rthconductor to carry the fault c urrent durin gthe perio d the fault persists, with out an ytherm al and mechanical dam age to th econductor;b) T he relat ive importan ce of the instal la t ionfor which th e ea rthing system is bein gdesigned;c) The likely increase in the near future in thefault level in the are a whe re the earth conductor has been instal led;d) Operating t ime of the protective devices;e) Corrosion of the earth cond uctor;f) Factor of safety for w orkm ans hip in joi nt ing, e tc; andg) M axim um permissible tem peratu re raise forthe buried part of the grid, which may betake n as 450G for copper an d steel cond u

ctors.20.6.2.3 Sizing

a) Th e cross-section of the ar ea of the gridconductor shall not be less than the varuestipulated in 12.2.2.1 where the value of k isto be taken as 80 for steel. This is based ona reasonable assumption that 3 secondsduratio n could not be adeq uate to bakeout the grou nd moisture around th e electrode especially as only a part of the currentwou ld be flowing across electrode-so il interface,b) K t is a coefficient wh ich takes int o ac cou ntthe effect of number n spacing D, diameterd and depth of burial h of th e grid conductors .

Kt = 12-K L Ds16hd + 1 In(4-) (v) (4-)up to n-2 terms

c) K e is a coefficient wh ich is similar to A\dependent on the mesh width and the num

ber of parallel conductors given by the em-phirical re lat ionship.1 1*.' -1 i-L-

1-4- +

3DD+h ^ 2D

up to n te rms+

( All lengths in metres )d ) K\ is an irregularity factor to allow for non-uniformity of ground, dependent on thenum ber of paral le l conductors in the groundused in the mesh.

K { = 0-65 + 0-172 x nw h e re n = num ber of paral le l conductors .

e) Mesh potential : Mesh potential is the potential difference in voltsfrom grid cond uctor toground surface at centre ofmesh gr id .Mesh E = K t K1 P -L-

where/ = fault current in amp eres, andL =s Length of buried conductor.

f) The duration of fault for calculation ofstep, touch and mesh po tential shall bethe actual breaker fault c learing t ime.20.6.3 Construction

20.6.3.1 General It is essential for th esafety of personnel and plant that an earth systemshould rem ain effective throu gho ut the life of theplant. It is difficult in many cases to make a checkof continuity after installation . T he system, the refore, has to be robust and protected from m ech anical damage and corrosion, where necessary.Any joints should be capable of retaining low resistance after many passages of fault current.20.6.3.2 Laving conductors Buried barecopper or steel conductors forming part of the earthing system should be at a bout 600 mm dee p

which, in addit ion to giving protection to the conducto r a nd conne ctions, should ensure th at it willnorm ally be below frost l ine. Alum inimum shouldonly be used for above ground connections.N O T E If th e indig eneo us soil is hosti le to copper,that is , acidic with a pH value of less than 6 or alkalinewith a^ H value of more than 10, suitable surroundingsoi l should be imported.

W here an adequa te earthing instal la t ion is provided, the subsidiary connections from the mainearth grid to equipm ent may be la id at a depthand by routes most appropriate to site connections.For convenience in connecting to equipment, theymay be la id at a dep th of about 250 mm , an d

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1 8 : 3 0 4 3 - 1 9 8 7as they a re, therefore , in g roun d mo re subject toseason al or p rogressiv e ch an ge s of resistivity, itmay be assumed th at they mak e negl igible contribution towards reducing station earth resistance.On the other hand, they do serve to reduce surfacegradient within the station site. Conversely wherethese co nnec tion are also requ ired to improve theear th value of the s tat ion, tbe 600 mm depth isrequired. The above recommendat ions deal mainlywith stations on no rm al sites. W he re grou nd conditions restrict the installation depth or where thesoil resistivity is excessive, additional measuresmay be required beyond the s tat ion boundary toimprove the overal l ear th value.

T he ear thing ins tal lation wi thin the s tationwill , however, bond the station plant and restricttouch potentials to acceptable l imits.W here bare me tal cond uctor is bur ied unde rme tal fencing, an d the fencing is inde pen den tly

earthed, the conductor should be insulated bythreading through non metal l ic pipe extending forat least 2 m each side of the fence or alternativelyinsulated conductor may be used.When laying s t randed conductor for ear thingpurpo ses, care should be take n to avoid birdca gingof the s t rands .

20.6.3.3 Fixing conductors In fixing alum iniu m or copp er cond uctors to structures, etc,insulated elips should be used to avoid dril l ing andprevent electrolytic action. Galvanized clips shouldnot be used. Fixing should be spaced not mo rethan 1 m apar t .Ea r th cond uctors in t renches containing poweran d/o r mu lti-core cables should be fixed to thewalls ne ar th e top ( for exa mp le, 100 m m fromthe top ) .

Co ppe r e arth strip supported from or in contact wi th galvanized s teel should be t inned to prevent elect rolyt ic act ion.Sha rp b ends required in aluminium s t r ip shouldbe formed by the use of a bending machine.Aluminium earthing conductors will give satis

factory performance in contact wi th concrete,cement , plas ter and br ickwork, and may be bur iedin co ncrete or p laster, p rovided it rem ains dryafter sett ing. In outdoor installations, the conductor wil l weathe r to a grey appe arance an d inmarine or industrial atmospheres slight surface pitt in g m ay o ccur. Th is will not affect perform ancesince the sections are relatively larg e. T h e interfaces of all 'mechanical ' joints should be protectedwith a suitable electrical join t com pou nd, pa rticularly any bimetall ic joints. All bimetall ic jointsshould then be encapsulated in a grease impregnated tape, mast ic compound or bi tumast ic paint ,e tc , to exclude moisture.In gen eral , aluminium should only be used

ctrodes m ad e above grou nd with bim etall ic joints.Alu min ium ca n be used below gro un d only if efficiently protected or sheathed against contact withsoil and moisture.20.6.3.4 Jointing conductors

a) General All crossings of conductors in themain ea r th gr id should be jointed. Com pression type joints may be used for stranded conductors. Non-conductor strip shouldbe dril led for a bolt having a diam etergrea ter th an one -third of the wid th of thestrip. If this diameter will be exceeded, thana wider flag should be jointed to the strip.b) Aluminium to aluminium W hen possible,joints on strip cond uctor should be arc welded using either the tun gsten ine rt-gas arc( T IC ) or metal iner t gas arc ( M IG )tech niqu es. Oxy -acetylene gas welding orbrazing may also be used.Ra ng es of com pression fittings an d toolsare avai lable for round conductors . Ro undcond uctors can also be flattened andpunched with suitable tools to form a terminal .

Ro und and rectangular conductors canbe joined with bol ted clamps.Rec tangular conductors can be joinedor termina ted by dril l ing and bolting.W hen mak ing a bol ted type joint , thesurface of the aluminium should be cleaned

thoroug hly by wire brushing and greasedor an approved joint ing compo und appl iedimm ediately to both mat ing surfaces .Bolts should then be t ightene d an d allexcess grease or com poun d wipe d off anddiscarded.To ensure adequate contact pressureand avoid overstressing, torque spannersshould be used. Th e conductor manufacturer s l i terature should be consulted forfurther details for the joints and procedures.Cold pressure welding and explosivebon ding can be used for jointing rec tangular conductors . T he approp r iate ma nufacture r sho uld be con sulted for details ofthese procedures .c) Aluminium to copper Join ts between aluminium and copper should be of the boltedtype an d be installed in the v ertical pla neat a min imu m d is tance of 150 mm aboveground level.The rat ing surface of the aluminiumshould be cleaned thoroughly by wire brushing and greased or an approved joint ing

comp ound appl ied and the copper t inned.Grease or an approved joint ing compo und

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IS t 3043 - 1987the aluminium . After bolt t igh tenin g bytorque spanner, excess grease or compound should be wiped off an d discarded ,and th e join t protected from the ingreaseof moisture by the application of suitableplastics compound or irradiated polyethylene sleeve with m astic l ining. A lternatively,the joint ma y be protected by a bitum asticpaint .

Aluminium conductor connect ions toequip me nt should, whe re possible, be in thevertical plane . Surface pre para tion of thealuminium a nd the making of the jointshould be as previously described. T h efinished joint should be prote cted by abi tumast ic paint .d) Copper to copper Th e following me thodsmay be used:1) Brazing using zinc-free brazing materialwith a melting point of at least 600G;2) Bolting;3) Rivet ing and sweat ing; and4) Explosive welding.Earthing conductor connections to equipmen t should, as far as prac ticab le, bemade onto vertical surfaces only. In thecase of painted metal, the paint should becarefully removed. Earthing conductorsshould be t inne d wh ere conn ected togalvanized steelwork. No connection pointshould be less tha n 150 mm above grou ndlevel. In any position, subject to corro sion,the finished joint should be protected by

bitumastic paint.e) Loop s for portable earths Loops of plainaluminium or copp er should be p rovidedon the earth con ductor at each loc ationwhere por table ear thing leads may be applied. The loops should not be less than 180mm long and 75 mm clear of the ear th conductor; they should be at a convenientheight and should be formed separately,not by bending the earth str ip itself. Loopsshould be jointed to the ear th conductorusing a method given in 20.6.8.4 ( d ) .f) Steel For steel, i t is reco mm end ed to useonly welded joints .2 0 . 7 E a r t h i n g o f H i g h V o l t a g e C a b l e S h e a t h s

20.7.1 Three-Core Cables Modern high vol tagepower cables are generally provided with a polyme ric insulating oversh eaths. T he sheath of solidtpye cables ar e generally directly earthe d at theirterm ination s and joints , the cable sheaths beingbonded at joints . T he sheath ear t h connect ionsof pressure type cables are generally m ad e via aremova ble l ink in 'a lockable box to perm itperiodic testing of the overshea th insulation, th ejoints being insulated, but the sheaths bon dedthrough. Th e tes t requirement also me ans tha tinsulating g lands should be provided at the cable

termination boxes of transformers, switchgear, etcan d at cable sealing ends or joint s .20.7.2 Single-Core Cable Tails Th e shea ths o fsingle-core cables hav e a lon gitudinal induc edvol tage, the m agn i tude of which is di rect ly pro portional to the current f lowing in the core. Whenbot h ends of a single-core cab le are bonded toear th , a current f lows in the sheath and the thermal effects of this sheath current derates thecapaci ty of the cable core. W here this derat ing isunacceptable and the value of the s tandinginduce d voltage is acceptab le, i t is usual t oea rth th e shea ths of the single-core cables at thetrifurcating box o r in th e case of single-core m ains ,the end of the trefoil formation, the cable glands atsealing ends or plant cable boxes being of the insulated type. Th e acceptable level of the max imumsheath voltage is generally taken as 65V with fullrated current f lowing in the cable, but where theratio of fault curre nt to full rate d curre nt isso high th at the voltage developed across a n insu

lated g land is una cce ptab le, i t is necessary to de rate the permissible vo ltage to some level lowerthan 65 V.20.7.3 Single-Core Cable Mains The choice ofterminat ion and ear thing ar rangements for s ingle-core cable m ains is a mat te r of economics . T h epossible methods of earthing are as follows:a) Solid Bonding In this system, the sheathbond ing and ear thing ar rangem ents aresuch that the sheaths are maintaine d nearear th potent ial throughout thei r length.b) Single Point Bonding This method is asdescribed in 20.7.2 for single core tails, and

is subjected to practical l imitations of cablelengths permissible.c) Cross-Bonding In this meth od, the cableleng th is divided in to thre e equa l sections( or into a multiple of three such sections )and at each section junct ion , a n insulat ingjoint is provided. At these joints , the sheathof each cable section is bonded to the sheath of a different phase cable of the nextsection through lockable l ink boxes. By suitable connection, the phaser sum of thelongitu dinal shea th voltage is zero, and a tthe cable terminations, the sheaths of allthree cables are bonded to ear th . I t i s usualto provid e a three-ph ase star-connected setof cable protections at each intermediate insulating join t; these protectors are non -linearresistors presenting low impedance to surgecurrents . The cross-bonding method permitsthe full ra ting of the cable to be ma intai ned ,but incurs considerable cost in the provisionof insulating join ts , l ink boxes, protectors,etc.

2 0 .8 M i s c e l l a n e o u s M a t t e r s i n P o w e rS t a t i o n s a n d S u b s t a t i o n s If two or mo restations are adjacen t o n w hat m ay be con sideredto be one site, the earthing systems and the stationsshould be interconnected to form a single earthing

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I S : 3 0 4 3 - 1 9 8 7s y s t e m . W h e r e t h e s t a t io n s a c t u a l l y a d j o i n , t h ee x t r e m i t i e s o f t he i r e a r th in g sys t e m s sho u ld bec o n n e c t e d t o g e t h e r s o t h a t t h e w h o l e a r e a is e n c l o s e d b y t h e e a r t h i n g s y s t e m . W h e r e t h e s e p a r a t i o n i s t oo l a r g e t o t r e a t a s a d jo in ing s t a t i on s , a ni n t e r c o n n e c t i n g e a r t h c o n d u c t o r o f s u b s t a n t i a lc r oss - se ct i on sho u ld b e r un t o e n sur e t h a t , a s f a ra s p r a c t i c a b l e , f a u l t c u r r e n t s a r e d ive r t e d f r omc a b l e s h e a t h s a n d a r m o u r . T h i s i s o f p a r t i c u l a rim por a tnc e w he r e f a u l t c u r r e n t f l ow ing in ones t a t i on i s p r o v id e d f r om the a d jo in ing s t a t i on , f o re x a m p l e , w h e r e a s w i t c h i n g s t a t i o n a d j o i n s p o w e ror t r a ns f o r m ing s t a t i o n si t es so t h a t a n e a r th f a u l ti n t h e sw i t c hge a r c a use s c ur r e n t f l ow be tw e e n th etw o s i t e s i n o r d e r t o r e a c h t he sys t e m n e u t r a l a tt h e g e n e r a t o r s o r t r a n s f o r m e r s . S u c h i n t e r c o n n e c t i on s be tw e e n s i te s c a n i n c lud e l i nks su i t a b ly d i s pose d t o a s s i s t i n t e s t i ng .

E x c e p t w he r e spe c i a l i nsu l a t i on is c a l l e d f o r,sh e a t hs o f a ll m a in c a b l e s shou ld be c on ne c t e d t ot h e s t a t i o n e a r t h s y s t e m . W i t h m u l t i - c o r e c a b l e st h e c o n n e c t i o n i s g e n e r a l l y m a d e a t t h e t e r m i n a t i o n .

W h e r e h i g h e a r t h - f a u l t c u r r e n t s a r e t o b e e x p e c t e d , a n d a n a p p r e c i a b l e r i s e o f p o t e n t i a l o f t h es t a t i o n sys t e m w i th r e spe c t t o t he g e ne r a l b ody o fth e e a r th m a y e n sur e , spe c i a l c a r e i s ne c e ssa r yw i t h c o n n e c t i o n s o t h e r t h a n m a i n c a b l e s o r l in e se n t e r i n g t h e s t a t i o n , s u c h a s w a t e r p i p e s a n d t e l e p h o n e o r p i lo t c a b l e s , w a t e r p i p e s s h o u l d i n c l u d ea n i nsu l a t e d se c t i on ; po lym e r i c p ip ing i s o f t e nsu i t a b l e . I n se ve r a l c a se s , i so l a t i ng t r a ns f o r m e r sm a y b e n e c e s s a r y f or t e l e p h o n e c o n n e c t i o n s . B r i t i s h T e l e c o m p r o v i d e s i s o l a t io n e q u i p m e n t a t t h e i rc i r c u i t t e r m i n a t i o n s w h e n t h e p o t e n t i a l r i s e e x c e e d s43 0 V ( 650 V f o r h ig h r e l i a b i l i t y l i ne s ) . P i l o tc a b l e s s h o u l d b e p r o v i d e d w i t h i n s u l a t e d g l a n d sa n d so d i spose d a s t o m in im iz e t he poss ib i l i ty o ff a u t t t c u r r e n t s be ing c a r r i e d by t he she a ths .

W h e r e c a r r i e r -c u r r e n t e q u i p m e n t is e m p l o y e d ,a f u r t h e r e a r t h - e l e c t r o d e , n o r m a l l y a d r i v e n r o d ,s h o u l d b e p r o v i d e d a t o r i m m e d i a t e l y a d j a c e n t t oe a c h s t r u c t u r e s u p p o r t i n g t h e c o u p l i n g c a p a c i t o r s .T h i s e a r t h e l e c t r o d e i s a n a d d i t i o n a l o n e fo r t h eh i g h f re q u e n cy e q u i p m e n t a n d s h o u l d b e b o n d e di n t o t h e m a i n e a r t h i n g s y s t em . T h e s t r u c t u re ss u p p o r t i n g t h e c o u p l i n g c a p a c i t o r s s h o u l d b e e a r t h e d i n t h e n o r m a l w a y .2 1 . E A R T H I N G A S S O C I A T E D W I T HO V E R H E A D P O W E R L I N E S2 1 . 1 T y p e o f S u p p o r t A n y c o n s id e r a ti o n o f" w h e t he r m e t a l w o r k a s s o c ia t e d w i t h o v e r h e a dp o w e r l i n es s h o u l d b e e a r t h e d a n d / o r b o n d e d h a st o t a k e a c c o u n t o f t h e t y p e of s u p p o r t . S o m e o v e r h e a d l i n e s a r e s u p p o r t e d b y l a t t i c a t o w e r s o f m e t a l l i c c o n s t r u c t i o n , o t h e r s b y p o l e s , w h i c h m a y b eof s t e e l , w o od , c on c r e t e o r o f f a b r i c a t e d c ons t r u c t i o n , f o r e x a m p l e , g l a ss - re i n fo r c ed p l a s t ic s ;b r a c k e t s a t t a c h e d t o b u i l d i n g s a r e al s o u s e d t o

2 1 .2 I n s u l a t i o n F a i l u r e Fol low ing a n i nsu l a t i on f a i l u r e , a vo l t a ge m a y e x i s t be tw e e n a ny supp o r t i n g m e t a l w o r k a n d e a r t h . T h e p u b l ic a r ege ne r a l l y p r o t e c t e d i f no m e ta lw o r k w i th in 3 m ofth e g r o un d i s l i a b l e t o be c o m e l i ve on f a il u r e o finsu l a t i on . I f t he sup por t s a r e c lose t o bu i ld ings ,e t c , t h e pa r t i c u l a r c i r c um s ta nc e s ha ve t o bec o n s i d e r e d .2 1 .3 L a t t i c e S t e e l S t r u c t u r e s T h e r e w il lof ten be sa t i s fac tory e ar th in g of la t t ice s tee l s t ructu r e s , po l e s o f m e ta l l i c c ons t r uc t i on a nd r e in f o r c e dc on c r e t e po l e s t h r ou gh the i r c on t a c t w i th t h egr ou nd . I n a r e a s o f h igh e a r th r e s is t iv i t y , spe c ia le a r t h i n g a r r a n g e m e n t s m a y b e n e c e ss a r y; a n o v e rh e a d p r o t e c t i v e c o n d u c t o r a t t a c h e d a t e a c h s u p p o r ta nd c onne c t e d t o t he ne u t r a l o f t he supp ly a nd o fthe l i ne m a y be t he m os t e c onom ic a l so lu t i on .T h i s c on du c to r if po s i t i one d a bo ve t he l i ve c ondu c to r s , w i ll a lso p r o v ide a m e a su r e o f li gh tn ingp r o t e c t i o n .2 1 . 4 P o l e s o f N o n - c o n d u c t i n g M a t e r i a l

21 .4 .1 General W he r e a po l e i s o f non- c ondu c t in g m a te r i a l , f or e xa m ple w ood o r g l a s s - re in forced plas t ics , the pole wi l l ac t aga ins t the f lowo f l e a k a g e c u r r e n t a n d c a n b e e x p e c t e d t o p r e v e n tda ng e r ne a r g r o un d le ve l du e t o l e a ka g e a c r oss o rf a i l u r e o f a ny i nsu l a to r suppor t i ng a l i ne c onduc to r ,e x c e p t w h e r e t h e r e is i n t e r v e n i n g e q u i p m e n t o rm e t a l w o r k t h a t is o r m a y b e c o m e l i v e .For t he r e a sons g ive n i n 2 1 . 4 . 2 t o 2 1 . 4 . 5 , t h e r ea r e a d v a n t a g e s in n o t e a r t h i n g t h e p o l e - t o p m e t a l w o r k o f s u c h p o l e s a n d i n n o t m a k i n g b o n d i n gc o n n e c t i o n s t o i t .

2 1 . 4 . 2 Omission of Bonding W h e r e i n s u l a to r sa r e a t t a c he d t o a po l e o r t o non - c ond uc t ing c r oss -a r m s , e t c , a t t a c h e d t o t he po l e , om m iss ion o fbo nd ing o f po l e - to p m e ta lw or k g ive s a g r e a t e rim pu l se w i ths t a n d vo l t a ge , so t h e r e i s l es s r isk o ff a u l ts du e t o pha se - to - p ha se f l a shove r . T o r e duc er isk of fi re , wh er e w oo de n c ross-a rms a re used,c a r e shou ld be t a ke n t o m a ke c lose , f i r e c on t a c tb e t w e e n t h e c r o s s - a r m a n d t h e i n s u l a t o r p i p e .2 1 . 4 . 3 Omission of Earthing I f po l e t op m e ta l w o r k i s no t e a r th e d , t r a ns i e n t f a u lt s due t o b i r ds ,

f ly ing b r a n c he s , e t c , b r i d g in g t h e c l e a r a n c eb e t