Is 3043 Revision 1 Earthing

IS : 3043 - 1987

( Reaffirmed (R wmmd l o2001 ee) )

Indian Standard

CODEOFPRACTICEFOREARTHING (First Revision) Second Reprint FEBRUARY 1998

UDC

621.316.99

: 006.76

(Q Copyright 1988 BUREAU MANAK

OF BHAVAN,

INDIAN 9 BAHADUR

NEW

Price Rs 225.00

DELHI

STANDARDS SH AH 110002

ZA AR

MARG

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Indian Standard

CODE COD E OF PRACTICE PRACTICE FOR EARTHING irst irst Electrical

Installations

M-3 batyam, Mlmbsr~s

ision Sectional

Committee,

CtiTlll4ll SHBI M. L. DONQBE 88 Sian Circle,

Bombay

ETDC

400022 Rafi8s0iting

Engineer-in-Chief’s Engineer-in-Chief’s Branch, Defence ), New Delhi

SHBI P. ANANTHABAM~

20

Army

Headquar ter8

( Ministry

SHRI S. K. SH AI~~ABI ( Alfrrnetr Alfrrnetr Tata Consulting Consulting Engineers, Bombay P. D. BAQADE BAQADE SFIRI R. K. KAUL KAUL ( Alfrr Alfrr netr Siemens India Ltd, Bombay Bombay SHR I V. S.BHATIA SEEI M. M. SEETHnA ( Alturnutr Federation of Electricity Undertaking of India, Bombay SH BI K. V. CHAUBA SHEI K. S. JO SHI ( Alfrnatr ) Larsen 8 Toubro ( Construction Group ), Madr as SHB1R.R. CJKOUDHUBI SHR I N. BALA BALASUBB SUBBAY AYAX AX~I ~I AN ( Alrraur~ Railway Board ( Minirtr y of Railways ), New Delhi C~xxx ELIZCTEIOALEN~IHEEB ELIZCTEIOALEN~IHEEB DEPUTY DIBECTOB STADABDS STADABDS ( ELEO ELEO )-DI. RDSO ( Ahnat* ofTami1 Nadu, Madras Chief Electrical Inspector to Government CHIEF ELECTRICAI, IN~PEOTOB TO GO-WENT ox TAMIL NADU ELICCTBIO ELICCTBIO AL INJ BPECTOB T~ OEX~I OAL TO GO~ EBNYWT or TALL NADU ( ABmralr ABmralr Central Public Works Department, New Delhi CHIEF NO~EBIU ( ELEC )-I SUPEBINTENDENT SUEVEYOB or Wosne ( E~lec )-I ( AItrnorr Larsen 8 Toubro Ltd. Bombay SHBI Dirvmrnm Dirvmrnm NATE SEBIT.P:R. hBYA(dll#n~Utr) Jyoti Ltd, Vadodara SEBI K. W. DHABYADEIICABI Da V. N. MALL~CB ( Al&a& Al&a& Corporation Ltd, New Delhi Rural Electrification SEEI G. L. Dnr SETEI(~~&TMU) SEBIS.K. Delhi Electric S upply Undertaking, New Delhi SEBI R. C. KHANXA Sam

%~BIP.~.~AWHSEY(

Affsrotr)

M~ ~YB~B ( HYDRO -ELECTBIC -ELECTBIC ) DIRECTOR ( HED )-I Allsmolr EB S. PANEERBELVAY Sam V. ANABDZAXAN ANABDZAXAN ( Ahaatr SERI K. I’. R. PILLAI ( Alfanofr SHRI C. R. R. M-ON SHRI V. RADHA KRISENAN SHSI H. S. RAO PROF G. RAVEENDRAN NAIB SHRI S. R. SARDA SARDA SERI R. SATHIYABAL SHRI K. K. MONDAL Altmatr ) SHBI H. K. SITABAM SHR I S. K. P ALIT ALIT ( Al& Al&rsutr rsutr SHBI P. SBINIVASA POTI SERI D. S. TAWAB~ SEBI S. . HABIDAS HABIDAS ( Allrr Allrr ~f~ SHRI G. N. THADAX~I

Central Tamil

Electricity Nadu

Authority,

Electricity

Fact Engineering

New Delhi

Board,

and Dedgn

Madr as Organiiation,

Udbyogamandal

Bbarat Heavy Electricala Ltd, Hyderabad Crompton Greaves Ltd, Bombay Chief Electrical Inspector to the Government of Kerala, Trivandrum Mabar asbtra State Electricity Board, Bombay Tariff Advi Advisory sory Committee Committee ( General Insura nce ), Bombay Calcutta

Electric

Supply

Kamataka

Electricity

Electrical

Engineer

Engineers

India

Ltd.

Corporat ion Corporat

Ltd, Galcutta

Board, Bangalore to Government New Delhi

of Mabar aabtra ,

Bombay

of























1s t 3943 - 1987 ( Cont inued from page 1

Panel for the Revision Revision

of IS

3043,

Convener

SHBI N. BALA~UBRAYANIAN Paos SHH I SHH I .%a~ SHRI

G. V. G. R. K.

Members RAVEENDRAN NAIR SATEYANATHAN SATEYANATHAN S. THAHUR SATHIYABAL P . R. P ILLAI ILLAI

ETDC

20

P38

Refiasenting Larsen

& Toubro

Conrtruction

Group ), Madras

Chief Electrical Electrical Inspector to the Government of Kerala, Trivandrum Tamil Nadu Electricity Board, Madr as Chief Electrical Inspector, Government of Madhya Pr adesh, Bhopal Tariff Advisory Advisory Committee, Committee, Madr as Fact Engineering and Design Or ganization, ganization, Udyogamaodal Udyogamaodal























IS t 394i$ - 1987

CONTENTS Page 0.

FOR BW~RD

..

1. SCOPE

SEClWN

J

GENERAL GENERAL GUIDELI NES

2. TERMINOLOOY 3. EXCHANO~OP

INFORMATION

..I

STATUTORYPR OVISIONSFOR EARTH~NIJ 4. STATUTORYPROVISIONSFOR EARTHED 5. FACTORS INFLUENCING THE CHOICE OF EARTHED SYSTEMS 6. SYSTEM EARTHING 7: EQUIPMENT EARTHING

SECTION 2

AND UNEARTHED .I.

.

. .

9. EARTH ELECTRODE CURRENTDENSITYATTHESURFACE CE 10. CURRENTDENSITYATTHESURFA

. . OF AN EARTH

ARRANQE~~~NT FORPROTECTWE 13. EARTHING ARRANQE~~~NTFORPROTECTWE

.W. AND PROTECTIVE .

AND ._

ON CONSUMER’S CONSUMER’S

EARTHFAULTPROTECTION IN INSTALLATION 18. EARTHFAULTPROTECTION 19. SELECT~~NOFDE~~CE~F~RA~T SELECT~~NOFDE~~CE~F~RA~TOMAT~CDISCON OMAT~CDISCONNEC~ION NEC~ION

SECTION

4

34 OF

SUPPLY .

SUBSTATIONS POWER STATIONS, OVERHEAD LINES

EARTHING IN POWER STATIONS

ND

POWER LINES

39

AND 43

SUBSTATIONS

21. EARTHING A~SOCIATEUWITH A~SOCIATEUWITH OVERHEAD

32 33 33

AND PROTECTIVE CONDUCTORs

FAULT FAULT PROTEC PROTECTION TION PREMISES

27 32

EQUIPOTENTIA L BONDING CONDUCTORS 16. EQUIPOTENTIAL

EARTH

27

32

PURPOSES

ARRANGEWNTFORFUNCTIONAL PIJRPOW 14. EARTHING ARRANGEWNTFORFUNCTIONAL COMBINED PROTEOTIW3 ARRANOEMENTS FOR 15. EARTHING FUNCTIONAL PURPOSES TYP~CALSCH~MAT~COFEARTHING ING 17. TYP~CALSCH~MAT~COFEARTH

16 27

ELWTRODE

EARTHELECTRODES 11. VOLTAGE GRADIENT AROUND EARTHELECTRODES CONNECTIONSTO EARTH ELECTRODE'I- EARTHING 12. CONNECTIONSTO CONDUCTORS

3

11 15

CONNECTIONS CONNECTIONS TO EARTH

8. RESISTANCETO EARTH

SECTION SECTION

.

10

.

52























Page

SLCTION 7 24. 25.

MEDICAL ESTABL ESTABL ISHMENT

P R O T E C T I V EM EM E A S U R E ST ST H R O U G H E A R T H I N G YENT S U P P L YC YC e A n l r a T s a x s n c s

SECTION SECTION 8

A ND ND

MEDICAL E S T A B L I S F X -

ARA BTER

.. .

64

..

65

STATIC AND LIGHTNING PROTEC PROTECTION TION EARTHING

( Under consideration. consideration.

SECTION SECTION 9

Clauses 26 and

reserved for Section

MISCELLAN MISCELLAN EOUS EOUS INSTALL ATIONS A ND CONSIDERATIONS

AL L YH YH A Z A R D O U SA SA R E A S 28 E A R T R I N G I N P O T E N T I AL

.. .

29.

..

T E ~ B C O M U N I C A T I O N I R C U I T SA SA N D A P P A R A T U S

3‘0. BUILDINGS BUILDINGS I T E S 31.

..

MXNES N D Q U A R R I E

32. STRE ET LIGHTING FURNIT~RB

AID

OTHER

ELEC TRICALLY SUP

71 71

LIES STRB~.

73

33.

E A R T H I N G O F C O N D U C T O R S O R S AF AF E W O R K I N G

..

34.

MAINTENANCEO MAINTENANCEO F E A R T H E L E C T R O D E S

.. .

SECTION SECTION 10

69

74 76

MEASUREMENTS MEASUREMENTS AND CALCULATIONS

C A L C U L A T I O NO NO F E A R T H F A U L T G J R R E N T J

..

M E A S U R E M E N TO F E A R T H R B S I S T I V XT XT Y

.. .

37.

M E A S U R E M E N TO TO F E AR A R T H E L E C T R O D ER ER E S I S T A N C E

..

76 77 79

38.

M E A S U R E M E N TO F E

..

80

R T H L O O P I M P E D A NC NC E

SECTION 11 DATA PROCESSING INSTALLATIONS

40.

E A R T H I N G R E Q U I R E M E N T S O R I N S T A L L AT AT I O N SO SO F D A T A P R O C E S S I N G EQUIPMENT .. .

80

EXAMPL

83

O F U SE SE O F T R AN AN S F O R M E R S

..























IS I 3043 - 1987

Indian St andar

CODE OF PRACTICE FOR EARTHING (First Revision) 0.

FOR

0.1 his Indian Standard

( First Revisi on ) was adopted by the Bureau of I ndian Standards on 6 August 1987, after the draft fin alized by the Electrical Installations Sectional Committee, had been approved by the Electrotechnical Divis ion Council. 0.2 The Indian Electricity Rules, together with with the supplementary regulations of the State Elettricity Departments and Electricity Undertakings, govern the electrical installation work in generating stations, substations, industrial locations, buildings, buil dings, etc, in the country. To ensure safety of lif e and apparatus apparatus against earth faults, i t was felt necessary to prepare a code of practice for earthing. Thi s code of practice is intended to serve as a consolidated guide to all those who are concerned with the design, installation, inspection and maintenance of electrical systems and apparatus. 0.3 The subject of earthing covers the problems relating to conduction of electricity through earth. The terms earth earth and earthing have been used in this code irrespective of reliance being placed on the earth itself as a low impedance return path. of the fault current. As a matter of fact, the earth earth now rarely serves as a part of the return circuit but is bein g used. mainly for fixing the voltage of system neutrals. The earth connect ion improves service continuity and avoids damage damage to equipment and danger to human life. 0.4 The obj ect of an an earthing earthing system is to provide as nearly as possib le a surface under and around a station which shall b e at a uniform potential and as nearly zero or absolute earth potential as possibl e. Th e purpose of this is to ensure that, in general, all parts of apparatus apparatus other than live parts, shal l be at earth potential, as well as to

EWORD 0.5 Earthing associated with current-carrying current-carrying conductor is normally esse ntial to the security of the system and is generally generally known as system earthing, while earthing of non-current carrying metal work and conductor is essentiaC to the safety of human lif e, animals and property, and is generally known as equipment earthing. 0.6 Si nce the publi cation of this standard in 1966, considerable experience has been gained through the i mplementation of its various stipulations. Moreover, several new concepts have been i ntroduced the world over, on the understanding of functional and protective earthing with a view to take into account a variety of complex problems encountered in actual practice. In the context of increased use of electric power and th e associated need for safety in the design of install ations, it had b ecome n ecessary to prepare an overall revision of the earlier version of the Code. 0.7 In this Code, the terms ‘earthing’ ‘earthing’ and ‘grounding’ are used synonymously. However, this Code introduces several new terms ( see 2.15, 2.17, 2.28, etc ) and distin
























IS

t 3043

1987

measures. Details of such protective measures and guidance on their choice is the subj ect matter of debate in the process of revision revision of I 732*. Earth f ault/eakage protection sought to be achieved through equipotential bonding and automatic disconnection of supply is envisaged envisaged to prevent a touch voltage from persisting for such a duration that would be harmful harmful to human beings. Guidance on achieving achieving this protection protection is covered in Set 3 of the Code. 0.9.2 While detailed detailed guidelines are covered in .specific portions of the Code, the followin g shall be noted:

b)

For solidly grounded systems, it shall be suffi cient to check whether the characterischaracteristics of protective protective device for automatic automatic disconnection, earthing arrangements and relevant impedances of the circuits are properly coordinated coordinated to ensure that voltages appearing between simultaneously accessible, exposed and extraneous conductive parts are within the magnitudes that would not cause danger; For systems where the earthing is deemed deemed to be adequate, adequate, it shall be checked whether the main overcurrent overcurrent protective device is capable of meeting the requirements in the wiring code; and Where the main overcurr overcurrent ent protective device did not fulf il the requirements or where the earthing is considered inadequate, then a separate residual current device would b e necessary to be installed, the earth fault loop impedance and the tripping characteristics SO chosen that they comply with safe touch voltage limits.

0.10 The revision of the Code aims at at consolidating ‘in one volume all the essential guidelines needed for preparing a good earthing design in

0.11 For convenience convenience of identifying areas of interest by any specific users of the Code, the information contained in this standard is divided into different Sections as follows: Section 1 General guide1 nes; Section 2 Connections to earth; Section’3 Earth-fault protection in consumer’s premises; Section 4 Power stations, substations and overhead lines; Section 5 Industrial premises; Section 6 Standby and other private generating plant; Section 7 Medical establishments; Section Static and lightning protection grounding; Section 9 Miscellaneous installations and considerations; Section 10 Measurements and calculations; and Section 11 Data processing installations. 0.12 In the preparation preparation of the Code, Code, assistance has been taken from the following: IEC Pub 364 ( and Parts ) Electrical installations in buildings. International Electrotechnical Commission. BS Document 84121243 84121243 Draft standard code of practice on earthing revision of CP 1013: 1965 ). British Standards Institution. ANSI/ EEE Std 142-1982 142-1982 I EEE Recommended practice practice for grounding grounding of industrial and commercial power systems. American National S tandards tandards Institute ( USA ). 0.13 For the purpose of deciding whether a particular requirement of this standard standard is complied complied with, the final value, observ@ or calculated,























IS t 30 43 - 1 987 SECTION

GENERAL

GUIDELINES

A wiring system in which one or more insulated conductors are completely surrounded throughout their length by a conductor, conductor, for example, example, a sheath which which acts as a PEN conductor. 2 .1 .1 2 E a r t h e d

TER MINOLOGY 2. f o l lo lo w For the p u r p o s e o f t h i s s t a n d a i d , t h e fo i n g d e f in in i t i o n s s h a l l a p p l y . 2 .1 .1 Ar Ar c . S u p p r e s s i o n

Coil ( Pe ter son

Coil)

Concentr ic

iring

An earthing reactor so designed that its reactance is such that the reactive current to earth under fault conditions balances the capacitance current to earth flowing from the line s so that the earth current at the fault is li mited to practically practically zero.

2.13 E a r t h i n g C o n d u c t o r - A protective conductor connecting the main earthing terminal ( see 2.2 ) ( or the equipotential bonding conductor ofan installation when there is no earth bus to an earth electrode or to o t h e r m e a n s o earthing.

C o n d u c t o r - A protective 2.2 Bonding ductor providing equipotential bonding.

2 .1 .1 4 E l e c t r i c a l ly ly I n d e p e n d e n t E a r t h E l e c t r o d e s - Earth electrodes ele ctrodes l ocated at such a distance

con-

2.3 Class I E q u i p m e n t

- Equipment in which p r o t e c t i o n a g a i n s t e l e c t r i c shock does not rely on

basic insulation onl y, but which includes means for the connection of exposed conductive conductive parts to a protective conductor conductor in the fi xed wiring of the installation. NOTE For information on clasaificat~o ment w i t h r e g a r d t o m e a n s p r o v id id e d f or or

a g a i n s t e l e c t r i c shock

( J#IIIS J#IIIS

9409-1980.

equip-

p r o t e c ti ti o n

).

2.4 Class I I E q u i p m e n t Equipment in which protection against electric shock does not rely on basic insulation only, but in which additional safety precautions precautions such as. supplementary insulation are provided, there being no provision for the connection of exposed metalwork of the equip ment to a protective conductor, and no reli ance upon precautions precautions to be taken in the fixed wiring ot he installation. The term used to describe a device or circuit circuit to indicate indicate that a voltage voltage is not applied. applied.

2.5 Dead -

from one another that the maximum current likely lik ely to flow through one of them does not significantly af fect the potential of the other(s). other(s).

2.15 Equipoten$ial Bonding - Electrical connection putting various exposed conductive parts and extraneous extraneous conductive conductive parts at a substantially equal potential. NOTE In a building bonding conductors shall conductive parts: a) Protective b) Earth c)

installation, interconneiz t

equipotent ial the following

conductor;

continuity

conductor;

Risers of air-conditioning air-conditioning systems ( if any ).

and systems

and

heating

Condu ctive P ar t A conductive part of equipment which can b e touched touched and which is not a Jive part but which may become live under fault conditions.

2 .1 .1 6 E x p o s e d

2.6 D o u b l e f n s a l a t i o n Insulation comprising both basic and supplementary insulation.

2.17 Extraneous Condctive Part - A conductive part liabl e to transmit a potential including earth potential md n ot f orming part of the electrical installation.

2.7 E a r t h -

ir c u i t 2.18 F i n a l C ir

The conductive conductive mass of the earth,

A circuit connected direc-























2 .2 .2 3 P E N C o n d u c t o r A conductor combming the functions of both protective conductor and neutral conductor.

NO= 2 i tionsspecifically

This term intended

2.33 S u p p l e m e n t a r y

applies for livestock for these animpls.

h s a la la t i o n

-

in

loca-

Independ

2.24 P o r t a b l e E q u i p m e n t Equipment which is moved while in operation or which can easily be moved moved from one place to another while connected to the supply.

insulation appli ed in addition to basic, insulation, in order to provide protection against electric shock in the event of a failure of basic insulation.

2.25 P o t e n t i a l G r a d i e n t ( t a P o i n t ) The potential difference per unit length measured in the direction in which it is maximum NOT= - When an electric force is due t o poten-

2.34 S w i t c h g e a r - An assembly of main and auxiliary switching apparatus apparatus for operation, regulation, protection or other other control of electrical installations.

tial difference, difference,

it is equal to the potential potential

NOTE 2 -Potential per unit length.

gradient

gradient.

is expressed expressed

in volts

v e Co Co n d u c t o r 2.26 P r o t e c t i ve conductor used as a measure of protection against electric shock and intended for connecting any of the followin parts:

a) b) c) d)

Exposed conductive parts, Extrantous Extrantous conductive parts, Main earthing terminal, and Earthed point of the source or an artif icial neutral.

or c e d I n s u l a t i o n 2.27 R e i n f or Single insulation applied to live parts, which provides a degree of protection against electric shock equivalent to double insulation under the conditions specified in the relevant standard. Noble - The term ‘singl e insulation’ does not imply that the insulation has to be one homogeneous piece. It may comprise several layers that cannot be tested singly as supplementary or basic insulation.

R e s i d u a l C u r r e n t D e v i ce ce A mechani; cal switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions. 2.20

2.29 R e s id id u a l O p e r a t i n g

Carrent

-

Residual

definit ion of the N O T E - A more comprehensive term ‘Switchgear’ can be had from IS 1885 ( Part 17 )-1979..

Voltage by which an installation ( or part of an install ation ) is designated. 2 .3 .3 5 V o lt lt a g e ,

Nom ina l

lt a g e - The potential difference 2.36 T o r c h V o lt between a grounded metallic structure structure and a point on the earth’s surface separated by a distance equal to the normal maximum horizontal reach, approximately approximately one metre metre ( see Fig. 1 ).

The potential difference 2.37 Step V oltage between two points on the earth’s surface, separated by distance of one pace, that will be assumed to b e one metre in the direction of maximum potential gradient ( ste Fig. 1 ). 2.38 Equipotential Line or Contour - The locus of points having the same potential at a given time. 2.39 Mutual Resistance of Grounding Electrodes - Equal to the voltage change in one of them produced by a change of one ampere ampere of direct current in the other and is expressed in ohms. 2.40 Earth Grid - A system of grounding electrodes consisting of of inter-connected inter-connected connectors























IS:

3043 -1987

POTENTIAL RISE ABO”ErREMOTE EARTH DURING SHORT CIRCUIT

STEP

TOUCH

VOLTAGE

V.OLTAGE

FIG. 4.

STATU STATUTORY TORY EARTHING

PR OVISIO NS FOR

AT A

GROUNDED

AT A

GROUNDED

STRUCTURE,+’

STRUCTURE

STEP AND TOUCH VOLTAQES 4.3 Ear th electrode electrodess shall be be provided provided at generat generat ing stat ions, ions, substa tions a nd consum consum er premises























Is : 3043 - 1967

fault current or its duration duration until it has been ascertained that the existing arrangement arrangement of earth etc, are capable of earth bus-bar, electrodes, carrying the new value of of earth fault current which may be obtained by this addition. 4.9 No cut-out, link or switch other than a linked switch arranged to operate simultaneously on the earthed or earthed neutral conductor and the live conductors, shall be inserted on any supply system. This, however, does not include include the case of a switch for use in controlling controlling a generator or a transformer or a link for test purposes. purposes. 4.10 All materials, fittings, etc, used in earthing shall conform to Indian Standard spec$ications, wherever these exist. 5. FACTORS INFLUENCXNG OFEARTHEDORUNEARTHED SYSTEM 5.1 S e r v i c e C o n t i n u i t y

THE

CHOICE

A number ofindustrial plant systems systems have been operated unearthed at one or more voltage levels. This is basically guided by the thought of gaining an additional degree of service continuity varying in its importance depending on the type of plant. Earthed systems are in most cases designed so that circuit circuit protective devices will remove the faulty circuit from the system regardless of the type of fault. However, experience has shown shown that in a number of systems, greater service continuity may be obtained with earthed-neutral earthed-neutral than with unearthed neutral systems. 5.1.1

le F a u l t s t o G r o u n 5.2 M u l t i p le 5.2.1 While a ground fault on one phase of an unearthed system generally does not cause service interruption, the occurrence of a second ground fault on a different phase before the first

switching and motor control control equipment. It is generally general ly recognized that protection protecti on under such circumstances circumstances is possible by fast and sensitive detection of the arcing f ault current and interruption within IO-20 cycles. In sol idly earthedneutral systems, this is possibl e as an arcing arcing fault would produce a current in the ground path, thereby providing an easy means of detection and arcing fault tripping trippi ng against phase-to-ground breakdowns. ca t i o n o f F a u l t s 5.4 L o ca 5.4.1 On an unearthed system, a ground fault does not open the circuit. Some means of detecting the presence of a ground fault requires to be installed In earthed system, an accidental accidental ground fault is both indicated indicated at least partially located by an automatic interruption of the accidentally accidentally grounded circuit or piece of equipment.

5.5 S a f e t y 5.5.1 Whether

or not a system is grounded, protection of personnel personne l and property from hazards require thorough thorough grounding grounding of equipment and structures. structures. Proper grounding grounding results in less likelilik elihood of accidents to personnel. Other hazards hazards of shock and fire may result from inadequate grounding of equipment in unearthed and earthed systems. However, However, relatively high fault currents associated -with solidly earthed system may present a hazard to worke rs from exposure to hot arc products products and flying molten metal. This protection is, however, however, reduced because of use of metal-enclosed equipment. 5.6

bnor mal

o lt lt a g e

Hazards

5.6.1 The possib le over-voltages over-voltages

on the unearthed system may cause more frequent failures of equipment than is the system, system, if earthed. fault

























IS I 3043 - 1983 grounding is not likely lik ely to reduce reduce the total magnitude of over-voltage over-voltage produced by lightning or switching surges. It can, however, distribute the voltage between phases and reduce the possibility of excessive voltage stress on the phase-to-ground phase-to-ground insulation of a particular phase. A system ground connection even of relatively high resistance can effectively eff ectively prevent static voltage build-up ( see Set 8 ). Even under conditi ons of an I-IV line breaking and falling on an LV system, an ef fectively grounded LV system will hold the system neutral close to the ground ground potential thus limiting the over-voltage. over-voltage. An unearthed system will be subj ected to resonant over-voltages. over-voltages. Field experience and theoretical theoretical studies have shown the wokId over that arcing, restriking or vibrating ground faults on unearthed systems can, under certain conditions, produce surge voltages as high as times the normal voltage. Neutral Neutral grounding is effective eff ective in reducing transient b uild up by reducing the neutral displacement from ground potential and the destructiveness destructiveness of any high frequericy voltage oscillations following foll owing each arc initiation or restrike. 5.7 cost 5.7.1 The cost diff erential between earthed and unearthed neutral neutral system will vary, vary, depending on the method of grounding the degree of

earthing is designed to give protection, the protective gear will operate to make the faulty main or plant harmless. In most cases, such operation involves isolation of the faulty main or plant, for example, b y circuit-breakers or fuses. 6.0.4 In the case of underground underground systems, there is no diff iculty whatever but, for example, in the case of overhead-line systems protected by fuses or circuit-breakers fitted with overcurrent overcurrent protection only, there may be diff iculty in arranging that the value of the system earth-resistance is such that a conductor falli ng and making good contact with the ground results in operation of A low system-earth system-earth resistance the protection. is required even in the cases where an arcsuppression coil is installed, as its operation may be frustrated by too high an earth-electrode resistance. 6.0.5 Earthing may not give protection against faults that are not essentially earth faults. For example, if a phase conductor on an overhead spur line breaks, and the part remote from the supply falls to the ground, ground, it is unlikely unlik ely that any protective gear relying on earthing, other than current balance protection protection at the substation, will operate operate since the earth-fault current circuit includes the impedance of the load that would be high relative to the rest of the circuit.























IS:3043 1987 6.1.2t is also recognized that, in practice, system system may be an admixtur e of type for the purposes of this code, ea rt hing systems systems are designated as follows: TJV-System (for 240 Vsingle phase domestic/ commercial supplv Systems where there are separate neutral and protective protective conductor s thr oughout the system. system. A system system where the metallic metallic path between the installation installation and the source source enerx v is the sheath and a rmouring of the suppry suppry cable ( see see Fig. 2 ).

b) Indian ThLS System (for 415 V three+hase domestic commercial supPry ) - An independent earth electrode electrode within the consumer’ consumer’ pre mises mises is necessary ( See Fig. 3 ). c) Indian T&C-System The neutral and protective functons are combined in a single single SOURCE

OF ENERGY

couductor thr oughout the system system ( for example earthed concentric wiring ( see Fig. 4 ). TN-C-S ystem - The neutral and protective functions are combined in a single conductor but only in in part of the system system (seeFig5).

e) T-TN-S

System (for 6*6/1X kV three-phase The consumers installation, bulk supjly ) a TN-S system system receiving power at a captive a delta connected substation through transformer transformer primary ( see Fig. 6 ). TT

System (for 415V three-phase it‘dustrial suM@ ) - Same as 6.1.1b ) ( s e e Fig 7. ).

t3)IT

System Fig. 8 ).

Same

as

6.1.1 c) ( see























ls

3843 -

1987

electrode

has

SOURCE OF ENERGY

3-

CONSUMER

conductive All exposed conductive to be provided in addition. FIG. 4

\EXPOSEO

parts are connected

INDIAN TN-C

SYSTEM

to the

I-

CONDUCTIVE

PEN

( NEUTRAL

AND

conductor.

INSTALLATION

For 3 IY consumer, consumer,

local earth

ROTECTIVE FUNCTIONS COMBINED IN

A S I N GL GL E C O N D U C T O R T H R O U O H O U T

S YS YS T E M )























SOURCE OF ENERGY

es/l

1 kV Three

Fro. 6 SOURCE

OF ENERGY

phase bulk supply

T-TN-S

SYSTEM























IS : 3043 - 19 87 6.2 M a r k i n g tor

o f E a r t h i n g /P /P r o t e c t i v e

Conduc-

6.2.1 The earthing earthing and protective protective conductor shall be b e marked as given in T able 1 ( see also IS 11353-1986* ). TABLE DESIGNATION 05' C~NDUOTOB

1

MARKI NG

OF CONDUCTORS CONDUCTORS

IDENTI~IOATI~N c_-_;*L----,

Alphanumerit Notation

Earth

E

Protective conductor

PR

BY

CoLona

Graphical Symbol l.

No colour other than colour of the bare conductor Green

and yellow

6.2.2

Use of Bi-Colour Combination - Green and The bi-colour combination, green and yellow ( green/yellow ), shall be used for identff ying the protective conductor and for no other purpose. This is the only colour code recognized ior identifying the protective conductor. Yellow

Bare conductors or busbars, used as protective conductors, shall be coloured by equally b road green and yellow stripes, each 15 mm up to 100 mm wide, close together, either throughout the length of.each conductor or in each compartment

shock voltages exposure exposure to persons in the area;

2) to

provide current current carrying carrying capability, both in magnitude and duration, adequate adequate to accept the ground fault current permitted by the overcurrent overcurrent protective system without creating a fire or explosive explosive hazard to buildin g or contents; and 3) to contribute to better performance of the electrical system.

When there is unintentional contact b etween an energized electric conductor and the metal frame or structure structure that encloses it ( or is adjacent, the f rame or structur tends to become en ergized to the same same voltage level as exists on the energized conductor. To avoid this appearance of this dangerous, dangerous, exposed shock hazard voltage, the equipment grounding conductor must present a low impedance path from the stricken frame to the zero potential ground junction . The impedance should also be sufficiently suff iciently low enough to accept the full magnitude of the line-to-ground fault current without creating an impedance voltage drop large enough to be dangerous. 7.0.2

Voltage Exjosure

7.0.3 Avoidance of Thermal Distress Th earthing conductor must.also f unction to conduct

























IS : 3043 - 1987 SECTION SECTION 8. RESISTANCE 8.0 Nature Nature

CONNECTIONS

TO EARTH

of Earthing Earthing

Resistance

8.0.1 The earthing resistance

made up of:

2

of an electrode is

a) resistance of the ( metal ) electrode, b) contact resistance between the electrode electrode and the soil, and c) resistance of the soil from the electrode surface outward in the, geometry set up for the flow of current outward from the electrode to infinite earth; The first two factors are very small fractions

of

TO

EARTH

concentration of salts dissolved in the contained water. Grain size and distribution, and closeness of packing are also contributory factors since they control the manner in which the moisture is held in the soil. soil. Many of these factors vary locally and some seasonally so that the table should only betaken as a genera1 guide. Local values should be verified by actual measurement, and this this is especially important where the soil is stratified as, owing to the dispersion of the earth current, the effective resistivity depends not only on the surface layers but also on the underlying geological formation.























IS : 3043 3043 - 1987 8.2 Where there is any option, a site should be chosen in one of the following types of soil in the order of preference given:

a) b) Cl

Wet marshy ground ( ~6~8.3 ); Clay, loamy soil, arable land, clayey soil, clayey soil or loam mixed with small small quantities of sand; Clay and loam mixed with varying proportions of sand, gravel and stones; Damp and wet sand, peat.

Dry sand, gravel chalk, limestone, granite and any very stony ground should be avoided, avoided, and also all locations locations where virgin virgin rock is very close to the surface. 8.3 A site should be chosen that is not naturally well-drained. A water-logged situation is not,

for the insertion of vertical vertical electrodes or where strip electrodes are laid radially under shallow areas of low resistivity resistivity overlaying rock strata, bentonite packing will will increase the contact efficiency with the general mass of g ground. round. fe c t 8.6 E f fe

of of -

M o is is t u r e

Conten t

on

Ear th

Moisture content is one of the controlling factors in earth resistivity. resistivity. Figure 9 shows the variation variation of resistivity resistivity of red clay soil with percentage of moisture. The moisture content is expressed expressed in percent by weight of the dry soil. Dry earth weighs about 1 440 kg per cubic metre and thus 10 percent moisture content is equivalent to 144 kg of water per cubic metre of dry soil. It will be seen from Fig. 9 that above about 20 percent moisture, the resistivity resistivity is very little affected, while below 20 percent the resistivity resistivity increases very abruptly with the decrease in moisture conResistivity























IS:

3043.1987

frozen, presents a decided increase in soil resistivity and has the effect ofshortening the active length of electrode in contact with soil of normal resistivity. Treatment of Soil 8.8 Artificial Multiple rods, even in large number, number, may sometime fail to produce an adequately low resistance to earth. This condition arises in installations involving soils of high resistivity. The alternative is to reduce the resistivity of the soil immediately surrounding the earth electrode. To reduce the soil resistivity, it is necessary to dissolve in the moisture, normally contained in the soil, some substance which is highly conductive in its water solution. The most commonly used substances are sodium chloride

8.8.1 With

average or high moisture content, these agents form a conducting electrolyte throughout a wide region surrounding the earth electrode. Approximatel y 90 percent of the resistance between a driven rod and earth lies within a radius of about two metres from the rod. This should be kept kept in mind when applying the agents for artificial treatment of soil. The simplest application is by excavating a shallow basin around the top of the rod, one metre in diameter and about 30 cm deep, and applying the artificial artificia l agent in this basin. The basin should should subsequently be filled several times with water, which should be allowed each time to soak into the ground, thus carrying the artificial treatment, in electrolyte form, to con-























IS r3043 r3043 A 987

8.8.2 The reduction in soil resistivity effected by salt is shown by the curve in Fig. 10. The salt content is expressed in percent by weight of the contained moisture. It will be noted that the curve flattens off at about 5 percent salt content and a further increase in salt gives but little decrease in the soil resistivity. The effect of salt will be different for different kinds of so11 and for various moisture contents but the curve will will convey an idea of how the soil conductivity can be improved. Decreasing the soil resistivity- causes a corresponding decrease in the resistance of a driven earth electrode. 400 kg

cl

3co 300

the other two, thus a pipe, rod or strip has a much lower resistance than a plate of equal surface area. The resistance is not, however, inversely proportional to the surface area of the electrode. 9.2 R e s i st st a n c e

of Common

Types of Ear th-

Electrodes

The approximate resistance 9.2.1 Plates earth of a plate can be calculated from: --

to

ohms where p-

resistivity of the soil ( assumed form ) ,( in n.m); n.m); and

uni-

= area of both sides of the plate ( in ms ).























IS : 3043 - I987 TheresistanceR(inQ) TheresistanceR(inQ) ofal1s2m plate is given approximately by the formula:

R--A For conventional conventional sizes, the resistance is approximately inversely proportional to the linear dimensions, not the surface area, that is a O-9 m O-9 m plate would have a resistance approximately 25 percent higher than a l-2 x 1.2 m plate. The current .loading capacity capacity of a 1.2 m x 1.2 m plate is of the order order of 1 600 A for 2 s and 1 300 A for 3 s. Plate electrodes shall b e buried such such that its top edge is at a depth not less than 1 5 m from the surface of the the ground. However, the depth at

Driven rods generally generally consist of round round copper, steel-cored copper or galvanized steel ( see 9.2.8 13, 16 or 19 mm in diameter from 1 220 to to 2 440 mm in length.

























lSr39U-1987























IS:3043-1987

with length for a soil resistivity of 100 a.m. The effect of conductor conductor size and depth over the range normally used is very small,

cessary in most circumstances subject to the prdvision of earthing facilities that are satisfactory before these bonding connections are made.

If several strip electrodes are required for connection in parallel parallel in order to reduce the resis

For existing installations in which a water pipe is used as a sole earth electrode; an independent























S

deliberately coated or wrapped against corrosion. In the latter case, cathodic protection is used to supplement the coating and guard against localized loca lized corrosion due to coating flaws or faults. Protective system current drain is proportional to the area of

3043 - 1987

tinned or not, is entirely satisfactory ( subject to the precautions given in this subclause ), the average loss in weight of specimens 150 mm x 25 mm 3 mm buried for I2 years in no case exceed exc eed 0.2 percent per year. Corresponding Corresponding average























ISr3043-1987

d) It may require require connection to earth at points to discharge unwanted induced currents and voltages from other sources sue as overhead lines.

buried bare metal connected to the pipeline, pipeline, and thus drawing a corrosion protection current that may be required required by the pipeline 9.4 Typical

installations

of pipe earth

electrode























CAST

IRON

Cl

OR COVER









































































































































































































































































































































































































































































































































































































































































. ._.. ._

MOVER

RCO

GENERATOR

LOAO

L.V.

PLANT-

PORTABLE

INOEPENOENT EARftl ELECTROOf

FIQ. 28

METHO D OF CONNWZ TINQ RUIDU L CURR CURR BNT DEVI~ DEVI~ B ( r . c. d. ON THE OUTPUT O F A PO RTABLE RTABLEOR OR MOBIL GBNBRATOR

23.3.2 Single High Voltage Censrator Earthing ( Synchronous Machines with Star Connrctcd AltsmplC?S). isolation (jam 23.3.2.1 Cmnator opsrating in isolation The star-point of the mains or other su~iers generator should be connected ( ia a resistor, if necessary ) and through a bolted link for test purposes to an ear thing conductor and the inde-

pendent earth electrode. 23.3.2.2 Standby generator ( without paralleling In addition addition to the earthing requirefacility ) ments described described for a set operat ing in isolation from other other supplies, supplies, the presence of an incoming incoming electricity board supply makes necessary necessary the interlocking of the standby supply circuit break ers to prevent inadvertent inadvertent connection ( see Fig, 29 ).























lS:3043-1987 23.3.2.3 Standby generator ( capabk of parallel operation with an incoming incoming supply ) - The operation

of a private generator ge nerator ( or generators) in parallel with an electricity board high voltage system is subj ect to the parallel parallel and technical agreement agreement of the electricity board. In most cases where parallel parallel operation operation with an incoming in coming electricity b oard is required, a earthing contactor is necessary between the generator generator star point and the bolted test link ( see Fig. 30 ). The contactor contactor should b e interlocked with the incoming supply circuit breaker so that it is open during during periods of parallel operation operation but closes at all all times. In the event of the el ectricity supply being lost during a period of parallel operation, operation, the earthing contactor should be arranged to close automatically. automaticall y. The form of generator earthing earthin g ( direct or resistance resistan ce ) is dependent dependen t upon the system parameters and th machine machin e manufacturer’s recommendations 23.3.3

Multiple High Voltage Generator Earthing

23.3.3.1 Generators operating in isolation from operate other supplies - When it is required to operate

two or more generators gen erators in parallel and the method

of energy source source earthing is direct or resistance earthing, then earthing contactors should be installed between each generator generator star-point star-point and the earthing earthin g conductor each electrode ( as described in 2362.3.1 2362.3.1 ). T he contactors contactors need to to be interlocked so that only one can b e closed to maintain a single energy source source earth. If a neutral earthing transformer is to be used used for energy source source earthings, it should be connected as shown shown in Fig. 31 except that that in the case of an isolated i solated generating system, the earthing contactors is not required. 23.3.3.2

parallel facilio

Standby generators ( without mains When the generating sets ar )

not to be operated operated in parallel with the mains supply, and have direct or resis resistance tance earthing, the standby generator generator circuit-breakers and mains circuit-breaker need to be interlocked.

If a neutral earthing transformer transformer is used the requirements are the same as described described for a single standby generator generator in 23.3.2.2; 23.3.2.2; as shown in Fig. 31, but without the earthing contactor.

HV STANDBY GENERATOR LOAO

EARTHING CONTACTOR

b pt.-

-

IEARTHING ,lNCOMlNG SO THAT ‘ANY TIME

CONTACTOR INTERLOCKED WITH MAINS SUPPLY CIRCUIT BREAKER ONLY ONE CAN CAN BE CLOSED AT























I$ 23.3.3.3 Standby generawrs ( capable capable of parallel @ration with an incoming incoming mains mains supprU) supprU) When the

generating sets have direct or resistance earthing and are used as standby standby to the mains, earthing contactors contactors are sded if parallel running is

NEU TiIAL

3043 - 1987

requirement. These should be interlocked with the incoming mains supply circuit-breaker so that they are open during during parallel operation of the set with the mains, but one is closed at all other times ( see Fig. 32 ).

EARTHING

v-t

FIQ. 31

INCOilING

MAINS

SUPPLY

LO’AD

L EARTHING CONTACTOR JNTERLOCKED SO THAT THE CONTACTOR CANNOT BE.CLOSfD DURING PARAL LEL OPERATION WITH THE INCOMING MAINS s UPPLY

MULTI MULTI PLE HIOH VOLTAGESTANDBY VOLTAGESTANDBYGENERATING GENERATING ETSWITH ETSWITH NEUTR NEUTR AL EARTHING TRANSFORMERUITABLE TRANSFORMERUITABLEFOR PARALLEL OPERAT OPERAT IONWITH EACH OTHER AN WITH THE INCOMINC~ MAIrw Supply























IS I 3043 - I987

If more than one patient is present in an area, connection connection of the various potential equalization centres to a central potential equalization busher, which should preferably be connected to the protective earth system of the power powe r supply for the given area. In its completed form, the t he equipotential bonding network may consist partly of fixed and permanently installed bonding and partly of a number of separate separa te bondings which are made when the equipment is set up near the patient. The necessary terminals for these bonding connection should be present on equipment and in the installation.

PR OTECTIVE MEASURES THR OUGH EARTH EARTH ING IN MEDICAL ESTABLISHESTABLISHMENTS 24.0 General In the context of this Section “installation”, means any combination of interconnected electrical equipment within a given space or location intended to supply power to in medical practice. electrical equipment used in 24.0.1 For the the purposes reference may also be made Set 4 )-1985*.

Section, of this to SP : 30 ( Part 3/

24.0.2 As such, some parts of o f the installation may be present in the patient’s environment differences, that could l&ad to where potential excessive excessiv e currents through the patient, must be avoided. For this purpose a combination combination or earthing of equipment and potential equalization in the installation seems to provide the best solution. disadvantage of such a system is that in the cas of an insulation insulation fault in circuits directly connected to supply mains, the fault current may cause a considerable voltage drop over the protective earth conductor of the relevant circuit. Since reduction of such a voltage drop by the application of increased cross-sectional areas of protective conductors is usually impractical, available solutions are the reduction of the duration of fault currents to earth by special devices or the application of a power supply which is isolated from earth. 24.0.3 Generally a power supply system sys tem including a separated separ ated protective conductor is required. ( TN-S System ) in medical establishment ( see 6.1.1 ). In addition the following provisions required, depending upon the nature examinations or treatments performed:

may be of the

a) Additional requirements concerning protective conductors and protective devices to restrict continuous voltage differences b) Restriction of voltage differences by supplementary equipotential bonding. During During the application of equipment with direct contact to the patient, at least a potential equalized zone around the patient shall be provided with a patient centre bonding bar to Ivhich the protective and functional earth

e)

Restriction of the duration of transient voltage difference differen ce by the application of residual current operated operate d protective devices ( earth leakage circuit-breakers circuit-breakers ).

Continuity f 1 Continuity

of power supply to certain equipment in the case of a first insulatio fault to earth and restriction of transient voltage differences by application of isolat-

g)

Monitorjng of first insulation insulation fault to earth in an IT Systems ( see 6.1.1 ) ( the secon sec on dary side of an isolating transformer ) with sufficiently high impedance to earth. Now-Additional safety measures are required besides earthing earthing described described in this Section. These cover fire safety, safety supply systems and interference suppression. Ref erence may be made t@ NEC Part 3, S ection 4 )*.

24.1

Safety

Provisions

24.1.1 Safety measures from the point of view of earthing are divided int0.a number of provisions as given in Table 10. 24.1.2 Provision PO shall be applicable to all buildings containing medically used rooms. Provision Pl shall be applicable for all all medically used rooms. Other requirements be complied with, if:

of this

Section,

need

not

a room is not intended for the use of medjcal electrical equipment, or

b)

patients do not come intentionally intentionally in contact with medical electrical equipment during diagnosis or treatment, or























TABLE

10

SAFETY

PROVISIONS

( Clrrusr 24.1.1 PBOVIaIONa (1) PO

PRINCIPAL REQUIREMENTS Duration limit

(2) of touch voltage restricted

INSTALLATION MEASURES to a safe

TN-S,

Touch voltages in restricted to a safe

(3) TT or IT system ( see 6.1.X )

Pl

As PO but additionally patient environment limit

P2

As Pl but additionally Resistance between extraneous conductive parts and the protective conductor conductor busbar of the room room not exceeding 0’ 1

P3

As Pl or P2 but additionally Potential difference between exposed conductive parts and the protective conductor busbar not exceeding 10 mV in normal condition ( se# Note

As PI or P2: Measurement necessary, corrective action possibly necessary

P4

As Pl or P2. Additional protection against electric shock by limitation of discormectiog time

Additional to PI or P2 Residual operated protective device

P5

Continuity of the main s s upply maintained in case of a fi rst insul ation fault to earth and currents to earth restricted

Additional to Pl, P2 or P3 Isolated supply system with isolation monitoring

Additional to PO Suppl y system with addiadditional requiremen requiremen ts for protective earthing, etc Additional to Pl Supplementary tential bonding

equipo-

current

NOTE - Normal condition means ‘without any fault’ in the install ation.

241.3 Guidance on the application provisions are given i n Tabl e 11.

of the

limit

The conventional LL ) is fixed at 25 V ac.

25.1.2.1

touch

voltage

24.1.4 A typical example of an installation in a hospital is given in Appendix C of NEC (Part 3, Section 4 )*.

25.1.2.2 Protective conductors inside medically used room shall be insulated; their insulation shall be coloured coloured green-yellow.

25. S U P L Y C H A R A C T E R I S T I C S

25.1.2.3 25.1.2.3 Exposed conductive parts parts of equipment being part of the electrical installation used in the same same room s hall b e connected to a common protective conductor.

PARAMETERS

AND

25 0 E xchan ge of Informa tion 25.0.1 Proper coordination

shall, be ensured between the architect, building buil ding contractor and the electrical engineer or the varrous aspects of installation design. The necessary special features of installation s shall be ascertained before hand with reference reference to Table Il. l a t i o n M e a s u r e s f or or S a f e t y 25.1 C i r c u i t I n s t a l la P r o v i s io i o n s - ( S e e Tabl e 10, co1 3 ). 25.1.1 Provision PO General 25.1.1.1 All buildings in the hospital area which contain medically used rooms shall have

25.1.2.4 25.1.2.4 A main equipotenti al b onding with a main earthing bar shall be provided near the main service service entrance. entrance. Connections shall be made to the following parts by bonding conductors: a) lightening conductor; b) earthing systems of the electric power distribution system; the central heating system; d) tne conductive water supply line; e) the conductive parts of the waste water line; f-1 the conductive parts of the gas supply; and rame-work of the !a the structural metal f rame-work

























25.1.2.5 Each medically

TABLE TABLE

II

EXAMPLES EXAMPLES OF APPLICATION SAFETY PROVISIONS

used room or room group shall shall have its own protective conductor bus bar, which which should have adequate adequate mechanical and electrical properties and resistance against corrosion.

Cp

( Claurs 241.3 241.3 MEDICALLY USED ROOM

POTECTIVE MEASURES ----_--.~_-_-_ ----_--.~_-_-_--~ --~ 5’

PO/P1 1. Massage

room

2. Oper ating n

Ward,

wash room

5. ECG,

25.1.2.6 The impedance ( < ) between the protective conductor bar and each connected protective conductor contact in wall sockets or terminals should sho uld not exceed 0.2 !.2, if the rated current of the overcurrent-protective device is 16 A or less. In case of a rated current exceeding 16 A, the impedance should be calculated using the formula:

General

4. Delivery

room

EEG,

EMG

6. Endoscopic

8. Labour

room

room

7. Examination ment room

or

trea t-

room

9. Operating room 10. Orology being theatre

sterilization room (not an operating )

in all cases 5 shall not exceed 0.2 Q. ( I, = rated current of overcurrent protective device in amperes ).

diagnostic 11. Radiological and thera py room, other than mentioned under 20 and 24 12. Hydrotherapy

room

13. Physiotherapy

room

14. Anaesthetic

room

Xl

15. Operating 16. Operating room

theatre

x1

17. Operating

plaater

preparation

recovery

18. Operating room 19. Out-patient room 20.

Heart

room

Xl xl x1

operating

catheterization

This busbar may be located in the relevant power distribution box. b ox. The leads connected to terminals of such a protective conductor bar shall be identified and shall be similarly designated on drawings of the installation system.

Xl

of the protective conNOTE - The measurement ductor impedance should be perfor,med with an ac current not less than 10 A and not exceeding 25 A from a source of curr ent with a no-load voltage voltage not exceeding V, for a period of at least 5 a.

25.1.2.7 The cross-sectional

area of the protective conductor shall be not less than the apprepriate value shown in Table 7. The cross-sectionai area of every protective conductor which does not form part of the supply cable or cable enclosure shall be, in any case, not less than: a) 2.5 mm*, if mechanical protection is provi-























IS!9943-1997 conductive s urface area larger than Co02 m* or linear dimension exceeding 20 cm or smaller part that may be grasped by hand. Additionally, the following foll owing requirements apply: Such connections connections need not be made made to:

b)

1) Extraneous Extraneous conductive conductive parts i nside of walls ( for example, structural structural metal work of buil dings ) having no direct connection to any accessible conductive part inside the room, and 2) Conductive parts in a non-conductive enclosure; In locations locations where the position of the patient can be predetermined this provision may be restricted to extraneous extraneous conductive conductive parts within the patient patient environment ( see Apppendix B of NEC (Part 3, Section 4 ); and In operating theatres, intensive care rooms, heart catheterization catheteri zation rooms and rooms intended for the recording of bioel ectrical action potentials all parts should be connect ted to the equipoten tial bonding bar via direct and separate conductors.

25.1.3.3 25.1.3.3 The foll owing requirements shall be fulfilled: a) The imnedance between extraneous conductive p&ts and the equipotential bonding bar shall not exceed 0 1 Q. The merrcrurement of this impedance NOTE should be performed with a current not less than 10 A and not exceeding exceeding 25 A during not leas than not exceeding

b) All

6 V ac.

equipotential bonding conductors shaIl be insulated, the insulation being coloured green-yellow.

g) An adequate number (under consideration) of equipotential bonding terminals other than those for protective conductor contact or pins of socket outletsshould be provided provided in each room for the the connection of an additional protective conductor conductor of equipment or for reasons of functional earthing of equipment. 25.1.4 Provtsio~rP3 Restriction of Touch Voltage in Rooms Rooms Equipped for Direct Cardiac Applzcation The continuous current through a resistance of 1 000 connected between the equipotential bonding b ar and any exposed conductive conductive part as as well as any extraneous extraneous conductive conductive part in the patient environment shall not exceed LO MA in normal condition for freq uencies from dc to 1 wz. 25.1.4.1

For a description of patient environment, see Appendix B of NEC ( Part 3, Section 4 ). Where the measuring device has an impedance and a frequency characteristics, the current may als o be indicated as a continuous voltage with a limit of 10 mV between the parts mentioned above. During the test, it is assumed that fixed and permanently installed medical electrical equipment is operating. ‘Normal conditions’ means without any b) ‘Normal fault in the installation and in the medical electrical equipment. it NOTE - To comply with this requirement, may be necessary necessary to apply one or more of the following methods: Extraneous

conductive conductive

parts may

be:

a) connected to the equipotential bonding bar by a conductor of a large area in order to reduce the voltage voltage drop across such conductor, b) insulated insulated ao that it is not possible unintentionally, and

to touch them























IS I 3043 - 1987 25.1.5.5 Electrical equipment, for example, general lighting luminaries, installed more than 2.5 m above f loor level, need not be protected by a residual-current residual-current protective protective device.

25.1.5.6 25.1.5.6 Fixed and permanently installed electromedical equipment with a power consump tion requiring an overcurrent overcurrent protective protective device of more than 63 A rated value may be connecte d to the supply mains by use of a residual-current residual-current protective protective device with I n N Q 300 mA. 25.1.6

ProvisionP5: ProvisionP5: Medical IT System

25.1.6.0 The use of a medical IT-S ystem for the supply of medically used room room for example, aperating aperating theatres, may be desirable for diff erent reasons:

a)

b)

Cl

A medical I T-S ystem increases the the reliability of power supply in areas where an interruption of power supply supply may cause a hazard to patient patien t or user; A medical IT-S ystem reduces an earth fault current to a low value and and thus also reduces the touch voltage across a protective conductor conductor through through which this earth fault current,may current,may flow; A medical IT-S ystem reduces leakage currents currents of equipment to a l ow value, where the medical IT-System is approxiapproximately symmetrical to earth. It is necessary to keep the impedance to earth of the medical IT-S ystem as high as possible. This may be achieved achieved by: a) restriction of the physical dimensions of the medical isolating transformer, b) restriction of the system supplied by this transformer, c) restriction of the n umber of medical electricalequipment connected to such a system, and

In case. of fault in parts of of IT-S ystem, the nected by the device.

a short-circuit short-circuit or a double earth earth opposite opposite polarity of the medical defective system shall be disconrelevant overcurrent overcurrent protective protective

If more than one item of equipment can be connected to the same secondary winding of the transformer, at least two separately protected circuits should be provided provided for reasons of continuity of supply. 25.1.6.3 Overcurrent protective devices shall be easily accessible and shall be marked to i ndicate the protective protective circuit. 25.1.6.4 25.1.6.4 An insulation monitoring device shall be provided provided to indicate a fault of the insulation to earth of a live part of the medical IT-System. 25.1.6.5 Fixed and permanentl y installe ins talle equipment with a rated rated power input of more than 5 kVA and all X-ray equipment even with a kVA rated power input of less than shall be protected by Provision P4. Electrical equipment, for example, example, general lighting, more than 2.5 m above fl oor level, may be connected directly to the supply mains. requirements for for insulation insul ation 25.1.6.6 General requirements monit monitoring oring devices devices A separate separate insulation resistance

or impedance impedance monitoring device shall be provided for each secondary system. It shall comply with the requirements of (a) to (d) below: It shall not be possible to render render such device inoperative by a switch. It shall indicate visibly and audibly audibly if the resistance or impedance of the insulation falls below the value given in 25.1.6.7 and 23.1.6.8. 23.1.6.8. A test button shall be provided provided to enable checking the response of the monitor to a fault condition as described in 25.1.6 25.1.6.4. .4.























Is : 3043 -1987

reading calibrated in total hazard current with the value of 2 mA near the centce of the metre

scale.

The device shall not fail to alarm for total hazard hazard currents in excess of 2 mA. In no case, however, shall the alarm be activated activated until the fault faul t hazard current exceeds O-7 mA. SECTION

8

STATIC

AND

9

MlSCELLANEOlJS

28. EARTHING IN POTENTIALLY DOUS AREAS 28.1 Earthing

During the checking of the response of the monitor to a fault condition the impedance between the medical medical IT-S ystem and earth shall shall not decrease.

LIGHTNI NG

the general pr inciples inciples NOTE - F o r t h e time being, of static and lightning protection earthing, together with the relevant rules for such purposes as contained in 7689-1974 ‘Gulde for contr ol of undesirab le static electricity’ and IS 23091969 ‘Code of pra ctice for the pro tection of buildings and allied structur es’against es’against lightning (jrst rsuisien )’ are considered as valid in this section.

SECTION SECTION

The value of 2 mA or 6.7 mA are based experience with 110 to 120 V power power supplies. For a 220-240 V power supply, it may b e.necessar to increase these values to 4 and 1’4 mA because of the higher leakage current of equipment. NOTE -

on practical

PROTECTION PROTECTION

A simultaneous is in progress.

and Bonding

Earthing Earthing should be in accordance accordance with the relevant sections of this code. The connection between metal part to be grounded and the grounding conductor conductor shall be made secure mechanically and electrically electrically by using adequate metallic fitting. The grounding grounding conductors conductors shall be suff iciently strong and thi ck, and the portions of conductor which are are likely to to be corroded or damaged damaged shall be well well protected protected.. Grounding conductors conductors which shall not reach a hazardous high temperature due to the anticipated antici pated maximum earth fault faul t current flowing shall be used 28.1 l

28.1.2 28.1.2 Protection Protection against lightning shall be provided in accordance accordance with S ection 8. Specif ic guidelines for installations i n hazardous hazardous locations locations

review/revision review/revision

of

these

standar ds

For completeness f the earthing code, it is proposedto include relevant earthing an d bonding details details for control of static electricity and lightning lightning protection in Section 8 in due cour se.

INSTAL LATI ONS

HAZARHAZAR-

EARTHING

AND

CONSIDER CONSIDERATIONS ATIONS

elsewhere, all utility and process pipeli nes should be bonded to a common conductor by means mean s of earth bars or pipe clamps and connected to the earthing system at a point where the pipelines enter or leave the hazardous area except where conflicting with the requirements of cathodic cathodic protection. In addition, addition, i t i s recommended recommended that steel pipe racks in the process units and off-site areas should shoul d be grounded at every 25 m. 28.2 Permissible

Type

of Earthing Earthing

System

Guidance on permissibl e power systems is given below: 28.2.1

if a power system with an earthed neutral is used, the the type TN-S system with separate separate neutral (M) and protective conductor (PE) throughout the system is preferred. The

neutral

and the protective conductor























IS L 3043 - 1987

used to indicate the first earth fault. However, equipment in Zone shall be disconnected instantaneously in case of the first earth fault, either by the monitoring device or by a residual current operated device. For power systems at all voltage levels installrd in Zone 0, due attention should be paid to the limitation of earth fault currents in magnitude magnitude and duration. duration. Instantaneous earth fault protection shall be installed. It may also be necessary to provide instantaneous earth fault protection devices for certain applications in Zone 1. 28.2.2 Potential Equalization - To avoid dangerous sparki ng between metalli c parts parts of structures, potential equalization is always required for installations in Zone 0 and Zone 1 areas are as and may be necessary for installations in Zone 2 areas. Therefore, all exposed and extraneous extraneous conductive conductive parts shall be connected to the the main or s upplementary equipotential bonding system. The bonding system system may i nclude normal protective conductors, conductors, conduits, metal cable sheaths, steel wire armouring armouring and metallic parts of structures but shall not include neutral conductors. conductors. The conductance conductance be$ween “metallic parts of structures shall correspond to a cross-section of at least 10 mm* of copper. Enclosures are not to be separately connected to the equipotential bonding system if they are secured to and are in metallic contact contact with structural parts or piping which are connected to the equipotential bonding system. For additional information, see relevant section of this code. However, there are are certain pieces of equipment, for txample, txample, some intrinsicall y safe apparatus, apparatus,

If equipment requires ‘both a protective earth and a functional earth connection, it is preferred that the two earths should be separated within the equipment so that that power system fault currents cannot flow in the functional earthing conductors. conductors. The functional earthing system and conductors conductors can then be designed solely in accordance accordance with the requirements of the telecommunication system. Alternatively, the protective protective and functional earth may be connected together within the equipment but in this case the functional earth system and conductors conductors should be suitable for the current current they may carry under power system fault conditions. The general recommendations recommendations for lightning protection apply to earth systems for telecommunication lightning protection. The telecommunication functional earth should be obtained from a point which even under power system fault conditions is unlikely unlik ely to have a dangerous potential to remote earth. The consumer’s earth terminal of a TN system is.suitable, otherwise a suitable earth electrode system, separate from the protective protective earth, should be provided. 29.2

Ttlecommunication Circuits Association with High Voltage Supply Systems Telecommunication cir-

cuits used i n any way in connection with or in close proximity proximity to high voltage equipment require special attention and due consideration should be given to to the safeguarding safeguarding of such circuits against rise in potential of the supply system earthearthelectrodes. When a telecommunication circuit is provided in a b uilding, where a high voltage system terminates and the telecommunication telecommunication circuit is part of or is electrically connected to a system outside the ‘earth-electrode ‘earth-electrode area’, precautions precautions should be taken to safeguard personnel and tel ecommunica-

























connected to the station earth-electrode earth-ele ctrode system. b) At stations where the neutral of the high voltage system is i s earthed, earthed, it is generally practicable practicable from a knowledge of the impedance of the earth-electrode earth-electrode system and of the maximum earth-fault current to estimate the rise of earth potential that will occur upon upon the incidence of a f ault. Where Where the estimate does not exceed safe values no precaution precaution additional to that described described in (a) is necessary. This Thi s limit li mit may may be extended to higher values if all the power lines contributing to the earth earth fault current current are in the ‘high-reliabi lity’ category. category. If the the estimate i s above safe limits (see 20.5.1), 20.5.1), the following foll owing additional precautions should be observed. Where the telecommunication circuit lies within the ‘earth-electrode ‘earth-electrode area’, area’, it should be run in insulated cable capable of .withstanding the application of a test voltage of 2200Vdc(orac50Hzpeak)or(1500+2U) V dc ( or ac 50 Hz peak ), where is the estimated estimated rise of earth potential, whichever is the greater, greater, between conductors. and earth for 1 min. It is preferred that the cables have no metallic sheath,

it is unlikely unlik ely that the supply authority will offer an earth terminal where the supply system has multiple earthed neutral. If the supply is at a voltage higher than 415 V, the developer developer will have to provide provide the neutral earthing on the low voltage system. 30.2 The main protection protection against electrical hazards hazards on a construction construction site is the use of reduced low voltage system sys tem for power tool s ( 110 110 V b etween phases and 55 V to mid-point earth or 65 V to star-point earth ) and safety extra low voltage f or supplies to headlamps, etc. The earth fault loop impedances on a reduced reduced voltage system or on a 24(3/415 V system syste m serving s erving fixed equipment should allow disconnection within the safe duration. 30.3 Early discussions discussions with the electricity board are essential so that agreement agreement on the type of facilities, can be supply, including earthing obtained. Where the supply is provided provided from the low voltage di stribution system, the increasing use of protective protective multiple earthing ( PME will usually prevent prevent an earthing earthing terminal being provided b y the electricity electricity board because of the developers inabi lity to comply with the requirements of the PME approval during construction work.























IS : 3043 - 1987

that is not the the property property of the supply authority, authority, or if the consumer generates generates electricity privately, privately, then the consumer should should provide and maintain the earth electrodes that have the neutral or midvoltage points bonded to them. If the supply transformer ( or generator generator ) is distant from the consumer’s consumer’s premises, provision of an earth terminal at the premise, should be requested. Where this is possible, the earth terminal should be made available by means of an additional earth conductor in the supply supply cable or overhead line. armouring NOTE - The supply cable rheath and armouring mav serve the nurnose of this ear th conductor orovided that they a>e bonded to the supply source karth, neutral or mid-voltage mid-voltage point and meet the 50 percent conductivity requirement.

If the provision of such an earth terminal is impracticable, impracticable, then it is imperative that the earth electrodes at the supply source and consumerr’ premises are maintained such that their resistance to the general mass of earth is as low as possible, for example, less than 2 Q, and appropriate appropriate earth fault protection protection is provided In all cases, the aim should be to maintain earth electrode resistance, as low as is practicable, practicable, taking account of the site conditions, for example, example, soil rock resistivity. resisti vity. Except, however, for the

should have a cross-sectional cross-sectional area of not less than 14 mm’, in the case of flexibl e cable working at less than 125 125 V, the cross-section cross-section area need rot flexib le cable on be greater than 6 mms; also a flexible the surface surface of the mine supplying a load less than 3 kW need not have an earth conductor larger than the power conductors. Cables incorporating steel tape armour armour ( unless supplementing steel wire ), aluminium armour armour or copper sheathed ( mineral insulated ) cables cables are unsuitable for use below ground. ground. Generally single or double, doubl e, steel wire wire armoured cables cable s are used. The use of paper-insulated lead covered covered cable is also discouraged from use below bel ow ground owing to the poor mechanical strength of the paper insulating material. The foll owing are released from the requirements to be earthed, when used solely at the surface of the mine: a) any lamp holder, holder, that is, ef ficiently protected by a covering which is insulated or earthed and made made of fire resisting material; b) any hand held tool that is double insulated; c) any portable portable apparatus apparatus working at less than 50 V dc or 30 V ac; and d) any other non-portable non-portable apparatus apparatus working























lamma ble gas gas uld be pr esent in th e general general body of air ), th en limita limita tion of th e ma ximum prospective ear th fau1.t curren t is called for for on power system s workin workin g at vol volta ta ges between 250 an d 1200 V ( th e ra nge of of voltage voltage norma lly used for for coal coal windin g ma chiner y ser ved by flexible flexible tra iling iling cables cables ). In. these cases, t he maximum prospective prospective eart h fault current should be limited ( norma lly by impeda nce eart hing ) to 16 A at volta volta ges between 250 a nd 650 V a nd to 2 A at voltages voltages bet ween 650 an d 1 200 V. In eith er case, th e switchgear contorlling th e circuit circuit should be able to detect an d cut-off cut-off t he supply of electr electr icity icity with less tha n one-third one-third f the maximum prospec tive earth fault current flowing. prospect ive NOTE The ratio between maximum earth fault current and protection settings is known as the ‘tripping ‘tripping ratio’. ratio’. In practice it has been found found that in order to take account of voltage voltage depressions depressions occurring when a.short circuit coincides with an eart h fault the tripping tripping ratio should be set to at least 5 1. Multipoint earthing of a power circuit ( sometimes sometimes referred to as an ‘insulated’ ‘insulated’ or ‘free neutral system’ ) is allowed at any place in a mine, including places where flammable gas may occur, provided that a transformer is used which has a means to cut off the supply and prevent danger should a breakdown occur between th primary and secondary windingr. windingr. In these systems systems the maximum prospect ive e art h fault curr ent does not usually exceed 2 A and switchgear switchgear ia set to trip at less than one- fifth of this value. value.

or steel, therefore metals other thsa &H cables with ar mourin gs or meta llic. llic. cover cover made f soft soft metals such as aluminium and copper copper ( MICC cable ) can can not be used on th ese prem ises wher e th e voltages voltages exceed exceed 250 V dc or 125 V a c. Where a able is provided provided with a lead lead sheat h, in ad dit ion to th e requ ired met allic’ coverin coverin g, the conductan conductan e f the lead lead sheath may be taken as cont cont ribut ing to th at of th e tn eta llic coverin coverin For suchinstallations, suchinstallations, plumbed joints joints have to used where the lead sheat h is jointed jointed or term inated. Where flexib flexible le cable is used to supply porta ble appa ra tu s at voltages voltages exceeding exceeding 250 V dc or 125 V ac, such cable should be protected by one of the following: met allic covering covering ( flexible flexible wir e ar mour ing ) th at encloses encloses 31 31 th e conductors conductors an having a condu condu ctan ce of not less less tha n half tha t of the largest current carrying conducconductor, or where t his is impr acticable, ha ving a conductan conductan e not less tha n tha t f a 14 mm * cross-sectio cross-sectiona na l ar ea copper copper condu condu ctor.

b)

A screen of wires to enclose all th e condu condu ctor s ( collectively collectively screen ed type cable having a onductan onductan ce not less tha n t hat o























IS I 3043 - 1987

and protective conductor is required, that is, an SNE cable. The wiring on the load side of the protective device in the unit should consist of separate phase, neutral and circuit protective conductors. Exposed extraneous conductive parts of the item of street furniture being supplied should be bonded to the earthing terminal terminal within the equipment. The earthing tetminal is itself connected to the supply protective conductor. 32.3 An alternative method of supplying and protecting street furniture is by means of a T-C-S system. In such cases, a combined neutral and earth conductor cable is normally used, that is, a CNE cable. 32.4 Wiring on the load side of the protective device in the units being supplied should use, unless a special approval has been obtained, separate phase, neutral neutral and circuit protective conextraneous conductive parts ductors. Exposed should be bonded to the neutral terminal by conductor with a copper equivalent cross-section of 6 mm or the same as that of the supply neutral conductor if this is less. This requirement does not apply to small isolated metal parts not likely to come into contact with exposed metallic metallic or extraneous metal parts or o r with earth, for example, small metallic doors and door frames in concrete or plastics units should not be so connected. 32.5

In the the case of circuits feeding more than one

its resistance to earth is not under under the control of the lighting authority. In such circumstances, consideration should be given given to the use of residual current devices to ensure disconnection disconnection of faulty equipment. The use of metallic street light columns or the metal carcasses of control units, etc, as protective earth electrodes is not recommended. 33. EARTHING WORKING

OF CONDCTORS

FOR SAFE

33.1 General - This clause deals only with the broad principles of the earthing of conductors for safety purposes. It is intended to cover the the safety earthing of both light and heavy current equipment and is generally applicable to high voltage equipment; however, in some circumstances circumstances it may, where required, be applied as an additional safety feature to low voltage equipment. Where applicable, the use of safety earths should be part of overall safe system of work, which will include isolation, locking off, permits to work or similar documents and liaison between parties in in control of the supplies and in control of the work. To ensure that a safe system of work is clearly set out, a set of detailed detailed rules and procedures will be necessary in each particular case’. 33.2 S a f e t y Earthing - When maintenance or repair work, etc, is to be undertaken on or near























IS x 3043 0 1987

damage. In no case, even for the earthing of light current current equipment ( for example, high voltage testing equipment ), should the cross-sectional cross-sectional area of the earthing lead be less than 6 mm. mm. It has been found in some cases that a 70 mm copper equivalent earthing lead is the largest that can be conveniently handled. In such cases, where a larger size of lead is necessary to carry with safety, the maximum short-circuit current that can occur, it may be necessary to ase a number of leads of 70 mm2 or other suitable size in parallel Before earthing leads are applied, applied, it should be verified that the circuit is dead and, where appliindicator should be applied ( the indicator itself being tested immediately before and after verification ) before applying earth connections. Earthing leads should first be efficiently bolted or clamped to the permanent earthing system or to a substantial electrode of low resistance. Should no convenient convenient permanent earth electrode be readily available, a substantial copper earth-spike driven well into the ground ground can be utilized to provide provide a quick and convenient temporary earth electrode. Whilst such a spike is not generally adequate as a primary safety earth, it will give a degree of

point of any transformer that is to be worked on should be disconnected and directly earthed as well as the phase terminals. When liquid earthing resistors are are to be worked on, particularly particularly when they are drained for work inside, the central electrode should be shorted to the tank and not earthed remotely. This is especially important where two liquid liquid resistors are located side-by-side and one remains in commission while while the other is opened for maintenance. maintenance. When work is is to be carried carried out on equipment equipment that is capable of capacitively storing electrical energy, for example, cables and capacitors, such equipment has to be discharged to earth prior to work commencing. As, in some circumstances, charge can reappear on such apparatus without reconnecting it to a source of supply, it is important work that the equipment should remain earthed whilst is in progress. The cuttmg of a cable cable during the course of work may disconnect conductors from safety earths and precautions should be taken to prevent this happening. Overhead Relating to 33.4 P r e c a u t i o n s L i n e s -After a line has been made dead, isolated, discharged and earthed at all points of supply, working earth should be securely attached to each phase of the line at the point or points where work

























wires. Where the earthing of low voltage conductors is adopted, then the general principles set out in 33.2, 33.2, 33.3 33.3 and 33.4 should be applied and due consideration should be taken of fault current levels ( which can be as nigh or higher than on high voltage systems ), when the size of earth conductor is chosen. 34. MAINTENANCE TRODES

OF

EARTH

justify increase or decrease& .&is interval should not be l ess than once in two yeara.

but it

electrode connection, connection, wherever wherever available, shall be carried out to ensure their rigidity and other signs of deterioration.

ELEC-

34.4.4 In rural substations, particularly those

34.1 It is recommended that periodical check tests of all eart h electrodes should be car ried out. Records should be maintained of such checks.

tage lines, greater reliance should be placed on the electrode system, and therefore facilitiei for testing the resistance of the electrode to general mass of earth, annually or as required by experience, experience, should be provided.

34.2 Where earth-leakage circuit-breakers are employed, a check shall be kept on the associated -earth-electrode by periodically operating the testing device which is embodied in the earthedleakage circuit-breaker.

34.4.5 Where installations are are earthed to metal sheath of the supply cable, it shall be verified eriodically thaS the earth -fault loop is in a satis-























IS

33.3 solid Earthing 33.3.1 In this case, the fault current can be

computed

from:

1s 36.

3E x1-t

MEASUREMENT RESlSTlVITY

x,+

&I

3XGP

OF EARTH

outer electrodes electrodes and theearth as shownin shownin Fig. 33 and the voltage difference. V, observed between the two inner electrodes. electrodes. .The current I flowing flowing into the earth produces an electric field field pr oportional to its ‘density and to the resistivity resistivity of the soil. The voltage voltage Y measured between the inn er electrodes is, therefor e, pr opor tional to the field. Consequently, the resistivity resistivity will will be pr opor tional to the ratio of the voltage voltage to curr ent. The following equation holds for:

36.1 Resistivit y of the Soil 36.1.1 The r esistivi esistivity ty of the earth varies within extremely wide limits, limits, bet ween 1 and 10 000 ohm metres. The r esistivi esistivity ty of the soil at many station sites has been found to be non-unifor m. Variation of the resistivity resistivity of the soil with depth is more red ominant as compared to the variation with orix ontal distances. ide variat variat ion of resistivity resistivity

36433643 - 1987

4sx

lf

-2s

v 2s

*/ s’ + 4 t?’ -44ss+4Cs

..V) where p -

resistivity resistivity

of soil in ohm-metre,























TS:3043-1997

instrument is thus selected selected to obtain clear readings avoiding the two ends of the scale as far as possible. The readings read ings are taken while turning the crank at about 135 rev/min. Resistivity is calculated by substituting the value of thus obtained in the equation (3). In case where depth of burial is more than 1120th of spacing, equation (1) should be used instead instead of (3). 36.4.2 Correction or Potential Electrode ResistanceIn cases where the resistance of the potential electrodes (the two inner electrodes) is comparatively high, high, a correction of the test results results would be necessary depending on its value. For this purpose, the instrument is connected to the electrodes electrodes as shown in Fig. Fig. 34. The readings are taken as before. The correction is then effected as follows.

36.5 T e s t i n g o f S o i l U n i f o r m i t y During the course of above tests, it would be desirable desirable to get information about the horizontal and vertical variations variations in earth resistivity over the site under under consideration for the correct computation ot’ the resistivity to be used in the design calculations. The vertical vertical variation may be detected by repeating repeating the tests at a given location in a chosen direction with a number of different electrode electrode spacings, increasing from 2 to 250 metres or more, more, preferably in the steps 2, 5, 10, 15, 25 and 50 metres or more. If the resistivit variations are within 20 to 30 percent, the soil in the vicinity of he test Iocation may be considered uniform. Otherwise a curve of resistivity resistivity VCISUS electrode spacing shall be plotted and this curve further analyzed to deduce stratification of soil 365.1























IS:3043-1987 uniform. When the spacing is increased gradually gradually from l ow values, at a stage, i t may be found that the resistivity readings i s more or less constant irrespective of the increase i n the electrode spactaken as the resistivity for that direction. In a similar manner, resistivities for at least eight equally spaced directions from the centre of the site are are measured. These resistivities are plotted on a graph sheet in the appropr appropriate iate directions choosing a scale. A closed curve is plotted on the graph sheets ointing all the resistivity points plotted to get the polar resistivity resistivity curve. curve. The area insi de the polar resistivity curve is measured and equivalent circle of the same area is found out. The radius of this equivalent circle is the average average resistivity of the the site under consideration. consideration. The average average resistivity thus obtained may be used for the design of

37.1.1 37.1.1 If the test is made at power frequency, that i s, 50 c/s, the resistance resist ance of the voltmete should be h igh compared to that of the auxiliary potential electrode B and in no case should be less than 20 000 ohms. In most cases, there wil l he stray currents Nc,TE flowing in the soil and unless some steps are taken to elimin ate their effe ct, they may produce produce serious error in the measured value. If the testing current is of the same frequency as the stray current, current, this elimination becomes very diff diff icult and It is better to use an earth tester mcorporating mcorporating a hand-driven generator. generator. These earth testers usually generate direct current, and have and synchronous rotary current-reverser rectifier mounted on the generator shaft so that alternating current is supplied to the test ci rcuit and the resulting potentials are rectified for measurement by a direct reading moving-coil moving-coil ohm-meter. The presence of stray currents in the soil is indicated by a wandering of the























TEST

ELECTRODE FIG.

38. MEASUREMENT IMPEDANCE

36

POTENTIAL ELECTRODE

CURRENT ELECTRODE

METHOD OF MEASUREMENTOF MEASUREMENTOF EARTH ELECTRODE RESISTANCE OF

EARTH

LOOP

as well as the transformer winding. To test test tht overall earthing for any installation, depending

























IS I 3043 - 1987

C,.bI.I#...L.._

~UlrMtN

EQUIPME NT

-

wurt~ EQUIPME NT a_..._























18 I 3043 - 1987

incorporated b) When incorporated

in a multicore cable together with the supply conductors, conductors, the sum total cross-sectional cross-sectional area of all the conductors conductors shall be not less than 1 $ mms and the protective protective conductors conductors shall comply comply with Section 2;

b) I f the requirements of (a) cannot b e met, the requirements of 39.2.4.3 39.2.4.3 shall apply. 39.3 A d d i t i o n a l Systems

Requirem ents

f or or

IT























S

b) prevents excessive touch voltages on the equipment and ensures equipotential between the equipment and adjacent metal work or other electrical equipment, under normal and fault conditions; and

confine the earth within the unit.

leakage

NOTE - No further are nvcessary.

current special

3043

- 1987

in conductors conductors

installation

measures























S

3843 - 1987

I























Bureau of In dian St andards

BIS is a statutory

institution established establish ed under the Bureau Bureau of Indim Standards Acf, Acf, 1986 to promote harmonious development of the activities of standardization, standardi zation, marking and quality certification of goods and attending to connected matters in the country.

Copyright

Is 3043 Revision 1 Earthing

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