ACCORD & BNBC Stated about Earthing:
10.10 Earthing
10.10.1
General.
In
general all parts of equipment and installation other than live parts shall be
earth potential, thus ensuring that persons coming in contact with these parts
shall also be at earth potential at all times.
10.10.2 Circuit and System Earthing.
10.10.2.1 Circuit and system earthing shall limit excessive voltage from line surges from cross-overs with higher voltage lines or turn
lighting and keep non-current carrying enclosures and equipment at zero potential
with respect to earth.
10.10.2.2 The value of
the earthing resistance shall be in accordance with the protective and
functional requirements of the installation and be continuously effective.
10.10.2.3 Where a
number of installations have separate earthing arrangements, protective
conductors running between any two of the separate installations shall either
be capable of carrying the maximum fault current likely to flow through them or
be earthed within one installation only and insulated from the earthing
arrangements of any other installation. In the latter circumstances, if the
protective conductor forms part of cables the protective conductor shall be
earthed only in the installation containing the associated protective device
10.10.3 Methods of Earthing.
10.10.3.1 General.
The three main elements required for an
earthing system are earth conductors, earthing lead and earth electrodes.
10.10.3.2 Earth Conductors.
10.10.3.2.1
Earth conductors are the
part of the earthing system which joins all the metal parts of an installation.
10.10.3.2.2 In all cases the grounding conductor shall be made of
copper or galvanized steel or other metals or combination of metals which will
not corrode excessively and, if practical, shall be without joints or splice.
If joints are unavoidable, they shall be made and maintained so as not to
materially increase the resistance of the earthing conductor and shall have
appropriate mechanical and corrosion resistant characteristics.
10.10.3.2.3 Aluminium or copper clad aluminium conductors shall not
be used for final connections to earth electrodes.
10.10.3.2.4 The earth conductor shall have a short time capacity
adequate for the fault current which can flow in the grounding conductor or
conductors for the operating time of the system protective device. In case of
copper wire being used as earth conductors, the size of the wire shall not be
less than half the area of the largest current carrying conductor supplying the
circuit.
10.10.3.2.5 BNBC Table 8.2.11 gives the minimum sizes of copper earth
conductors corresponding to the sizes of associated copper circuit conductors.
No size smaller titan 14 SWG shall be used anywhere as earth conductor.
BNBC Table 8.2.11
Minimum Cross-sectional Area of Copper Earth Conductors in Relation to the Area
of Associated Phase Conductors
|
Cross-sectional Area of Phase Conductor(s)
(mm2)
|
Minimum Cross-sectional Area of the Corresponding
Earth Conductor (mm2)
|
|
Lessthan16
16orgreaterbutlessthan35
35orgreater
|
Same as cross-sectional area of phase
conductor but not less than 14SWG
16 Half the cross-sectional area of phase conductor
|
10.10.3.3 Earth Lead.
10.10.3.3.1 The earth conductor shall
be brought to one or more connecting points according to size of installation;
the copper wire earthing leads shall run from there to the electrodes.
10.10.3.3.2 Earthing lead can either be of copper wire or of
copper strands.
10.10.3.3.3 Earthing leads shall be run in duplicate down to the
earth electrode so as to increase the safety factor of the installation. Copper
wire used as earthing lead must not be smaller than 8 SWG (12 mm2).
10.10.3.4 Earth Electrodes.
10.10.3.4.1 The earth electrode shall
as far as practicable penetrate into permanently moist soil preferably below ground
water table. The resistance of the electrodes shall not be more than one ohm.
10.10.3.4.2 The following types earth electrodes are recognized: Copper
rods, copper plates, galvanized iron pipes.
10.10.3.4.3 The following is a guideline for electrode size:
Copper rods shall have a minimum diameter of 12.7 mm,GI pipes shall have a
minimum diameter of 50mm, copper plates shall not be less than 600
(NOTE: The whole topic has been taken from ACCORD book)
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