Page 109 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Earth resistance required as per is specification

Distance of earth from building - 1.5 meters
Size of earth continuity conductor (ECC) – 2.9 mm2 (14 SWG) or half the installation conductor size
Earth resistance
• Large power station – 0.5 ohm
• Major power station – 1 ohm
• Small power station – 2 ohm

• In all other cases – 5 ohm
X section of earth lead should not be less than 161 mm2 for main connections & 64 mm2 for branch connections
in large connections

Copper strip of 25 mmx3 mm or 25mmx6 mm for connecting electrical apparatus
Copper wire of size not less than 8 SWG (12.9 mm2) & not more than 7/0.0915 MM (64.52 MM2) used in case
of smaller installations

Size of earth wire for domestic installations should be not less than 2.9 mm2 (14 SWG) or half the conductor size
Various specifications in respect to earthing as recommended by Indian Standards are given below.
• An earthing electrode should not be situated (installed) close to the building whose installation system is being
earthed at least more than 1.5m away.
• The value earth resistance does not remain constant but changes with the weather, as it depend upon the
moisture content of the soil
• The earth wire and earth electrode will be the same material.
• The earthing electrode should always be placed in a vertical position inside the earth or pit so that it may be
in contact with all the different earth layers.The cross section of earth lead of copper wire should not be less
than 8swg in case of small installations or half of the cross section of the largest current Carrying conductor.
• The cross of ECC should not be less than either 14SWG or half of the largest conductor.
• The neutral conductor of a three phase four wire system is earthed at generation and substations.
Terms used in earthing system

Earth
A connection to the general mass of earth by means of an earth electrode. An object is said to be earthed when
it is electrically connected to an earth electrode.

Earth continuity conductor
Any wire, clamp, cable, cable’s metallic covering, earth terminal of an electrical installation which is connected to
the earth electrode is earth continuity conductor.
Earth electrode

A metal plate , pipe or other conductor or an array of conductors electrically connected to the general mass of the
earth.
Earth terminal

A terminal provided on a piece of apparatus for the purpose of making a connection to earth.
Earth lead
The conductor by which the connection to the earth electrode is made.
Types of earthing
There are two major types of earthing
1 Systemearthing
2 Equipmentearthing

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CITS : Power - Electrician & Wireman - Lesson 20-25