Page 138 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Earthing - Types - Terms - Megger - Earth resistance Tester

Objectives: At the end of this lesson you shall be able to:
• explain the reasons for system and equipment earthing
• define the terminalogy related to earthing
• state and explain the methods of preparing pipe earthing and plate earthing, according to B.I.S.
recommendations
• explain the procedure for reducing the resistance of earth electrodes to an acceptable value.

Earthing
Connecting the non-condutive metal body/parts of an electrical equipment and system to the earth through a low
resistance condutor is called as earthing.
Earthing of an electrical installation can be brought under two major categories.
• System earthing
• Equipment earthing
System earthing: Earthing associated with current-carrying conductors is normally essential to the security of
the system, and is generally known as system earthing.

System earthing is done at generating stations and substations.
The purpose of system earthing is to:
• maintain the ground at zero reference potential, thereby ensuring that the voltage on each live conductor is
restricted to such a value with respect to the potential of the general mass of the earth as is consistent with
the level of the insulation applied
• protect the system when any fault occurs against which earthing is designed to give protection, by making the
protective gear to operate and make the faulty portion of the plant harmless.
Equipment earthing: Earthing of non-current carrying metal work and conductor which is essential for the
safety of human life, animals and property is generally known as equipment earthing.
Terminology
Trainees can be instructed to refer the international electro technical commission (IEC 60364-5-54)
website for the standard safety rules related with earthing installation for the further details.
Dead: Dead’ means at or about earth potential and disconnected from any live 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; and a conductor is said to be `solidly earthed’ when it is
electrically connected to an earth electrode.
Earth-continuity conductor (ECC): The conductor which connect the non-conductive metal part/body of an
electrical system/equipment to the earth electrode is called as earth contained conductor.
Earth electrode: A metal plate, pipe or other conductor electrically connected to the general mass of the earth.
Earth fault: Live portion of an electrical system getting accidentally connected to earth.
Leakage current: A current of relatively small value, which passes through the insulation of conductive parts/
wire.
Fig 1 shows the magnitude of current and its effect
Reasons for earthing: The basic reason for earthing is to prevent or minimize the risk of shock to human beings
and livestock. The reason for having a properly earthed metal part in an electrical installation is to provide a low
resistance discharge path for earth leakage currents which would otherwise prove injurious or fatal to a person
or animal touching the metal part

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