Page 108 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Earth continuity test-V
• AIM:For safety all the metal pieces or covering such as conduit, metal covers of switches etc must be solidly
connected to earth otherwise on damage of insulation, the leakage current will start giving severe shock to the
person touching it.
This test is carried out with the help of ‘EARTH MEGGER’.

Earthing
Earthing systems

Earthing
Earthing means an electrical connection to the general mass of earthto provide safe passage to fault current to
enable to operate protective devices and provide safety to personnel and Equipment.

Purpose of earthing
1 To save human body from danger of electric shock or death by blowing fuse, ie ,to provide an alternative path
for the fault current flow so that it will not danger user

2 To protect buildings, machinery and applications under fault conditions ie, to ensure that all exposed conductive
parts do not reach a dangerous potential
3 To provide safe path to dissipate lightning and short circuit currents
4 To provide stable platform for operation of sensitive electronic equipment’s, ie to maintain the voltage at
any part of an electrical system at a known value so as to prevent over current or excessive voltage on the
appliances or equipment’s.

5 To provide protection against static electricity from friction.
An electric shock is dangerous only when the current flow through the body exceed beyond a certain milli ampere
value.
The degree of danger is dependent not only current flow but also the time during which it close.

This table shows the body resistance at specified areas of contact:
Skin condition area Resistance value

Dry skin 100000 to 600000 ohm
Wet skin 1000 ohm
Internal body to hand 400 – 600 ohm
Ear to Ear About 100 ohm

95

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