Page 139 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Fig 1

Table 1 shows the body resistance at specified areas of contact.
Table 1

Skin conditon or area Skin conditon or area
Dry skin 100,000 to 600,000 ohms

Wet skin 1,000 ohms
Internal body-hand 400 to 600 ohms to foot
Ear to ear about 100 ohms

CASE 1:Metal body of apparatus when it is not earthed
Let us consider a 240V AC circuit connected to an apparatus having a load resistance of 60 ohms. Assume that
the defective insulation of cable makes the metal body live and the metal body is not earthed.

When a person, whose body resistance is 1000 ohms, comes in contact with the metal body of the apparatus
which is at 240V, a leakage current may pass through the body of the person (Fig 2).
The value of current through the body = V
R Body
240
= = 0.24 amps or 240 milliamps.
1000
This current, as can be judged from Table 1, is highly dangerous, and might prove to be fatal. On the other hand,
the 5 amps fuse in the circuit will not blow for this additional leakage current of 240 milliamperes. As such the
metal body will have 240V supply and may electrocute any person touching it.
CASE 2: Metal body of apparatus when earthed.
In case the metal body of the apparatus is earthed ( Fig 3), the moment the metal body comes in contact with the
live wire, a higher amount of leakage current will flow through the metal body to earth.

Fig 2 & 3

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