Page 140 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Assuming that the sum of the resistace of the main cable, metal body, earth continuity conductor and the general
mass of earth is to the tune of 10 ohms
V
The leakage current = = 240/10 = 24 amps.
R Total
This leakage current is 4.8 times higher than the fuse rating, and, hence, the fuse will blow and disconnect the
supply from the mains. The person will not get a shock due to two reasons. Before the fuse operates, the metal
body and earth are in the same zero potential, and across the person, there is no difference of potential. Within
a short (milli-seconds) time the fuse blows to open the defective circuit, provided the earth circuit resistance is
sufficiently low.
By studying the above two cases, it is clear that a properly earthed metal body eliminates the shock hazards to
persons and also avoids fire hazards in the system by blowing the fuse quickly in case of ground faults.
Types of earth electrodes
Rod and pipe electrodes (Fig 4): These electrodes shall be made of metal rod or pipe having a clean surface
not covered by paint, enamel or other poorly conducting material.
Rod electrodes of steel or galvanised iron shall be at least 16 mm in diameter, and those of copper shall be at
least 12.5 mm in diameter.

Pipe electrodes shall not be smaller than 38 mm internal diameter, if made of galvanised iron or steel, and 100
mm internal diameter if made of cast iron.

Fig 4

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