Electrical Safety and Earthing Questions and Answers

Answer: C

The leakage current must not exceed 1/5000 of the full load current.

Rule 48 of the Indian Electricity Rules, 1956 deals with the insulation of installations: the insulation resistance between conductors and earth must be such that the leakage current does not exceed one five-thousandth of the maximum current supplied to the installation. With a full load current of 50 A, for example, the leakage may be at most 10 mA.

1/1000 and 1/2000 would allow far more leakage than the rule permits, and 1/10000 is stricter than the rule requires.

Answer: B

The earth resistance of a large power station should be about 0.5 Ω.

The earthing of a station must carry very large fault currents to ground while keeping the voltage rise of the earthed metalwork, and the step and touch voltages around it, low enough to be safe. The larger the fault level, the lower the earth resistance has to be, so large power stations are designed for about 0.5 Ω or less.

About 1 Ω is the usual figure for major substations, about 2 Ω for smaller substations, and higher values such as 5 Ω are acceptable only for small installations.

Answer: A

The earth wire is made of galvanized steel.

The earth (ground) wire runs along the tower tops above the phase conductors to shield them from lightning, and it also earths the towers. It carries no load current, so conductivity matters little, while strength over long spans and resistance to corrosion matter a great deal. Galvanized (zinc-coated) stranded steel is strong, cheap and rust resistant.

Copper and aluminium are good conductors but costly and weaker for this duty, and plain iron would rust quickly in the open.

Answer: D

Danger notices are covered by IE Rule 35.

Rule 35 of the Indian Electricity Rules, 1956 requires the owner of every installation of medium or higher voltage to fix a permanent danger notice, in Hindi or English and the local language, with the sign of skull and bones, on each support of an overhead line and at other places where live parts are accessible.

The neighbouring rules deal with other matters: Rule 42 with accidental charging of metal work, Rule 43 with protective equipment such as fire buckets and first-aid boxes, and Rule 45 with precautions to be adopted by consumers, owners and electrical contractors.