If the wiring in a building has a 2.4 kW load, what will be the permissible insulation resistance to earth for a 240 V system of…

Question

If the wiring in a building has a 2.4 kW load, what will be the permissible insulation resistance to earth for a 240 V system of supply?

1. A. 0.08 MΩ
2. B. 0.05 MΩ
3. C. 0.02 MΩ
4. D. 0.12 MΩ (Correct answer)

Correct Answer

Option D — 0.12 MΩ

Detailed Solution & Explanation

The correct answer is 0.12 MΩ.

Key Points

• Full-load current of the installation: I = P / V = 2400 / 240 = 10 A.
• Indian Electricity Rules permit a leakage current no greater than 1/5000 of the maximum supply current: 10 / 5000 = 0.002 A.
• Required insulation resistance: R = V / I_leak = 240 / 0.002 = 120,000 Ω = 0.12 MΩ.

Additional Information

• The insulation resistance of a completed installation should in practice never fall below 1 MΩ, which is the value normally quoted for acceptance testing.
• Insulation resistance is measured with a megger, which applies a high DC test voltage (typically 500 V or 1000 V) between conductors and earth.