Question

For specific wiring in a building, the full load current is 7.5 A. what will be the permissible insulation resistance to earth for a 240 V system of supply?
1. 0.20 MΩ
2. 0.18 MΩ
3. 0.16 MΩ
4. 0.22 MΩ

Answer 1

Correct Answer - Option 3 : 0.16 MΩ

Concept:

Full Load Current(IFL): A full load current is a maximum current that electrical equipment is designed to carry under rated conditions.

Leakage current(Ilek): A leakage current is an electric current in an unwanted conductive path under normal operating conditions.

Note: According to IE Rule 48 The maximum permissible value of leakage current should not be exceeded beyond the \(\frac{1}{{5000}} \)times of full load current.

Insulation Resistance(IR): An insulation resistance (IR) is the total resistance between any two points separated by electrical insulation.

It is given by,

\(IR = \frac{{Supply\;Voltage}}{{{I_{lekage}}}}\)

Note: IR of a cable is also given as:

\(IR = \frac{\rho }{{2\pi l}}{\log _e}\frac{{{r_2}}}{{{r_1}\;}}\)

Where ρ is the resistivity of cable on Ωm

l is the length of cable in the metre.

r1 is the radius of core in metre

r2 is the radius of cable in metre

Calculation:

Given, I_FL = 7.5 A

According to IE Rule 48:

\({I_{lek}} = \frac{{{I_{FL}}}}{{5000}} = \frac{{7.5}}{{5000}} = 0.0015\;A\)

∴\(IR = \frac{{supply\;voltage}}{{{I_{lekage}}}} = \frac{{240}}{{0.0015}} = 160,\;000 = 0.16\;M{\rm{\Omega }}\)