SOLUTION

(a) The direct deflection method is used for the measurement of the insulation resistance of the cable. Following precautions should be taken while using the direct deflection method.

(b) Before testing the cable is immersed in water at least for 24 hours. The temperature of the water is maintained constant. The water enters through the pores if any, and soaks through the defects, and at the same time, the cable attains the temperature of the water.

(c) In the earlier phase of the measurement, the galvanometer must be properly shunted. It includes a series resistance of high value. The true value of insulation resistance can be determined by subtracting the value of series resistance from the observed resistance.

(d) While conducting tests on cables the galvanometer should be short-circuited before applying the voltage. The short-circuiting connection is removed only after sufficient time has elapsed so that charging and absorption currents cease to flow.

(e) After the application of voltage, a short circuit connection across the galvanometer is removed. Thus galvanometer is protected from the sudden initial spike of current which charges cable which acts as a capacitor.

(f) The battery voltage should be around 500 V and should be maintained constant. The galvanometer should be highly sensitive (not less than 1000 mm per microampere at a scale distance of 1 metre), should have high resistance and also that its deflection should be directly proportional to the current flowing through it and should be critically damped.

(g) To prevent leakage currents, the galvanometer circuit switches and circuitry must be well insulated.