Test of Cables

• In the test circuit a special transformer is used.
• This transformer has primary 230 V to 440 Volts rating when secondary voltage is of 5 to 15 volts.
• The test voltage is adjusted by regulator.
• By regulating the voltage a required continuous high current is sent to the test terminals.
• The high current is measured by C.T. Current produces heating effect and temperature increases. Temperature (T) is measured by thermocouple or thermometer.
• Ambient temperature is known (T_a).

∴ Temperature rise = T – T_a is found.

• The rated voltage with rated frequency is applied in the circuit.
• A continuous current is passed till steady temperature is reached. The readings of temperature of various conducting , insulating and structure parts are taken every after 1/2 Hr or 1Hr.
• The maximum temperature of the parts shall be within the prescribed limits.
• The thermocouple comprises a junction of two dissimilar materials.
• The emf induced depends upon temperature difference between the hot junction and the cold end.
• The hot junctions on fixed on the parts of isolator (contacts of isolators, terminals insulators etc.)
• The output of thermocouple is measured by potentiometer or digital voltmeter.
• The temperature is calculated for the calibrated value (°C per Milivolts)

2. Life Expetancy Test

1. Normal Stress Condition
2. Increased Stress Condition

The time duration for these test may vary from few hrs to 1000 hrs. A mathematical expression is derived from experimental test data. Ems = K. t -(1 / n)

Where Maximum voltage stress across the cable not

K = Constant which depends on type of material used for cable and the field conditions

t = Life of cable insulation

n = Life index