(i) Insulation Resistance Test
This test most often conducted to determine the quality of High voltage cable insulation is the insulation resistance test. It is performed as a routine maintenance test for cables already in service or as an acceptance test for new cables. DC over potential testing is another way of testing cable insulation. This test is performed primarily on medium and high voltage cables to test their dielectric strength and is not recommended for routine maintenance testing of low voltage cables. It is a simple, quick, convenient and nondestructive test that can indicate the contamination of insulation by moisture, dirt or carbonization. Before testing any cable, the circuit must be de-energized. Once that is done, it is usually best to disconnect the cable at both ends in order to test only the cable, and to avoid error due to leakage across or through switchboards or panel boards. For an acceptance test, cable less than or equal to 300 V may be tested at 500 V and cable greater than 300 V but less than 600 V may be tested at 1,000 V. For a routine maintenance test, test voltage should be restricted to 80 percent of the factory test voltage. The test voltage should be applied from phase to ground on each conductor with the shielding tapes and metallic jackets also connected to ground. While no general standard exists for minimum acceptable insulation resistance values for cables in service, a “rule-of-thumb” of one mega ohm of resistance (minimum) per 1,000 V of applied test voltage is accepted.
(ii) High Potential Test
The objective of the test is to ensure the insulation level of cable. For conducting the test any Two Phase are short using 4sqmm and it is earthed, another one terminal is connected to the test kit , Form the test kit one terminal is connected to the earth. Before conducting the test Insulation Resistance test has to be conducted to ensure the insulation resistance of the cable In the DC test, the voltage is quickly raised from zero to the specified level. The test is usually held for one minute. The current is measured in mill amperes. The reading of current is provided so that the person running the test can determine if the particular test set has sufficient capacity for the task at hand. If the cable withstands the one minute application, the test has been passed.
Failure results in short circuit and a ruined portion of the cable. The test set is designed to trip off immediately upon detection of the fault current. After conducting the HIPOT test, the cable has to be re-checking the Insulation Resistance, to conform the cable has no damage due to HIPOT test. The same test has to be conducted on all the terminal of the cable.