It is highly recommended and often mandatory to perform a VLF test on all HV cables following jointing or prolonged outages. This is done to prove the integrity of the cable prior to system energisation thus avoiding a high power failure of the cable when first connected to the system. Megavar recommends and performs regular routine maintenance testing of aged cables.
This has been demonstrated to reduce the chance of unscheduled failure to approximately 10% of the failure rate that might be encountered without such testing. Approximately 12% of cables fail during the test and cable repair can then be done in a planned manner.
Alternative cable diagnostic techniques may be preferred however this may not be viable on circuits with combinations of different types of cable, very long cable runs, and network cables with multiple taps