Not much. DC can no longer be used. It damages cable insulation and tells little about the cable insulation quality. Recognizing the problems associated with DC testing, many utilities have reduced their test voltages from 45 – 50 kV DC to 15 kV DC on 15 kV cable. 15 kV DC is barely above the peak AC stress a cable experiences in service and far below the twice-normal voltage occasionally experienced due to transients. The test is not meaningful.

Some have tried to develop partial discharge detection methods for field-testing cable insulation, with the goal being to determine the insulation quality and location of defects without risking failure during the test. None of the methods employed to date have proven very effective. PD testing may work fine on substation apparatus but it has severe limitations for cable testing. Experience has shown that PD testing lacks reliability, with results often times unrepeatable due to the many variables involved with cable testing: temperature, moisture, number and types of accessories, various cable types spliced together, and operator interpretation. The results are highly interpretive. Also, the equipment and/or the service is very expensive.

Many utilities have wasted hundreds of thousands of dollars hiring PD testing services, only to replace good cables and splices while leaving bad cable in the ground to soon fail.

Tan Delta, or dissipation factor, testing (analogous to power factor) is performed, where a VLF unit is used to energize the cable and loss readings are measured. This method works well, so long as there are few splices, joints, etc. along the cable run and there is only one type of cable under test, which is usually not the case since many cable runs are of mixed cable types, all with a different loss factor. This method is easy to perform with minimal training and allows users to grade their cables’ level of deterioration. If done properly, it is a non-destructive diagnostic test. High Voltage, Inc. does offer Tan Delta.

The surest way to weed out bad cables and accessories is to just perform a simple AC hipot test, just like we do with vacuum bottles, arrestors, hot sticks, switchgear bus and insulators, etc. Yes the cable may fail under test if it has a severe defect, but that’s the point of the test. If a cable can’t withstand three times normal voltage for 10 or 15 minutes, it’s bound to soon fail. Fail it when convenient, rather than waiting for it to fail on its own, often at the worst possible time.