When a voltage stress (proof test) is placed across a cable’s insulation, partial discharge can occur in defect locations if the voltage stress is sufficient to initiate the partial dischaarge, which is the purpose of a VLF AC voltage stress test.
If we know how fast electrical trees grow under a given voltage stress, then we know how long the voltage must be applied for significant defects that are triggered into partial discharge to grow to failure during the test.
The cable is repaired, retested, and once passing a VLF test, will deliver years of trouble free service. Proper VLF testing exposes serious cable defects, thus preventing in-service failures, while not harming healthy insulation nor aggravating minor defects that will not cause cable failure for many years.
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