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Why the AC Test Lasts One Minute, and When DC Is Allowed
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The one minute duration in dielectric testing comes from a compromise between two failure modes of the test itself. Too short and a slow partial discharge has no time to develop into a measurable event; too long and the test begins to age the insulation it is supposed to verify.
What the hold does
During the hold, leakage current stabilises as capacitive charging completes and as any moisture in the insulation polarises. A defect that will fail in service usually shows as an increasing current during the minute or as a sudden collapse. A single instantaneous flash test would miss the first pattern entirely.
When DC is permitted
DC testing is allowed where AC testing would stress capacitive components that cannot withstand the AC current, typically large filter capacitors or equipment with EMC filters to earth. The DC test voltage is higher than the AC value by a factor related to the peak of the AC waveform, and the hold is longer, because a DC test does not exercise the insulation in the same way.
Why the substitution is limited
A DC test does not stress a wet surface in the same manner as AC, and it does not reveal a defect that depends on the alternating field. Where a standard permits DC as an alternative, it does so for a defined class of equipment; using it as a general convenience weakens the evidence, and a test house will usually ask why.
