The relationship between the withstand voltage level using high direct voltage and the equivalent withstand voltage using power frequency voltage cannot be precisely stated because the relationship is composed of many factors.

The lack of a precise equivalence should not cause concern because the purpose of proof tests is to demonstrate that the insulation can withstand the overvoltages to be expected in service rather than to establish the precise value of electrical strength. The electrical strength has been found in cases studied to be associated with impulse strength.

Therefore, a direct voltage proof test may indicate ability of the insulation to withstand surges and short-time overvoltages approximating the same peak value. The test overvoltage value also provides for insulation deterioration in a further period of operation.

The proper high direct voltage proof test for insulation need not necessarily be related to the corresponding power frequency voltage proof test by the ratio of the electrical strength of sound insulation under power frequency voltage stress to that under direct voltage stress.

Some investigators point out that until a known equivalence can be established, the direct voltage test cannot be considered comparable in searching ability to the established power frequency voltage tests. Direct voltage acts to search out a faulty area in the insulation by establishing a leakage current from that area.

Although small currents may aggravate damage and lead to breakdown if the voltage is raised to a high enough level, this usually does not occur unless the weakness is significant and should be found. High temperature of the insulation usually increases the conductance of any solid insulation remaining in the fault path; dc conduction in fissures, however, may be reduced rather than increased by an increase in temperature.

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