How is the choice made between HPAC and HPDC?
There are two tests on the AT3600. An ac one "HPAC" and a dc one "HPDC"
Either of these tests will check the insulation of a transformer to make sure that one winding is insulated from another. Typically, the insulation of all primary windings to all secondary windings and all primary windings to core is checked to guarantee that the transformer is safe when connected to the ac line.
In most cases, ac Hi-Pot HPAC is used since this is the convention and it is easy to generate high ac voltages.
For example, 4kV ac may be the specified test voltage. One terminal is LO or 0V, the other is 4kV ac which means a sinewave of + 5.6kVpk to -5.6kVpk is applied across the windings.
Sometimes a dc Hi-Pot test is specified. This may be because the capacitance (between windings) of the transformer is large and would draw ac current, making the measurement of ac Hi-Pot false. To use a dc Hi-Pot test, the equivalent PEAK voltage is chosen. In this example, the test would be 5.6kV dc (or greater). By default, the AT3600 supplies a negative (-) voltage.
It does not matter if this is +5.6kV or -5.6kV. The electric potential difference applied is the same.
A benefit of the AT3600 is that you can swap HI and LO terminals if required and reverse this voltage at any time if required.
Please find attached Editor screen shots