Why we use a DC voltage testing device to test an electrical power cable instead of AC testing device?

thanks for investigation, that is depending on type of test

like impulse test "AC", Megger is DC....

Normally we use DC for cable insulation testing, its also used for generator short circuit test. AC may also be used for this test but DC is more effective, If we test using AC, we need to provide high voltage at a longer time wherase DC has more impact on cable because it produces a constant stress on the cable, and it is advised not to prolong the test as we do in case of AC supply cable testing.

if you are using analog meter for testing supply voltage, then no power is required (ac or dc)

for digital meters it required DC power to process the task  ....  :)