Answer Posted / vineeth kumar c g

in the case of insulation tester (megger) we need to take care of cable insulation resistance not insulation impedance, so we choose DC rather than AC.
In hipot test we give high voltage to the cable and if cable is twisted type then AC will induce an emf due to flux and this will change the value of impedence... (if you only need to measure resistance only)
skin effect may occurs if the frequency is higher than 50 and long length cables.
normally in site DC voltages are used for testing but in manufacturing industry they also use AC tester to calculate impedence also...

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