Solution:

• Direct current is the most suitable for insulation resistance measurement.
• When DC is employed the value obtained is resistance since the capacitive and inductive effects of the DC current are nil.
• When AC is employed the value obtained will be impedance of the system.
• The resistance actually shows the healthiness of the insulator, not the impedance. So DC is most suitable.
• The diagram below shows the arrangement for insulation testing.

<img src=’https://cloudfront.entri.app/media/inline_images/2019/10/05/10-30-08/insulation_resistance.jpg’ style=’max-width: 100%;’ />