How to make insulation resistance test ?
Channel: Alfanar Engineering Services
Date: 08/03/2016

The insulation resistance test is done to make sure that 
a. The insulation can withstand a constant stress for a specified time without breaking down.
b. Measure the insulation resistance which shall be in the range of MΩ, GΩ or TΩ
DC voltage is used instead of AC voltage as the DC represents a constant stress on the insulation. But AC is a constantly varying stress, which allows the insulation to recover to some extent when the voltage falls into the negative portion of the waveform. DC stress is constant, and will not allow the stress on the insulation to recover until the stress is removed it or it breaks down.

Tools : CHAUVIN ARNOUX CA.6545 Megameter . 
you can get more details about it via : 
http://www.chauvin-arnoux.com/en/produit/c-a-6545.html