Insulating resistance: insulating resistance is the opposition offered by an insulating material to the flow of current (electrons) through it when an high potential is applied across it.
Insulating resistance are measured by megger.
First the equipment whose resistance is to be measured is disconnected from supply. If the machine is a large one, there may be accumulated static charge on the machine. So we have to discharge it by connecting a wire between the terminals and ground for 15 minutes. Otherwise megger will give wrong reading.
After this we should remove the wire and we have to connect megger terminals (live & earth) to the motor terminal and earth. The rating of the megger should be selected properly. Then rotate the megger at rated speed of 160 rpm and take the readings.
