How to Check Insulation Resistance

Attach the one of the megohmmeter's leads to the device to be tested. Attach the other lead to the device on the opposite side of the insulation barrier. If your megohmmeter has a third lead, attach it to any component of the device you wish to exclude from this resistance test.

Select a low voltage on the megohmmeter unless you know the item to be tested can handle a higher setting. Press and hold the "Test" button for one full minute. Watch the reading on the display throughout the test. If you are testing a large insulation barrier, the megohmmeter's reading should drop during the test. The reading for a smaller barrier should stay consistent during the entire minute.

Compare your results with the specifications in your device's owner's manual to determine whether the insulation falls within the recommended range. You may also be able to find the acceptable range in the Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems booklet offered by the InterNational Electrical Testing Association (NETA).