Insullation resistance test

[Fredre]

Good day.
I have a question, if I may. Before doing an Insulation resistance test, one needs to make sure all the equipment is safe, right?
So here is my question, does one go around the entire plant, factory, house or building, and disconnect all equipment like PC's, light bulbs, dimmer switches, burglar alarms, etc.. Surely you will miss something and then run the risk of blowing it up. Could someone please explain the procedure for me of preparing the site for meggar test.
Many thanx in advance.
Kind regards.
Fredre

 

[big john]

Our standard procedure is to review the prints to see what is supposed to be connected to a feeder we want to test. We compare that to what equipment we see in the field. Then we open all breakers and remove all fuses connected to that feeder, including instrument transformer and surge protection equipment.

Then you do a phase to ground test at less at less than the 1-phase system voltage so 100V for a 208V system, 250 for a 277V system, etc. High numbers don't guarantee that you found everything connected to phase under test, but low numbers or a ground fault definitely indicates you need to do some more searching.

Lastly we do a test at full voltage.

But to be sure, we almost never meg branch circuits unless we're trying to find a fault, so we wouldn't be concerned with things like GFCIs. If you have to meg a conductor with a bunch of electronics like that hard-wired to it you need to stay below the nominal operating voltage. Even then the odds are good you will read through a connected load that will mess up your test, so you likely just have to unwire everything that could be damaged

 

[ron]

Do you want to test the whole building at once? Usually it is one feeder or one circuit at a time, and you confirm isolation from things that need it, as you go to each.

 

[zog]

I think every testing guy has fried a piece of equipment or two at some point.

 

[mbrooke]

One good time saver is doing at the branch circuits at once then isolating the one which did not pass.