Help me understand how a megger works

[Sanjayo]

Is it safe to do a 1000V insulation resistance line-to-load on a circuit breaker without affecting the equipment downstream? As far as I understand you have +500V on one side and -500V on the other, if you did 1000V across the breaker would that send voltage downstream on the load cable?

[a.lallana@epsii.com]

Is it safe to do a 1000V insulation resistance line-to-load on a circuit breaker without affecting the equipment downstream? As far as I understand you have +500V on one side and -500V on the other, if you did 1000V across the breaker would that send voltage downstream on the load cable?

Yes that is correct when you are meggering (injecting voltage) you are injecting voltage thru anything that conducts electricity on that circuit. you have to make sure you isolate anything that can be damaged, including electronics, people, and on breakers a lot of newer ones especially have Plugs for their Trip unit that will isolate it so it will not get damaged. but a 480v breaker should get tested at 1kv.

it is okay but not a great practice to test a breaker installed because you aren't really testing the insulation resistance of the breaker you are testing the insulation with least resistance which most likely is going to be a small cable downstream or some dirty insulators, so in theory if you test everything as is connected and it is acceptable resistance it is proving that the breaker is good.

in addition to this as a personal procedure i do is that if you are going to test a breaker installed you start with the megger in the lowest setting and verify the resistances is adequate and there are no shorts, then move to a higher and higher voltage until you reach the test voltage needed to test the breaker. i hope this helps.