Millivolt Reading

Bkessler

How does a breaker leak millivolts in the open position? And how accurate is this when testing breakers in a residential situation? Also is it good practice to test all of the breakers in a homes panel and replace ones with a high milivolt reading?

mdshunk

They don't. You're measuring 'phantom voltage' in this case.

Are you talking about a Fall Of Potential (FOP) test, maybe, where you measure millivolts from line to load on a circuit breaker in the closed position? If so, that's not leakage, that's resistance, and the FOP test is a test that has some value.

Bkessler

So the other day I was testing breakers with them open from ground to the load side of the breaker all showed about 0-8 millivolts and then had one with 30, I took it off the bus bar and it was showing the usual signs of overheating. Pitted and discolored. An old employer called this a voltage leak test. So what exactly am I looking at and is this good practice for every panel I stick my nose in?

mdshunk

I maintain that it was only a coincidence that the one with the 30 volt reading was also overheated. There are many other reasons for voltage to ground at the terminal on the load side on an open breaker. "Leakage" is dead last on the list.

If you were using a DMM, your numbers you thought you measured were totally bogus: http://www.nema.org/stds/eng-bulleti...ulletin-88.pdf

If you want a "real" test, you measure millivolts with the breaker "ON" from the lug feeding the panel bus to the branch breaker's load side terminal. This is called a Fall of Potential test. The higher the millivolts, the more resistive connections are (both inside the breaker package and the breaker-to-bus connection).

Bkessler

That makes sense, do you do this test everytime your working in a panel. And do you recommend replacing breakers from this test alone? And also what is to high of a reading or do you just compare to other readings in the same panel. Thanks bk