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#26569 - 06/15/03 01:21 PM Megging circuit breakers
Elzappr Offline

Registered: 12/20/01
Posts: 273
Loc: Oregon
Anyone know the scoop about how to safely meg a circuit breaker? I often wonder if all the electronics that are part of circuit breakers these days would mess up megger readings. Suppose, for example, someone wants to meg out a circuit from a supply bus bar, through a distribution panel circuit breaker, through the feeder conductors..stopping at the open branch circuit breaker. Is this a practice which might show misleadingly low resistance numbers due solely to the feeder circuit breaker's electronic trip package?
Are there limits to how high the test voltage should be? Would a 1kV test voltage mess up, or interact with the trip unit, in a 240 volt breaker?

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#26570 - 06/15/03 04:09 PM Re: Megging circuit breakers
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
It’s probably a little more common to use 500VDC on 240/208V stuff. As for readings, it’s primarily a matter of experience, and comparing pole-to-pole readings for lack of any IR history. In electronic-trip molded-case breakers, if there is no external communications or relaying, the meg tests generally shouldn’t damage anything—but get your mitts on the breaker instruction pamphlet for any warnings.

Biddle sells a booklet titled “A Stitch in Time” that covers the subject in a practical manner.

Obviously, megging a VFD would be suicide.


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