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Breakers not tripping

3192 Views 7 Replies 5 Participants Last post by  brian john
What would cause a breaker not to trip a on short circuit? HO calls said she just cut through a wire and it popped but did not trip any breakers. Can I add to this, I had to leave and could finish my thread. The same week I was working in a small machine shop and was putting cover back on 4x4 j-box and I use the old screws that were in the box but they were the wrong screws and they had pointed ends (sheet metal ) well I caught a wire and pop but the breaker did not trip here either. I just the grounds here and there weren't any. Bonding was the the conduit here. Same building my helper closes the cover on other tie box and pop but no trip here either. They were not F.P.E.. One was Seimens and I'm not sure of the other.
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I think that if he cut through it quickly enough, it could be below the time-current trip threashold of the breaker. Just a guess, mainly. Trip curves for plug-on molded case circuit breakers can be a little tough to get ahold of. The FPE guess is a good one too. They're famous for that.
I've accidentally cut wires, and had them not trip. Cutler Hammer if I recall. Never had this happen with Square D QO.

Of course, it is best not to cut any energized wires. :whistling2:
Depends a bit too on how much wire is between hither and tither. You're making a bit of a heating element with the longer wires, which may not appear like a hard fault for a bit.
Marcs got it.

Impedance of the ground path.
Time/duration of fault.
Though possible defective CB
I was thinking bad ground/bonding but actually it is more the duration of the fault current.
Combination of both items
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