Faulty Breaker

[Rufus]

I have just installed a brand new 200 A panel and wired 27 circuits in an
addition to our home. I applied energy to each circuit in turn by setting
the breaker and one of the lighting circuits (15A) tripped immediately. To
troubleshoot, I switched each of the wall switches on the circuit one at a
time and reset the breaker after each trial. I tripped the breaker perhaps
five times within 2 minutes.

I finally found the short (line to ground caused by aggressive tightening of
the cable clamp in one of the boxes coupled with a small burr on the inside
of the clamp).

A new problem, and the reason for this post, is that the breaker (Square D
QO series) was leaking voltage when it was off. With the breaker in place,
turned on and the line physically disconnected, I read 120 volts from the
neutral bus to the line terminal on the breaker, as I would exect. With the
breaker off, however, I read 60 volts from the bus to the terminal, which I
would definitely not expect.

Thinking that the frequent tripping may have created a high resistance path
through the breaker, I pulled it and idly flipped it on and off a few times
before checking the resistance. Breaker off, resistance infinite, breaker
on resistance 0. I reinstalled it and now find that it works properly, not
leaking any voltage while it is off.

Does anyone have a plausible explanation?

Thanks in advance

David

 

[Cameron Dorrough]

I have just installed a brand new 200 A panel and wired 27 circuits in an
addition to our home. I applied energy to each circuit in turn by setting
the breaker and one of the lighting circuits (15A) tripped immediately. To
troubleshoot, I switched each of the wall switches on the circuit one at a
time and reset the breaker after each trial. I tripped the breaker perhaps
five times within 2 minutes.

I finally found the short (line to ground caused by aggressive tightening of
the cable clamp in one of the boxes coupled with a small burr on the inside
of the clamp).

A new problem, and the reason for this post, is that the breaker (Square D
QO series) was leaking voltage when it was off. With the breaker in place,
turned on and the line physically disconnected, I read 120 volts from the
neutral bus to the line terminal on the breaker, as I would exect. With the
breaker off, however, I read 60 volts from the bus to the terminal, which I
would definitely not expect.

Thinking that the frequent tripping may have created a high resistance path
through the breaker, I pulled it and idly flipped it on and off a few times
before checking the resistance. Breaker off, resistance infinite, breaker
on resistance 0. I reinstalled it and now find that it works properly, not
leaking any voltage while it is off.

Does anyone have a plausible explanation?

Metal filings in the arc chamber, either from the contacts arcing during
your fault scenario or from dodgy QC during manufacture, would do this.
Taking the breaker out and operating it a few times, as you did, would
likely clear away any rubbish from the contacts and make it work again.

With luck, the rubbish should now stay where it is and the breaker should
live a long life - but for peace of mind (or if it starts tripping again) it
should be replaced.

HTH,
Cameron