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Old 02-07-2010, 11:25 PM   #1
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Short circuit?


While in the attic performing some repairs, I reached to hang my droplight from an attic fan frame, near the attic fan motor. The droplight shield is metal, and when it came in contact with motor pully, I was surprised by a significant arc between the light and the motor pulley. Made a nice burn mark on both items (light and pulley). The real surprise is that I can't find the short. The short did not trip the droplight circuit nor the attic fan motor circuit. I can't recreate it and I can't find any problems in either the fan motor or the droplight. Very puzzling. Any ideas?

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Old 02-07-2010, 11:28 PM   #2
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Short circuit?


which of them (droplight or fan frame) is grounded or are both?

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Old 02-07-2010, 11:29 PM   #3
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Short circuit?


Both are grounded...although I did not fully confirm that with the attic fan motor.
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Old 02-07-2010, 11:34 PM   #4
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Short circuit?


If the fan was not grounded, it could actually have a short right to the frame and not cause a problem. If the light was grounded, by touching the fan with the light, it then grounded the fan frame and viola` you have an arc.


if it was a loose wire, it may have burned it to the point it no longer makes contact with the metal frame. I would turn off the power to the fan and open up any junction box on the fan and check for a missing wire nut loose wire that may have touched the ungrounded metal frame.
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Old 02-07-2010, 11:43 PM   #5
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Short circuit?


Thanks...I agree. I failed to mention that an additional attic light was turned on. This light is on the same curcuit as the attic fan. My initial assumption was that if the fan frame or motor frame was energized (and not properly grounded) then I would have tripped the breaker feeding the fan motor when the droplight frame closed the short. Since the attic light stayed on (and no subsequent breakers were found to be tripped), then either the breaker failed, or something else is happening. By the way, the droplight stayed on as well.
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Old 02-07-2010, 11:59 PM   #6
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Short circuit?


unless the circuit the drop light was on was the one shorted, it would not trip as if never experienced high current flow.

and you could have a failed breaker but that would not cause the arc.

the breaker should have tripped if there was much of any arc


One thing most people do not do but should is exercise your breakers at least once per year. Just flip them off and on a dozen times each to keep them free. The manufacturers recommend this.

anyway, I would think the circuit in the attic is not grounded. You still need to look for what ever shorted though. Like I said, I would think what ever it was simply burned the connection to clear the short. That would help with finding it at least.
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Old 02-08-2010, 06:57 AM   #7
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Short circuit?


If the light bounced around a bit then it might not have made contact long enough to trip any breakers.

Get a voltmeter and measure the voltage (voltage difference) between the light and the fan. Also measure between the light and ground (try it with the plug reversed too if it fits) and between the fan and ground.

A long single conductor wire connected to a known ground downstairs and strung up into the attic or wherever you are can come in handy for measurement purposes.

>>> ... flip them off a dozen ...

Really? The manufacturers recommend that? I would think that would put excessive wear and tear on breakers.

I would flip the breakers off and on manually just once or, if you insist, twice for the annual test. Do not create a short circuit to test breakers with.
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Last edited by AllanJ; 02-08-2010 at 07:53 AM.
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Old 02-08-2010, 12:04 PM   #8
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Short circuit?


Quote:
Originally Posted by AllanJ View Post
>>> ... flip them off a dozen ...

Really? The manufacturers recommend that? I would think that would put excessive wear and tear on breakers.

I would flip the breakers off and on manually just once or, if you insist, twice for the annual test. Do not create a short circuit to test breakers with.
12 times a year (what it averages out to be) as excessive?

Do 6 then, or 3 or what ever you want but it needs to be done and no, flipping a breaker 12 times is not going to damage the breaker.

and this is not a test. It is intended to prevent the breaker from becoming stiff or actually stuck due to lack of use. It is called "exercising" for a reason. Just as a person exercises to maintain flexibility, your breakers need it too.

the last thing you want is a breaker that is stiff not opening when it is called upon to do so. I think we are all familiar with stab lok breakers and what a breaker that does not trip can lead to.

and if you happen to have SWD rated breakers, they are actually designed to be used as switches if desired (plus the obvious use as a breaker) where a breaker that does not have the SWD rating is just a breaker.


and most definitely, do no create a short circuit to check a breaker., manufacturers actually recommend replacing a breaker after a short circuit trip. It is thought more as a means to absolve them of liability but in reality, depending on how great the trip current is, there could be actual damage and replacing the breaker could actually be prudent.

I am having a hard time finding the specific recommendation in Square D's site but here is a snippet that was included in part of a Google found link:
Quote:
Exercise circuit breakers at least once a year to ensure proper operation. ..... QO(B). 30. 8910 DPA 32,33,34 ★. 30. 9080 GR6/GC6 ■ 8 ...

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