Blown fuse in branch circuit

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Old 02-22-15, 09:33 AM
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Blown fuse in branch circuit

I have a branch circuit box with two 30 amp fuses, one of which leads to shop and garage lights, garage door opener (connected via a GFCI outlet), and a jet aerator motor that runs full time in my septic setup. Woke up to find that this fuse was blown. Before replacing fuse I switched motor off, I believe lights were still switched on. Replaced with a 20 amp fuse (just to be cautious). When I flipped the switch in the circuit box, it sparked at the contact point and the fuse failed immediately. Would this be caused by a short circuit? I don't think there was any noticeable load on the fuse. This has never tripped in over ten years, so this is puzzling. Any help is greatly appreciated.
 
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Old 02-22-15, 09:39 AM
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When I flipped the switch in the circuit box, it sparked at the contact point and the fuse failed immediately. Would this be caused by a short circuit?
Yes, you have a dead short somewhere, probably in equipment connected on the circuit. Do not use 30 amp fuses. The fuses should be sized to the wire on the crcuit; 20 amp for #12 or 15 amp for #14.
 
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Old 02-22-15, 10:42 AM
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blown fuse in branch circuit

Thanks CasualJoe. I am suspecting the garage lights, there are six of them -- bare bulb ceramic fixtures operated by a single switch, but each also has the on/off pull string; these are old and do not work well. Also suspect possibility of squirrels getting into garage ceiling and chewing wires. I guess this will be a trial and error process with the fuses. Unless anyone has other suggestions.
 
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Old 02-22-15, 11:42 AM
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Once you have 15 (or 20 if #12) or amp fuses in there and a box more of fuses disconnect the first device after the fuse box and test. If it doesn't blow reconnect and disconnect the second device and test. Reconnect the second device and try the third.... and so on.
I am suspecting the garage lights, there are six of them -- bare bulb ceramic fixtures operated by a single switch, but each also has the on/off pull string;
I wouldn't. Nothing in the parts that make up the fixture likely to cause a short. It could be the wiring though.
and do not work well
Please explain more fully.
 
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Old 02-22-15, 12:28 PM
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Thank you ray. The part of the light fixtures that do not work well are the pull string on/off switches, often need to pull several times to switch, one or two of them I have not been able to switch on for some time as the pull string switches will not function at all. I infer from what you say this would not likely be a cause.

As you recommend, I plan to test by adding one item at a time. I would not leave the circuit live until I have identified and eliminated the cause. If I can't figure it out, an electrician will be in order.
 
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Old 02-22-15, 01:26 PM
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and do not work well
There is a high rate of failure for the switches but that would not cause a short. (Cheap in, failure out .)
 
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Old 02-22-15, 03:26 PM
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I narrowed it down to the six ceramic light fixtures (double checked; first I added the light fixture load first, fuse blew immediately; the second time added all other load first, did not blow till I added lights last). I removed all the light fixtures, at each fixture there was continuity between the black and white leads. My next test will be to disconnect the light fixtures from each other to try to identify where the short circuit is. Not sure what to do once I figure that out. Any advice is appreciated. Thanks.
 
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Old 02-22-15, 03:34 PM
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Some of the fixtures are bad, some of the cable is probably bad, almost certainly the cable does not have a ground required to meet modern code. Key-less fixtures (what you have but no switch) are only a couple of dollars each. I'd replace all the cables and fixtures.
 
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Old 02-23-15, 08:24 AM
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I narrowed it down to the six ceramic light fixtures (double checked; first I added the light fixture load first, fuse blew immediately; the second time added all other load first, did not blow till I added lights last).
You could spend a fortune on fuses before finding the problem. Try removing the fuse and screwing a light bulb into the fuse socket. If the bulb lights brightly you have a dead short. When you find the fault and correct it, the light bulb will go out. Then, install the proper fuse. Replace the old pull chain porcelain fixtures with new keyless porcelain fixtures.
 
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Old 02-23-15, 10:27 AM
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Wow! I haven't seen that method used in years.
Good test though.
Geo
 
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Old 02-24-15, 05:47 PM
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Casualjoe -- thanks for the light bulb idea. Awesome is a way overused word, but applies in this case. Took me a few seconds to grasp it and then it made me laugh when I realized how simple this was.
 
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