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Old 12-02-2009, 11:49 PM   #1
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Self-resetting open on branch?

We have a fairly-new (3yr) branch circuit dedicated to our microwave oven with one duplex receptacle. Fortunately, we added some low-wattage lights on that outlet, which reveals that our problem is probably not in the microwave:
Occasionally, power shuts off at this outlet, resetting itself after about 5 seconds.
Any idea of the likely cause?
I've never seen flicker; when power's off it seems to be completely off (no dim lights or hum); breaker hasn't blown; no problems on other branches. We'd only notice outages if the microwave is running, so no info there. Sometimes it'll fail half a dozen times in a row, a couple of minutes apart. Or it'll go for a month with no problem.

Guess I'll have to put something on this outlet that'll give me an audible indication when power's off, and hope I can be positioned at the breaker when we get one of these erratic outages. Unless this is a common failure mode that somebody can identify...

Breaker is Square-D HOM 15A single-pole, feeding #12 Cu. Oven rated 1.58KW, mfr calls for 15A ckt.
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Old 12-03-2009, 03:16 AM   #2
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A few possibilities:
  1. Bad breaker. Your breaker might be bad, with a loose or partial connection internally. Try swapping out the breaker with another one if you this may be the case.
  2. Bad neutral connection at panel. Check your neutral connections. Sometimes, it's easy to miss the terminal hole, and the wire may be lying next to the neutral bar, or the set screw may not be fully tight. Pull on the wire to ensure the connection is good.
  3. Loose connection on receptacle. If the terminal screw on the outlet is not fully tightened, you may experience problems like you have described. Check the connections on the outlet.
  4. Bad wire. Even if you don't have any splices in the wire between the panel and the outlet, I have seen bad internal spots of cable in the middle of the run. Rare, but possible.

Life is uncertain -- eat dessert first!!
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Old 12-03-2009, 08:58 AM   #3
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If this circuit is in a kitchen then you have violated the code. You are not permitted to have lights on that circuit.
The problem is a loose ocnnection somewhere on the circuit.
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Old 12-03-2009, 11:58 AM   #4
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Uh oh, the code police are out. Rerickson's going to the slammer for plugging an undercabinet light into a dedicated microwave receptacle. LOL

Their local code might require a dedicated outlet for a microwave but I doubt that it controls what the occupant actually decides to plug into it.
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Old 12-03-2009, 01:07 PM   #5
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Most likely either item 2 or 3 from KB's post...
"Life is hard. Life is harder when you're stupid." John Wayne
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