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Old 02-19-2007, 11:30 AM   #1
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Troubleshooting a short

After being left all alone in the rain for four months in the rain forest, we came back to find that there was a short in the AC system in our 1979 31ft Sovereign. A little trial and error in the "fuse" box isolated the problem to a white wire connected directly to the AC in, and whose matched black wire went through a breaker labeled "air conditioning". Now, the circuit I plug in would blow even with the breaker on OFF, so I'm guessing that there's a short between the "white" line and ground. The only way to avoid blowing the entire circuit was to disconnect the white line (and isolate it, naturally).

1) Does my diagnosis of a short between the AC circuit and the ground make sense?

2) any idea how to figure out where the short would be? I'll clearly need to go on the roof some sunny day and take apart some of the AC unit, as that's the most likely place for some bad water pooling, but any hints appreciated.

Cheers

--david
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Old 02-19-2007, 12:01 PM   #2
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Quote:
Originally Posted by davida
... Now, the circuit I plug in would blow even with the breaker on OFF, so I'm guessing that there's a short between the "white" line and ground. ...
What exactly 'blows' when the breaker is off? Normally, a fuse 'blows', and a breaker 'trips'. Can you rephrase?

If you follow the wiring back to the house/campground, you will note that the white wire (neutral) and the ground wire are electrically tied together at one point. So a short between the two connectors may or may not be noticeable.

Tom
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Old 02-19-2007, 12:10 PM   #3
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TomW, sorry for the poor language.

I'm currently plugging in to an outside outlet of my neighbor's via an extension cord. The breaker on his outlet trips as soon as I plug in, if and only if the AC line is connected (in the electrical box in the trailer).

Does that help?

--david
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Old 02-19-2007, 12:13 PM   #4
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Quote:
Originally Posted by davida
TomW, sorry for the poor language.

I'm currently plugging in to an outside outlet of my neighbor's via an extension cord. The breaker on his outlet trips as soon as I plug in, if and only if the AC line is connected (in the electrical box in the trailer).

Does that help?

--david
SO this is AC-alternating current, not AC-Air Condintioning, right?
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Old 02-19-2007, 12:14 PM   #5
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It sounds like your neighbor is supplying you power from a 15 or 20 amp service.

If ALL the breakers in the Airstream are switched off, does your neighbor's breaker still trip when you plug in?

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Old 02-19-2007, 12:29 PM   #6
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Quote:
Originally Posted by TomW
It sounds like your neighbor is supplying you power from a 15 or 20 amp service.

If ALL the breakers in the Airstream are switched off, does your neighbor's breaker still trip when you plug in?

Tom
Good point Tom, also how long is the extension cord and what gauge wire is it?

Make sure you are comfortable working around 110 A/C. Unlike 12 volt DC, the A/C can kill you. You might enlist some local help here.
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Old 02-19-2007, 12:54 PM   #7
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I clearly wasn't clear enough.

1) Yes, this is an extension cord off of 20A or so. However, it's worked fine for months. Only recently did connecting cause the breaker to trip. It's a heavy duty outdoor extension cord rated for 30A or so IIRC. The extension cord itself tests fine.

2) This is a "parked" unit -- I only run on 110V AC. (the batteries are disconnected, I think).

3) If I don't disconnect the white wire to the air conditioning loop, the breaker trips even with all breakers in the Airstream off. The breakers are all inline with the "black" wires, which is why I'm guessing that there's a short between the white wire and the ground. If I disconnect the white wire to the air conditioning loop, then I can flip the breakers on and everything works fine (lights, furnace, hydraulic jack, etc.).

Clearer I hope?
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Old 02-19-2007, 01:03 PM   #8
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Multiple issues

Quote:
Originally Posted by davida
... If I don't disconnect the white wire to the air conditioning loop, the breaker trips even with all breakers in the Airstream off. ...
That should not happen with good breakers.

That is also your starting point even though it is not your original problem. One or more breakers has fused contacts.

Remove the breakers and check each with an ohm meter. The bad breaker(s) will show continuity in the OFF position.

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Old 02-19-2007, 01:16 PM   #9
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I did test the continuity of the breaker associated with the air conditioning system (I was thinking like you that the breaker was the problem), and it is working ok. (I didn't test all of them, I'll do that next time I'm there).

Still, I'm not sure I understand. If my hypothesis of a short between the white line and ground is correct, wouldn't that current loop bypass all of the breakers which are on the black wire?

--david
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Old 02-19-2007, 01:23 PM   #10
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We don't want you quick-fried to a crackly crunch

Quote:
Originally Posted by davida
... If my hypothesis of a short between the white line and ground is correct, wouldn't that current loop bypass all of the breakers which are on the black wire?
No.

David, I think you have more than one problem. The best starting place is to be able to plug your airstream into shore power without blowing your neighbor's breaker.

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Old 02-19-2007, 02:03 PM   #11
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would love to...

I'd love to plug in to real shore power, but for that I have to build a shed, get the electric co to setup a meter and run a line from the pole, etc, and that'll take a couple of months =)

I get to the trailer only every few weeks, so I'm trying to "store up" as much info between visits as I can, especially since I don't have good net access there.

So far, I plan on checking each of the breakers, and testing for shorts between the suspicious lead and whatever wires I can get my ohmmeter on.

Oh, one other thought that could explain why my breakers don't trip but the neighbor's does is that it could be his is rated lower, so his trips first, no?
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Old 02-19-2007, 02:31 PM   #12
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First things first

Perhaps I have mis-read your post, but my interpretation was that you were tripping your neighbor's breaker even when all of your breakers were OFF. If this is so, then the same thing would happen even if you were plugged into a bonafide 30A shore outlet.

Ya know it makes me grit my teeth, but Inland Andy's right, "Safety first". And balance your running gear is a close second.

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Old 02-19-2007, 03:47 PM   #13
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I was drawing out the schematic on the board to try and understand what might be going on, and I just wondered something. If my neighbor's outlet was miswired and what my airstream thinks of as the neutral is actually live, wouldn't that explain why the breakers aren't having an effect?
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Old 02-19-2007, 03:53 PM   #14
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But it worked OK for a while...unless he changed something.
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