Isolating a short?

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  #1  
Old 02-17-06, 10:10 PM
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Isolating a short?

I came home tonight to a tripped circuit breaker. The breaker covers one bedroom and half of the living room. In total there are about 8 outlets and 4 lights on the ceiling. The only things on when it tripped were a cable box and an alarm clock. A TV and 2 lamps were plugged in, additionally.

I can't see the room from my fusebox. But when I try to reset the breaker, I see the lights in the house *briefly* dim and then it trips immediately. I take this to mean that I have a short.

The condo is 6 years old. I have owned it for 3 weeks. I had a full inspection which turned up no problems. The owners did replace a particular outlet before leaving, but when I removed it, everything looked okay.

Furthermore, with the breaker "off", I measured the resistance across the contacts of a couple of outlets. The resistance was 2 ohms between them, and 1.5 from either wire to ground. This seems like further evidence of a short.

My question is now...how the heck do I track it down!?
 
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Old 02-17-06, 11:06 PM
bolide's Avatar
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You start by disconnecting pieces.

First you disconnect the hot (black) from the breaker and the neutral from the panel buss.

Since you know how to check resistance, you can then test for resistance from black to white. Then check from black to ground (leave the ground connected in the panel).
(There should not be continuity from white to ground at this point; but check this also.)


Next find the first outlet (or junction box) fed on this branch.
Open it, take it apart, and figure out if the fault is between there and the panel or from there to the next outlet.

The recently replaced receptacle is a likely candidate.

After that, pick one in the middle of the bad section and take it apart.
 
  #3  
Old 02-17-06, 11:33 PM
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Finding a short

I assume that you have a good idea of what is on that circuit.
Make sure that everything is unplugged and all wall switches are off.
If you have no clue where the problem might be, start somewhere in the middle of the circuit. Take an outlet out of its box, making sure that it has no power to it. Take one pair of the wires off of the outlet (a white neutral and a black hot).
Set the ohmmeter to the Rx1 scale. Touching the probes together should read about zero ohms.
Measure the resistance between the hot and neutral that you disconnected and also between the pair still attached to the outlet. You should read infinity on one pair and about zero ohms on the other pair. The zero ohms is the side of the short. Now you have to figure out if that pair is leading towards or away from the panel.
Measure the resistance from the neutral to ground on each of those pairs. Assuming correct wiring, you should read near zero on one pair and infinity on the other. The pair that reads near zero goes back to the panel. Leave that outlet out and unwired.
Go to an outlet about halfway between that first outlet and the beginning or end of the circuit, depending on which direction the short is. Open an outlet there and repeat the above.
What you are doing is starting in the middle and seeing if the short is on one half of the circuit or the other half. Then you cut the first half in half, and so on.
When you have found and fixed the short, put all of those outlets back together and in their boxes.
Let us know what you find.
 
  #4  
Old 02-17-06, 11:42 PM
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Guys, I appreciate your suggestions. However, the outlets are connected in parallel right? It sounds like you're saying that if I remove an outlet, it will break the connection between the fusebox and any outlets 'after' it in the circuit.

Or do you mean remove more than the outlet but the whole box and untie the branches that are presumably inside the wall?
 
  #5  
Old 02-18-06, 12:15 AM
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OK, I removed the most recently replaced outlet and was appalled to see a neutral tied to some hots!?!? I disconnected everything in there, and my circuits are 90% back. One outlet and the outdoor light are all that still don't work.

Now I have one pair that's known good, and 2 other pairs that I believe go to one further outlet and the outdoor light.

I believe if I simply connect all 3 hot, and all 3 neutral, I should be okay. The 3 grounds are already tied.

I am surprised that this didn't cause a problem earlier...
 
  #6  
Old 02-18-06, 06:36 AM
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Testing for continuity on a residential electrical circuit is not as easy as it sounds. If you conducted your tests with any loads on the circuit then your tests are meaningless. By loads I mean any light bulbs screwed in or any devices plugged in. Further, if the wires were still connected up in the panel, you will read continuity through other circuits.

Don't assume that all white wires are neutrals. It is perfectly acceptable to use a white wire as a hot wire in a switch loop. If a particular receptacle is to be switched (such as in a living room or bedroom with no overhead lights) then you just might have a switch loop.

It's possible that the switch loop got messed up, such as when a receptacle was replaced, and you didn't find the problem until the switch was turned on.

Anyway, as for your problem. As has been suggested, you need to divide and conquer. The best way is to start at the panel. Make sure the breaker works with no load. Then slowly add pieces of the load until you determine where the problem is.

Now that you have some or most of the circuit working, you need to look for an open. It's possible that the short circuits caused a weak connection to fail, causing an open. Check the last working location on the circuit and the first non-working location. The open will be at one of those two locations.

When you solve this problem, and if you haven't already done so, completely map out your electrical system. Your goal should be to determine which circuit breaker controls each and every receptacle, light and appliance in the condo. Further, you need to know exactly what is on each circuit breaker. This information is invaluable in a problem situation, and could save your life.
 
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