Need help with blown circuit

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  #1  
Old 12-05-04, 11:04 PM
hewie
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Need help with blown circuit

(I tried posting this earlier, but I seem to have failed. Hope this isn't duplicate post.)

I have a circuit which failed tonight, and need some help trouble shooting the problem.

My wife called me in from outside and said we blew a circuit breaker. So I went out and checked the panel with no tripped breakers. Here's the current status:

1. Votage at the breaker tests good.
2. Some lights on the circuit work, some don't.
3. All of the outlets show a "hot/ground reverse" using a cheepie 3-light circuit tester. I double checked this with a volt meter, and it looks like the ground is hot on this circuit.
4. I disconnected a low voltage light from this circuit since it's sister has had numerous problems with no changes.
5. There are no GFI's on this circuit as far as I can find. (House was built in '79, so the only GFI's are the one's I've put in)

Any help I can get trouble shooting this would be appreciated.

Thanks,

Dave
 
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  #2  
Old 12-06-04, 05:37 AM
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Don't believe your tester just yet. It may not be telling you the truth. If some of the items on the circuit work and other don't then the problem is a circuit interruption somewhere. This could be a tripped GFCI or an open connection somewhere on the circuit.

If yo are using a digital voltmeter, put it away and forget what it told you. Digital voltmeters don't work on AC circuits when a wire is open.

You seem to be fairly sure that you don't have a GFCI issue. So the next thing I would do is begin searching the receptacles and other outlets for an open connection. You will need to check all the junction boxes, lights and receptacles. The open could be at a working or a non-working box, so check them all. It helps if you know how the wires are laid out, but if you don't, try to imagine the best way to lay out the circuit minimizing wire length to determine which wires to check.

Pay particular attention to back stabbed connections. If you find any, many people recommend moving them to the screw terminals since you have the receptacle open.
 
  #3  
Old 12-06-04, 07:49 AM
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You probably have an open neutral. A GCFI breaker or outlet will interupt the hot wire, so you have 120V between hot and ground in the circuit you don't have a tripped breaker or GCFI.

Start with all of the outlets on that breaker. I would actually first map out everything that doesn't work now. That shut off the breaker that feeds this circuit, and see if anything else lost power. I would start looking at the last outlet or fixture (in the logical layout of the circuit, if there is one) that worked while the breaker was on, but stopped when the breaker was turned off.

As racraft said, start with outlets, especially backstabbed ones. Take the outlet out of the box and actually pull slightly on the wire. Don't just look at it.
 
  #4  
Old 12-06-04, 10:02 PM
hewie
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Thanks for the help. Traced the fault to one outlet, pulled the outlet, and the neutral leaving that outlet had obviously been arcing pretty bad. Replaced the outlet (using the screw terminals) and everything is great.

I do have a question about the symptoms, cause after thinking about it, I'm a little nervous.

Here's what I saw using a good old fashion analog voltmeter ( I find it easier to use for good/bad troubleshooting): hot to neutral = no voltage; ground to hot = ~115V; ground to neutral = ~115V. What scares me is it appears down circuit of the faulty outlet, I had a hot ground. Am I not understanding something, or was this a close call. Will installing GFCIs alleviate this?

Thanks again,
Dave
 
  #5  
Old 12-06-04, 10:09 PM
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No, you did not have a hot ground. You had an open neutral. Both the hot and neutral were hot--the ground was not. When you have an neutral that is open between the outlet and the panel, but also have a load somewhere on the circuit (e.g., a lamp turned on), the neutral becomes hot. That is because the hot is connected to the neutral through the load, but since no current is flowing, there is no voltage drop through the load as there would be if current was flowing.
 
  #6  
Old 12-07-04, 08:06 AM
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Very excellent, concise, easy to understand explanation by moderator John.

Opens sometimes present the most confusing symptoms, but if you pencil out a little sketch of the circuit, Ohm's law will give you the answer every time.

The comment about using DVM's is accurate, but as we saw in this case, even a conventional VOM has much higher internal resistance than the lamp or motor on the circuit, so will develop almost 100% of the voltage drop in the circuit, hence the reading of about 115 v gnd to neut.
 
  #7  
Old 12-07-04, 12:40 PM
hewie
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Thanks for the help

Thanks for the insight. I really appreciate the service this forum provides and everyones help in understanding what I had going on.

Thanks again,
Dave
 
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