How much and what testing to find a short circuit?


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Old 03-07-11, 12:19 PM
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How much and what testing to find a short circuit?

We have three flood lights around our house that are in standard Red Dot receptacles using par 38 CFL's. These are on an Intermatic T101 timer. These are also on their own circuit breaker. A few weeks ago the breaker went off, and if there was any power to the circuit the breaker went off again immediately. The timer is not the problem since it controls garage flood lights, and these continue to work. The house flood lights have worked for the past four years since we have been here. My electrician came out, disconnected all the lights, and the breaker still went immediately off after applying power. The wiring is completely hidden behind the exterior walls. That is as far as he got. So the conclusion was to replace the flood lights by locating new flood lights. My question really is what further testing could have been done to locate the short even if it might not be possible to rebuild that original circuit? This of course leads to the question of how much are you willing to pay to have your electrician do additional testing that still might not be successful? I would have thought that somehow the electrician could have determined that the short is someplace in the circuit between such and such light.
 
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Old 03-07-11, 01:57 PM
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My electrician came out, disconnected all the lights, and the breaker still went immediately off after applying power.
Did he disconnect the breaker at the panel to determine if the breaker was bad?

So the conclusion was to replace the flood lights by locating new flood lights.
If the flood lights are good you don't need to replace them just the cable to them.

I would have thought that somehow the electrician could have determined that the short is someplace in the circuit between such and such light.
If it is a dead short even an ohm meter should show it. However in some cases you might need a Megger. You could do the initial tests yourself with a multimeter. You could also use the process of elimination.

If you disconnect the cable to the first light from the next cable and leave only the end to the panel connected and it tripped the breaker you would know it was bad if it didn't you would connect it to the next cable leaving the light out of the circuit and disconnecting the far end from the next cable. Same procedure test to see if breaker trips. Keep repeating till all sections of cable have been added and tested without lights. If it still doesn't trip add the lights one by one.

Ben and I tied for reply time. His explanation for elimination testing is easier to understand then mine. Be sure not to connect the lights till after testing the whole cable in case one of them is the culprit.
 
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Old 03-07-11, 01:57 PM
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It is pretty easy to determine which section of the wiring is bad. You start by unhooking everything, then reconnecting the circuit one box at a time until the breaker trips when energizes (or test continuity between hot and ground). At that point you know the bad section is between the last two boxes you hooked up. From then you can try to find the exact problem (rodents, box clamp, nail puncture, etc.)

I see no reason to replace the light fixtures themselves if you have shown they are not the culprit.
 
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Old 03-07-11, 05:00 PM
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I am sure that the breaker was not tested. The only testing that may have been done was some continuity testing. The impression I received was that if something did not show up under visual inspection, then it was too hard. We are not planning to replace the Red Dot fixtures. Since we cannot pull cable to them unless we do it on the outside of the house, we will just remove them and re-use them. The thing that annoys me is that from my point of view there was not enough testing done or even tried. Once I take care of all of this, I will go to the big boss electrician and make my opinions know. So far I am out one hour of labor.
 
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Old 03-07-11, 05:08 PM
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Most of what Ben and I suggested can be done by you. If your comfortable working inside the breaker box you can even check the breaker without a meter.

We are not planning to replace the Red Dot fixtures. Since we cannot pull cable to them unless we do it on the outside of the house, we will just remove them and re-use them.
Actually there are a couple of ways to do that which would not require moving the lights or running cable on the outside of the house. Maybe a bit of conduit from the ground if it is a slab house.
 
 

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