Light trips circuit breaker when connected to box

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Old 12-20-17, 11:54 AM
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Light trips circuit breaker when connected to box

I'm a newbie on this site, replacing conventional lights with Model CLL56-2WW LED fixtures and all went well on first 3 installs.

The 4th is wired just like the rest- white to white, black to black, ground to ground and works fine when the light is unmounted (hanging from wires).

However, when I connected it to the box, the Cbreaker tripped- I imagined I had somehow damaged or inverted a wire, however when I dropped the fixture all looked well and the light worked after the circuit breaker reset.

So I imagined that maybe a surge had taken place and tried the whole thing over again- This time there was a spark at the screw that connects the light base to the box before the breaker flipped. Light still works fine when unattached to box.

So I stopped and can't imagine what is going on-

I was thinking that since this is an old house, maybe black and white are reversed- However, if this was true, why didn't the previous fixture short and why did the other new installs work?

I do have a nice LCR multi-meter and I would really appreciate any thoughts to diagnose and fix!

Adding photos:



Also note that image 2 is much simpler than it looks- The black wire is wrapped around white wire near white wire nut (from being pushed up into box) and then runs behind the ground nut. He should have pulled down more and they would have pulled away from each other for a clearer photo- all 3 wires are totally independent and appear to be in good condition.

And note singe mark in photo 3 where bolt sparked.

Thanks in advance!
Mickey
 
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Old 12-20-17, 12:09 PM
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Turn off the breaker to that circuit. Carefully examine the wires along their full length that you can see. I suspect you cut or nicked the insulation and it's shorting out when you try to pack it back into the enclosure.
 
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Old 12-20-17, 12:54 PM
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Light trips circuit breaker when connected to box

Thanks Pilot Dane, but that's where I started- nothing I can see or feel on white or black wire and ground is copper that's picked up paint over the years.

I've got a good LCR metter, but can't figure out what I should look for-
  • White to black should give me 120v, but not sure how that helps
  • Should black to box and ground wire be 0 ohms, ie 0 resistance since they are all connected?
  • If so, white to box should be 120v? If not, does this tell me that the power is wired backwords?
Are these the right questions or are there better things to look for?
 
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Old 12-20-17, 02:07 PM
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Is the house wiring ground wire connected to the metal light mounting bar?

When testing for voltage make sure your meter is on the correct setting and you can check for voltage with the circuit hot. If checking for resistance or continuity then the breaker for that circuit should be OFF to avoid damaging your meter or at popping it's internal fuse.

It looks like your box is plastic so you shouldn't really be read current between your enclosure and conductors and it even if they were touching.

When the circuit is on/hot then you should get 120VAC between hot/black and neutral/white and between hot/black and ground/bare copper. With all the wires out in the open, not connected and not touching anything black/hot is the only wire that should be hot or have current. You should get zero voltage between neutral/white and ground/bare copper.
 
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Old 12-20-17, 02:17 PM
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Thanks!

Good input PilotDane- Will test tomorrow and post results- Much appreciated!
 
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Old 12-21-17, 02:55 AM
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A thought.

Possible that when you drive the screw/s into the fixture to secure it to the box you are driving the screw into a conductor up in the box. Make sure your wires are out of the way of where the machine screws used to secure the fixture to the box will go. If you are doing this it may be hard to see where the screw is going into the wire. Take a flashlight and look carefully or pull down the fixture support bar and rearrange the wires to be sure no wire will be in the path of a machine screw.
 
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Old 12-21-17, 04:37 AM
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Please explain the following statement:

Should black to box and ground wire be 0 ohms, ie 0 resistance since they are all connected?
 
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Old 12-21-17, 05:46 AM
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A metal box is conductive. Plastic boxes are not (for the most part. Black boxes might show some very minor conductivity because of the carbon used to tint the plastic black but it's pretty minimal at household voltages.) So, even though a hot conductor may be touching a plastic enclosure the box is generally not energized.
 
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