Spark when turned breaker on


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Old 11-09-08, 03:09 PM
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Spark when turned breaker on

Hi, I just installed a outdoor motion floodlight on my detached garage. I put in a new circuit in for it and another existing outdoor light.

OK, so I got everything hooked up and made my last connections at the sub-panel. White and ground to the bar on the side, it's a three wire sub panel, and hooked the black to the new breaker. Installed the new breaker with it off, and flipped it on and had a loud and bright spark come from somewhere near the top of the neutral/ground bar, and it flipped the breaker off.

I noticed that a ground wire (from a different circuit) and the main neutral wire from the main panel were really close together but not quite touching. Seemed to be in the area of the spark, so I moved the ground further away from the main neutral. Flipped the breaker on and everything seems to be working OK.

Do you think this was the problem or should I be worried about something else? I double checked all of my connections before turning the breaker back on, and everything seemed OK. Like I said everything is working OK now. Any ideas or thoughts?
 
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Old 11-09-08, 03:29 PM
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Can you post pictures of your electrical panel and your wiring work?

Modern subpanels have separate ground/neutral bars and the neutral bar is not bonded to the metal case, but your subpanel has the grounds and neutrals connected to the same bar and this is grounded to the metal case - correct?
 
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Old 11-09-08, 03:33 PM
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Thanks for the reply. I have an old picture before installing the new breaker, I'll get new picture later if needed. It is an old Murray panel, and it is a sub panel in a detached garage.

 
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Old 11-09-08, 03:40 PM
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Where was the arcing? (Best to see new pictures)
 
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Old 11-09-08, 04:15 PM
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New pic with the details of what I did highlighted. I checked all the screws on the bar and they seemed tight. I'm sorta guessing that's where the arc came from, it was just a bright flash in that area.

 
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Old 11-09-08, 07:14 PM
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Your hot wires all pass right through the area in question and along side all of your grounds. A good arc like your are describing will have left its mark. Carefully inspect the back of those black wires and look for any flash/burnt marks related to your arc.
I like to group my blacks and keep them away from that ground buss, but maybe that's just me.
Bud
 
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Old 11-09-08, 08:20 PM
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I will just turn off the subpanel power off for short while while you are checking hot conductors to make sure there is no visble nicks on the insluating materals to make a flashover.

Merci,Marc
 
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Old 11-10-08, 05:32 PM
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I looked over all of the wires and found one white wire in the area of the arc had a little melted insulation where it looks like it was resting on the main neutral wire. It's not melted to expose the wire just a little cup in it.

I found no burn marks or any nicks or cuts on any wires.
 
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Old 11-10-08, 07:28 PM
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I am going to guess at what caused the arcing...

I would say that when you installed the light or when you or someone else installed the existing light, that when placing (cramming) the wiring back into the electrical box, a portion of a ground wire was touching a hot wire.

Or if stranded wire was used, maybe one strand of a hot wire was touching a grounded metal box after wires crammed back into box or a strand of ground wire was touching a hot.

Anyway you turn on the breaker, there is a dead short from hot to ground, then the "hot" electricity travels through the new ground wire you installed, and the shortest path to ground/neutral is where the ground wire overlaps the neutral wire, but a slight gap there, so arcing????

Then this causes the one strand of wire to melt away and thus the lights now work!

Perhaps not a real good connection on the neutral bar with the ground or main neutral?

And it would be a longer path to ground/neutral going all the way down and back up again.

I would pull your light fixtures including the existing fixture and inspect the wiring. Also any junction boxes/switches associated with the new wiring. (With electricity off of course.)

If you find any burn marks including on the sides of the metal electrical boxes, let us know what you find. Post pictures if you feel like it....
 
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Old 11-11-08, 02:59 PM
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Thanks, I'll leave the breaker for the lights off and take a look this weekend. I'll let you know what I find.
 
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Old 11-14-08, 02:36 PM
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OK, I found the problem.

I ended up having a nick in the hot (black) wire under the cable clamp on a junction box. I had burn marks on the clamp and a little hole burned through it. Not sure how it got nicked, it melted through the insulation on the wire and the yellow jacket around all the wires. I was surprised that it melted a pin hole in the clamp.

The wire was discolored (instead of a shiny copper color it turned to a dull color) were it connected to the breaker. Is this because of the heat?

Thanks for the help guys, replaced the wire (was only 3-4 feet) and everything seems to be working great.
 
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Old 11-14-08, 03:09 PM
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Good detective work! And good thing you asked and stuck with it until the problem was found.

Now we can all sleep better tonight! (Unless that dog next door to me keeps me awake...)

P.S. I like to leave about an inch of the romex jacket remaining on the wire going into an electrical box.
 
 

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