Short-circuited Romex

fmtjfw

Senior Member
I've been presented with the following problem:

20x20 2-stall garage, wooden construction, NM-B wiring. Attic over garage, finished ceiling. Wiring either notched into rafters or run on top of rafters and stapled using uninsulated staples. Attic then floored with OSB which is nailed down with 1 to 1? inch nails. Fiberglass batting with paper vapor barrier attached to fiberglass.

Wall boxes plastic. Junction boxes metal. Connectors wirenuts and buchanon crimps. About 10 duplex receptacles, 2 3-way switches. 2 8ft 2 tube 1 pin flour. fixtures. 2 garage door openers. Receptacles and lights on one circuit. Openers on another.

Homeowner plugged in a battery charger. Lights went out. Breaker tripped and would reset for only a second or so before tripping again.

Another electrician there before me. Checked ballasts, seemed ok. Heard "arcing" in attic. Told the homeowner to move stuff in attic in prep for taking up floor.

------

So I show up reset breaker, trips 2 seconds, helper in garage reports light flicker, no more "arcing" noise. Look up in attic at j-boxes no evidence of arcing found. Look at box the battery charger was plugged into, nothing apparent there. Look at 3-ways nothing apparent there. Take up some flooring, find no evidence of arcing, do find mouse poo and chewed paper but no nibbles on cable where I looked. Found 1 run of NM that had "drawn" the brown goo from the paper over the insulation where it touched.

Pulled neutral for the circuit at the panelboard. Turned on breaker. Breaker held. So not a ground fault to grounding wire.

Assuming it is a hot--neutral fault.

Here's my plan.

1) try another breaker (easy)
2) terminate white/black at panelboard using a 1kOhm resister
3) pull hot to lights
4) check resistance of lights
5) pick a receptacle ? way around wall disconnect, look for 0 or 1Kohms or open. if zero found move one left and right repeat....


This is the first time I've gotten into this and I'll applying how I would trace a shorted fire alarm pair.

Looking for better, practical system.

Thanks
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
your plan seems like a lot of work. If you have access in the attic, i would just cut the Romex were it drops down at each point and see if the short is in that leg. You can always add a j box up there to splice it back together. It is also possible there are already splice boxes up there where you can just disconnect the circuit and test it without cutting anything.

if the Romex is daisy chained from box to box you could open the chain inside the outlet boxes and test it there. Even easier.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Make sure that nothing is plugged in or turn on then start in the middle of the circuit and disconnect the end half of the circuit. Test and see if there is a short- with an ohmmeter not using the breaker. Keep splitting the circuit in half til you find the culprit.
 

wolfman56

Senior Member
>>>>Homeowner plugged in a battery charger. Lights went out. Breaker tripped and would reset for only a second or so before tripping again.<<<<

Does this mean that everything was OK before that?
If so then either the battery charger was bad, (probably not), or when plugging it in it moved just enough to short to the ground wire. If that happened then it could still be shorted even after unplugging the charger.

I would start by pulling that receptacle out, move all the wires apart and check the breaker.

RW
 

fmtjfw

Senior Member
your plan seems like a lot of work. If you have access in the attic, i would just cut the Romex were it drops down at each point and see if the short is in that leg. You can always add a j box up there to splice it back together. It is also possible there are already splice boxes up there where you can just disconnect the circuit and test it without cutting anything.

if the Romex is daisy chained from box to box you could open the chain inside the outlet boxes and test it there. Even easier.
Yea, I'm planning to split it at each receptacle, if it does loop through. If it is "tee-tapped" they it will have to be in J-boxes. The attic is still stuffed. I'm hoping I can work in the garage mostly.
 

fmtjfw

Senior Member
Make sure that nothing is plugged in or turn on then start in the middle of the circuit and disconnect the end half of the circuit. Test and see if there is a short- with an ohmmeter not using the breaker. Keep splitting the circuit in half til you find the culprit.
Wish I knew the layout. I'm familiar with the difference between an "N" search and a "log2(N)" search, very powerful. (Comes from my Comp Sci background.)
 

fmtjfw

Senior Member
>>>>Homeowner plugged in a battery charger. Lights went out. Breaker tripped and would reset for only a second or so before tripping again.<<<<

Does this mean that everything was OK before that?
If so then either the battery charger was bad, (probably not), or when plugging it in it moved just enough to short to the ground wire. If that happened then it could still be shorted even after unplugging the charger.

I would start by pulling that receptacle out, move all the wires apart and check the breaker.

RW
That was what was reported to me by the homeowner. I've verified that the short is between the hot and neutral wire, not the ground(ing) wire -- see original post. I've pulled the receptacle out, no burns no crosses. I have not disconnected the receptacle. Keep the ideas coming. Thanks.
 

220/221

Senior Member
Location
AZ
1. It's not the breaker so don't even try to replace it.

Troubleshooting 101 says,:



1. Determine exactly what is out. Test all lights and receps.

3. UNPLUG EVERYTHING and turn switches off.

4. Determine a likely and logical path for the circuit.

3. Take the circuit apart midway. If it clears, put it back together and move toward the end. If it does not clear, move to the beginning.

5. After repeatedly tripping the breaker, searching for the fault, once you clear the fault, replace it :)


Two electricians have attempted this job? Seems odd. Seems like an hours work, maybe two.
 
Last edited:

kwired

Electron manager
Location
NE Nebraska
>>>>Homeowner plugged in a battery charger. Lights went out. Breaker tripped and would reset for only a second or so before tripping again.<<<<

Does this mean that everything was OK before that?
If so then either the battery charger was bad, (probably not), or when plugging it in it moved just enough to short to the ground wire. If that happened then it could still be shorted even after unplugging the charger.

I would start by pulling that receptacle out, move all the wires apart and check the breaker.

RW
That was my thinking also, if this worked before and quit when the battery charger was plugged in, something had to happen as a result of that event. Short circuit current could have damaged another point in the system that was working before the event, could even be the breaker is damaged and will not reset.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
As others have said, divide and conquer is the easiest way to narrow it down. On another note, is there a power attic ventilator? Since they are automatic, it would not be too far fetched that it shorted out about the same time the battery charger just happened to be plugged in. That would explain the arcing sound in the attic, and the hot to neutral short.
 

readydave8

re member
Location
Clarkesville, Georgia
Occupation
electrician
I try as much as possible to find short with ohmeter due to the fact that one time working on a short in a bedroom, I walked up from the basement after resetting breaker to find the room on fire. Not saying I never reset a breaker before finding out why it tripped, just have become a little more likely to try ohmeter first.
 

cowboyjwc

Moderator
Staff member
Location
Simi Valley, CA
I still would be concerned about the arcing sound in the attic. It would not set well with me to have not found some visible form of damage. Arcs you can hear several feet away are going to do some damage somewhere.
As others have said, divide and conquer is the easiest way to narrow it down. On another note, is there a power attic ventilator? Since they are automatic, it would not be too far fetched that it shorted out about the same time the battery charger just happened to be plugged in. That would explain the arcing sound in the attic, and the hot to neutral short.
Well after I thought about it, I'm going to throw these two together. The arcing sound could be a motor trying to start.

I would go along with 220/221 also as to taking it apart and seeing if it holds. You also have to know how a building works as well as how the electrical works. You could hav a nail in the cable and when a bunch of stuff is piled on it and the weight press down on the OSB it could be pushing the nail down. If he removed the stuff like he was asked that could have relieved just enough pressure for the nail to remove it's self just enough to clear the arcing noise, but of course you still could have a short because the insulation is damaged.
 

fmtjfw

Senior Member
Two hours on Saturday "nailed" it.

Two hours on Saturday "nailed" it.

Here's what we did:
  1. insert a 200ohm resistor between hot and neutral in shorted circuit at panelbox, left floating
  2. break circuit apart at "random" receptacle, read open on one side, 200 on other
  3. Move to another outlet, repeat
  4. Finally find a hot/neutral that read 80ohms after isolation
This leads to an outdoor receptacle behind a concrete flower pot that is cracked and home owner doesn't want to move. Finally convince the owner to let us move it -- gently. There is a GFCI. Remove the GFCI from the box. Install new GFCI. Hook up all the places we had disconnected wires, put devices back in boxes. reconnect hot and ground in panelboard. Problem solved!

IMG_0245.jpg
 

fmtjfw

Senior Member
Thanks guys for all your suggestions!

Thanks guys for all your suggestions!

I've tried to cover all, redundant ones omitted.

"If you have access in the attic, i would just cut the Romex were it drops down at each point and see if the short is in that leg. You can always add a j box up there to splice it back together. It is also possible there are already splice boxes up there where you can just disconnect the circuit and test it without cutting anything.

if the Romex is daisy chained from box to box you could open the chain inside the outlet boxes and test it there. Even easier."

** all in junction boxes or outlet boxes, only cutting required was some crimped connections.

"Make sure that nothing is plugged in or turn on then start in the middle of the circuit and disconnect the end half of the circuit. Test and see if there is a short- with an ohmmeter not using the breaker. Keep splitting the circuit in half til you find the culprit."

** more or less what we did.

"Does this mean that everything was OK before that?
If so then either the battery charger was bad, (probably not), or when plugging it in it moved just enough to short to the ground wire. If that happened then it could still be shorted even after unplugging the charger.

I would start by pulling that receptacle out, move all the wires apart and check the breaker."

** did that first visit, no joy.

"Has anyone actually put an amp meter or megger on this? A couple meters instead of flipping breakers with who knows how many amps. After a while some breakers quit tripping."

** went to the terminating resistor for several reasons

"I series the hot through a light bulb and start pulling apart connections till the light goes out."

** panelboard is 60ft as the electrician walks from the garage. Lamp in series still energizes the line. That requires PPE. I can work in meterman's gloves, but would prefer not to.

"This worked a whole lot easier when all we had to do was unscrew the fuse and screw in a light bulb. Danged CBs."

**I introduced that to the electricians course for students wiring motor controls. A lit bulb is better than frying the contacts on a pushbutton when closed on a dead short.

"That was my thinking also, if this worked before and quit when the battery charger was plugged in, something had to happen as a result of that event. Short circuit current could have damaged another point in the system that was working before the event, could even be the breaker is damaged and will not reset."

** It's a mystery to me why the GFCI fried -- when the battery charger was plugged into another outlet.

"I still would be concerned about the arcing sound in the attic. It would not set well with me to have not found some visible form of damage. Arcs you can hear several feet away are going to do some damage somewhere."

** we left the homeowner with a box of nail plates and the following instructions:
take up all the flooring
where ever a cable crosses a joist, cut a notch and nail a plate over it
We're withholding the bill until later so we can take about any odd things he finds.

"On another note, is there a power attic ventilator?"

** no attic power vent.
 
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