Residential troubleshoot

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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I had an interesting troubleshoot yesterday that I have to return to finish. The kitchen/living-room/foyer lighting circuit has a short circuit that suddenly began a couple of days ago. I checked every switch box involved, making sure every switch was switched off, and found nothing. I placed a receptacle in series with the breaker and black wire, and plugged in the customer's shop vacuum as an audible test. The vacuum runs at full speed, with no variation, confirming a direct short.

Nothing I did caused any change to the vacuum, so I then disconnected the white and bare conductors of that circuit's NM cable from the panel's neutral bus, and the vacuum still runs at full speed. This tells me the black wire is grounded externally from the circuit's conductors. The only box where another circuit is involved is the sink light/disposer switches, which I opened and pulled the switches out. Still no change. I figure the next step is to visually trace the circuit in the crawl.

To me, disconnecting the neutral and EGC in the panel eliminates a staple, nail, or screw through the cable. I plan to return tomorrow with my helper to try the visual trace. I hope to minimize the time and cost to the customer any more than necessary to restore the power. Can anyone suggest a possibility other than the black wire being faulted to something solidly grounded under the house (or worse, inside a wall) that can cause this set of symptoms?
 

414Mhz

Member
Location
Canada
Nothing I did caused any change to the vacuum, so I then disconnected the white and bare conductors of that circuit's NM cable from the panel's neutral bus, and the vacuum still runs at full speed. This tells me the black wire is grounded externally from the circuit's conductors. The only box where another circuit is involved is the sink light/disposer switches, which I opened and pulled the switches out. Still no change. I figure the next step is to visually trace the circuit in the crawl.

Did you open/separate the EGCs in that box ? Wherever your hot is shorted, it could be returning on the other circuits ground.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
The only box where another circuit is involved is the sink light/disposer switches
If that's true then I think there must be a current path that strays off the premises wiring. But you can check for another circuit having its neutral improperly tied to your problem circuit's neutral by checking if you continuity at the panel between the end of the lifted neutral and the neutral bar. [Edit: and as 414MHz suggests, you can check the lifted ground the same way.]

Cheers, Wayne
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
But you can check for another circuit having its neutral improperly tied to your problem circuit's neutral by checking if you continuity at the panel between the end of the lifted neutral and the neutral bar.
Next next step.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
180218-1637 EST

Larry:

From your description there is no current flowing back thru the neutral and/or the EGC for this circuit with the problem as measured at the main panel because these two wires are disconnected at the panel. Thus, the return current has to be thru a different path to the main panel or some other way to the transformer center tap.

Therefore, there is a substantial stray magnetic field generated by a relatively open loop. Open loop does not mean an open electric circuit, but rather wires separate such that their magnetic fields do not cancel.

With a coil of 1500 turns of #33 wire on a 5/8" core measuring about 5/8 x 1.25 x 1.5" on the outside I can get moderate voltages (millivolts) from an open loop with 5 A 60 Hz of current. About 5 mV at the 1 turn coil center. This test coil is about a 5 ft diameter circle. Next to my water supply line I read 15 mV with this coil.

With a coil like this and a meter that can resolve 0.01 mV (10 microvolts) you might be able to get some idea of where the short is.

Where could you find such a coil. A small 120 V transformer that was not potted and could be disassembled to get the primary, doesn't matter that secondary is also there. A 5 or 10 W 120 V primary transfotrmer might provide a reasonably number of primary turns.

Within my home I read about 10 to 20 microvolts from this coil in most places. This coil in proximity to wires in my main panel will produce multi-mV readings. I would use a 1500 W space heater as a means of getting about 10 to 12 A at 120 V.

Another technique that might help find the unwanted return path envolves measuring voltage referenced to the neutral-EGC buses in the main panel. #12 copper wire is about 1.6 ohms per 1000 ft, or 0.0016 ohms per foot. At a change of 10 A you will see a change of 16 mV per foot. Thus, with a voltage measurement and a changing 10 A current you may be able to find what other path or paths are being used. A steady 10 A may be easy enough with which to work.

.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Short from hot to panel metal perhaps? I would start by looking at where the cable leaves the panel first. That's easier than tearing boxes apart and a short to ground by sharp metal does happen.
First thing I checked once I returned to the panel. I loosened the screws in the cable clamp to make sure it wasn't pinched, and it now moves freely. Thusly, my helper and I will hopefully be able to ID the cable from below.
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
WP_20170518_003WEB.jpg

Larry, I had a similar bolted short on a residential kitchen lighting circuit last spring. Turned out, the teenager, of the house, had hit a threeway hard enough to break the toggle pivot shaft which caused one of the two cams that operate the contacts to stop turning. It happened that the cam that broke loose was allowing its contact to close.

Now, in a normal threeway this wouldn't show up as a bolted short, but, for what ever reason, the electrician in this high-end kitchen, several years earlier, had to remove the far threeway (the hot was fed to this one), and just have a single switch controlling this particular group of lights. The electrician did it, rather than changing out the threeway to a single pole switch, by feeding the hot back on what should have been a traveler. And, evidently, did this in a recessed light J-box. I never did verify where the hot to traveler splice was. . . but the error that the broken threeway toggle cam exposed was that the white traveler (that was left over) was unconsciously tied into the neutrals in the recessed light J-box.

When the good cam was operated it shorted internal to the threeway switch to the traveler that was landed with neutrals. Welded everything together. But the toggle still moved, just a little wonky.

In your case, a cross-circuit neutral tie would lead to the fault.

While you have the series receptacle at the panel running the vacuum on the short, you could test amperage on other neutrals while turning the vacuum on and off.
.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
I placed a receptacle in series with the breaker and black wire, and plugged in the customer's shop vacuum as an audible test. The vacuum runs at full speed, with no variation, confirming a direct short.

I'm not following you here. If you used the black to the breaker for the receptacle, where did you get the neutral and EGC for the receptacle?
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
One side of the receptacle is the hot from the breaker, the other side is the black leaving the panel. He's just using the receptacle to put the Shop Vac in series with the black (shorted to neutral or ground) conductor and the hot from the breaker where it normally lands. As long as the black leaving the panel is shorted it goes back to neutral or ground, providing the neutral to run the vac.

-Hal
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
One side of the receptacle is the hot from the breaker, the other side is the black leaving the panel. He's just using the receptacle to put the Shop Vac in series with the black (shorted to neutral or ground) conductor and the hot from the breaker where it normally lands. As long as the black leaving the panel is shorted it goes back to neutral or ground, providing the neutral to run the vac.

-Hal

A more traditional approach would have been to wire in a screw socket for a light bulb and have a helper holler when the light went off. :)
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
One side of the receptacle is the hot from the breaker, the other side is the black leaving the panel. He's just using the receptacle to put the Shop Vac in series with the black (shorted to neutral or ground) conductor and the hot from the breaker where it normally lands. As long as the black leaving the panel is shorted it goes back to neutral or ground, providing the neutral to run the vac.

-Hal

I gottcha!
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
A more traditional approach would have been to wire in a screw socket for a light bulb and have a helper holler when the light went off. :)
Read how to do that in my Electricians Handbook while still in school and a set of test lights were the norm. Used it within weeks and got a pat on the back from the Journeyman I was working with.

The amp meter to find the return path is a good hint. We were working in a packed tight panel a couple weeks ago that I used a thermal imaging camera to find which was the neutral of a MWBC by scanning the neutral terminations. Got me down to choice of two that were laying closely together.
 
Residential troubleshoot

Disconnect neutrals till you see the big spark and hear the vacuum shut off!

(Amprobe each neutral to find the vacuum load a little safer)


Sent from my iPhone using Tapatalk
 

Jamesco

Senior Member
Location
Iowa
Occupation
Master Electrician
I had an interesting troubleshoot yesterday that I have to return to finish. The kitchen/living-room/foyer lighting circuit has a short circuit that suddenly began a couple of days ago. I checked every switch box involved, making sure every switch was switched off, and found nothing. I placed a receptacle in series with the breaker and black wire, and plugged in the customer's shop vacuum as an audible test. The vacuum runs at full speed, with no variation, confirming a direct short.

Nothing I did caused any change to the vacuum, so I then disconnected the white and bare conductors of that circuit's NM cable from the panel's neutral bus, and the vacuum still runs at full speed. This tells me the black wire is grounded externally from the circuit's conductors. The only box where another circuit is involved is the sink light/disposer switches, which I opened and pulled the switches out. Still no change. I figure the next step is to visually trace the circuit in the crawl.

To me, disconnecting the neutral and EGC in the panel eliminates a staple, nail, or screw through the cable. I plan to return tomorrow with my helper to try the visual trace. I hope to minimize the time and cost to the customer any more than necessary to restore the power. Can anyone suggest a possibility other than the black wire being faulted to something solidly grounded under the house (or worse, inside a wall) that can cause this set of symptoms?

As someone else suggested make sure the EGCs of the 2 branch circuits in the switch box are separated from one another.

To me, disconnecting the neutral and EGC in the panel eliminates a staple, nail, or screw through the cable.
True if the neutral and EGC is not connected to the neutral and or EGC of another circuit. Or the EGC, of the faulted circuit, is not connected to a grounded object somewhere else beyond the electrical panel.

It is possible a screw or nail was ran through the NM cable between the hot and EGC. Was any body driving any nails about the same time the lighting circuit fault tripped the breaker? Like new siding on the outside of the house? Or something was added to the inside of the house nailed or screwed to the wall studs?

Someone suggested you check for the vacuum cleaner's load current on another neutral in the panel. I would add check the EGCs as well at the panel.

EDIT:
One thing for sure, while someone is crawling through the crawlspace you can be looking for the problem elsewhere in the house.:)
 
Last edited:

growler

Senior Member
Location
Atlanta,GA
I don't go into a crawl space unless I really need to or have time to kill. The problem is that you can look right at a problem an still miss it with a visual.

You know you have a short of hot to a return path. The next problem should be to figure out which section of wiring is shorted (that way you don't have so much wiring to check).

I would disconnect the hot wires at all known connections and see if the short goes away on the home run. If the homerun is good then try to find which section of wiring is bad.

This darn thing could be shorted to a water pipe in a wall ( if it's properly bonded).
 

Jamesco

Senior Member
Location
Iowa
Occupation
Master Electrician
I don't go into a crawl space unless I really need to or have time to kill. The problem is that you can look right at a problem an still miss it with a visual.

You know you have a short of hot to a return path. The next problem should be to figure out which section of wiring is shorted (that way you don't have so much wiring to check).

I would disconnect the hot wires at all known connections and see if the short goes away on the home run. If the homerun is good then try to find which section of wiring is bad.

This darn thing could be shorted to a water pipe in a wall ( if it's properly bonded).

I would disconnect the hot wires at all known connections and see if the short goes away on the home run. If the homerun is good then try to find which section of wiring is bad.

Yeah, if you have an idea of how the branch circuit was installed.
Logic would dictate start at outlet boxes closest to the source, panel, and start there isolating the hot conductor from down stream.

I would ask the home owners if they were adding/doing anything or having anything done by someone else at the same time they lost power to the lighting circuit.

Back in the 1970s I was helping a friend reside his house. We would demo the old siding from a side of the house and then install the new lap siding. Good thing his wife was home when we was nailing one of the siding boards to the house. She came outside and ask if we were doing anything that would have caused the power to go out in the kitchen, where she was doing something? Checked the electrical and found a breaker tripped. I tried to reset the breaker and it tripped instantly.

I looked at my fiend and I said I bet we drove a nail into the wiring. Like I said it was a good thing his wife was home. We only had to pull a couple of nails to clear the fault.
 
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