Basic trouble shooting dead short on old BX cable long story/question

[dhalleron]

I only do this part time even though I am licensed and insured. I don't even like service calls, I'd rather do a small new install now and then.

Customer told me the breaker stays on for a little while, then trips. She even replaced the breaker before calling me.

I wasn't thinking of a dead short from her description, so I put my amprobe on it and saw 35 amps flowing out the 15 amp breaker when I turned it on. Also heard the hum somewhere throughout the house so I turned off the breaker and put my other meter on it and saw about 3 ohms to ground. Didn't check the exact voltage of the circuit but ohms law told me it was in the right ball park, if not a little low.

So I already used 2 meters in my troubleshooting. I traced the circuit across the basement where I broke open a junction box with old brittle wire. 1 cable going up the wall had the short to ground or neutral on it. It appears to go to the 2nd floor landing where a 3 way switch is.

After breaking that junction, I was able to get the breaker to hold and the basement lights came on. Now all that doesn't work is most of the 2nd floor overhead lights and a couple receptacles. Just a matter of breaking more joints that are soldered and taped in old brittle wiring with wires too short to really pull out of switch boxes. Maybe more room in the ceiling boxes to see what is going on.

I ran out of time to finish after working on it for about 2 hours and needed to pull off and go back later.

The actual owner who this woman is buying the house from on a lease option asked me if I had a meter like they use to trace cable tv and network wiring shorts. They tell the distance down the line where the short is. I said no and I tried to explain to him the meter might tell me the short is 25 feet away, but which direction and in what box does the wiring hit first before it goes to all the other lights. Not much help in an old house where wiring is spliced and goes several directions.

I use the divide and conquer method. That's how I got the basement working. Now time to divide and conquer the second floor. Guess they are afraid my bill is going to be too high.

PS. I just remembered. I unplugged everything upstairs. I didn't take out all of the CFL lights. I wonder if one of them can cause a short like that? I was looking for a bare brittle wire touching somewhere.

Am I going about it the right way? Guess I don't know any other way.

 

[jusme123]

it would probable be cheaper/safer to rip out and replace, the problem is going to come back with wiring that old. Your putting a band aid on it, just make sure they realize this.

 

[GoldDigger]

Am I going about it the right way?

Yes.

A reflectometer (TDR) is at its best with point-to-point cable runs. If you have a situation where there are splices, parallel connections, etc. it will try to show you every splice and junction, including reflections back and forth between one junction and another, which will get very confusing very quickly. You would still have to divide first. What it could do for you, if you have separated a three or more wire connection, is tell you how far away the short is so that you have an idea whether to open up the wall two feet away or look for another box down the line.
However, with a really hard short, you may also be able to use a circuit tracer and follow the tone up or down the wall. Once you pass the short the tone may change in volume.

I doubt that a defective CFL would able to carry 35 amps. But stranger things have happened.

 

[eHunter]

I am going to echo GoldDigger's advice.
A tone and probe tracer may be your best bet at tracing where the cables are going in walls and joist space.
Once the tone probe reaches the offending fault the tone will exhibit a very sharp reduction in volume.
It will most likely be less hour and labor intensive(cheaper) to simply replace the cable instead of attempting a repair.

 

[growler]

Just from experience I think you are going to find a lighting junction box where the wires are just completely smoked. If the kitchen lights are not working that's a good starting place because the lights stay on more there with larger wattage lamps and tend to burn up sooner.

It's not a bad idea just to pull those fixtures down just to see the condition of the wire. I have seen them where all the fixture wires were burned but only one junction was completely smoked ( shorted).


Just tell the owner that you hope it won't be necessary to call in the Fire Marshal and have the house condemned. That should keep him at a distance.

 

[dhalleron]

I still might quote a complete replacement of wire. I know it really isn't going to get any better down the road.

This is a job where I did a panel change out/upgrade and added new circuits for furnace, central air, washer, dryer, sump pump. There is a lot of new wiring in the house.

The problem that I won't let happen again is that I did the new work about 2 years ago and the owner had his handyman/rehab person replace all the light fixtures, switches and receptacles. I told him from then on I did all of the electric work or none on a project of his. I did correct the handy man on a few things I saw him doing wrong.

So all fixtures, switches etc are new. Much of the wiring is new except for most of the old ceiling fixtures and existing switch and receptacle wiring. Getting new wire to 2nd floor lights where the problem is might be tough but it can be done. Replacing wire to the first floor ceiling fixtures that are now working, but using the same old wiring will be hard to do without some cutting and patching old plaster.

I do a lot of small jobs for this guy. He buys houses to rent or flip. I told him from now on when I work on a house I will try to give him a better idea of the condition of the existing wiring before they do a lot of work and get paint and such finished.

Thanks for the comments guys.

 

[GoldDigger]

The problem that I won't let happen again is that I did the new work about 2 years ago and the owner had his handyman/rehab person replace all the light fixtures, switches and receptacles. I told him from then on I did all of the electric work or none on a project of his. I did correct the handy man on a few things I saw him doing wrong.

As has been referred to in passing, with wire like this just pulling a fixture down and replacing it can cause a short, let alone working on it when there is not enough slack in the wires. So there is a very good chance that you will find your short at (or just outside ) a box.

 

[jusme123]

Just from experience I think you are going to find a lighting junction box where the wires are just completely smoked. If the kitchen lights are not working that's a good starting place because the lights stay on more there with larger wattage lamps and tend to burn up sooner.

It's not a bad idea just to pull those fixtures down just to see the condition of the wire. I have seen them where all the fixture wires were burned but only one junction was completely smoked ( shorted).


Just tell the owner that you hope it won't be necessary to call in the Fire Marshal and have the house condemned. That should keep him at a distance.

good advice growler!! Bird, squirrel, mice/rat, pigeon urine can also cause havoc on wire insulation (dries it out and makes extremely brittle)

 

[templdl]

I once was on a volunteer fire department for a number of years when we got a call for smoke in the attic of a home.
Turned out it was caused but a kinked BX cable buried under insulation. next to a ceiling joist which must have had an arcing fault which was slowly charring the joist. The joist was about 1/3 burned through but there was apparently no actual flames yet.
The home owners just had their attic insulated and evidently one of the installers must have kinked the BX when installing the insulation.
Needless to say that that the home owners were very fortunate that the burnt joist didn't flare extending into a very serious attic fire. This was an example were the faulted BX was not great enough in magnitude to trip the breaker on instantaneously and not drawing enough current to trip the breaker thermally.

 

[dhalleron]

Thanks again.

I can find the problem and/or replace the wire. I just ran out of time that day.

In the past with this old BX I have found it dried out at the box. Break the armor back a foot or so and the cloth insulation still has that wax coating.

I have attic access, so if I narrow it down to shorted brittle wiring in a light fixture box, I'll try to cut out the bad stuff and extend it. If I can't find good wire a foot back in the cable, I'll install new. Either way if I don't end up replacing all of the old BX, the owner will be made aware of possible future problems via note on my invoice.

Just wanted to see if if my divide and conquer method could be improved on with some advice. Plus the owner making me wonder if there was a magic meter for this sort of thing. I have my Klein ammeter, Fluke multi meter, tone tracker and a handful of other stuff. Even tried to use that dividing rod trick to find some underground wiring before.

 

[templdl]

PMdhalleron,
Is the failed BX terminated in a light fixture box? I may assume as such since you indicated that the insulation was trashed.
If so that light fixture may have been replaced with a fixture which required 90degC THHN wire.
Copper not only is an excellent conductor of electricity but heat also. The heat is conducted from the socket through the wire which cooks the old 60degC insulation. Even with the original fixtures it may happen especially when 60w light is replaced with a 75 or 100w bulb.
Just a thought to determine the cause of failure. If so it may be necessary to the inspect of other boxes in the structure for potential failure.

 

[dhalleron]

PMdhalleron,
Is the failed BX terminated in a light fixture box? I may assume as such since you indicated that the insulation was trashed.

Not sure where the failure is yet. I haven't been able to get back to finish trouble shooting due to my and resident's schedule.

Failure might be in a fixture box, I haven't pulled them all down yet.

I don't think it is a heat problem from recent time. The new fixtures are all using CFL lamps. The insulation was trashed in several boxes from way back all those years when incandescent lamps were used. Some of the junction boxes didn't have fixtures on them and insulation was brittle. Just plain old wiring.

Pretty sure the cause might be insulation falling off a brittle wire. Could have been borderline of falling off or rubbing through to short against a box or sharp BX edge and just normal vibration from walking or cars driving by pushed it over the limit.