things that make you go huh?

[M. D.]

The other day I was asked to check a 12/2 U.F. cable between a house and a yard light mounted on a tree ..The owner said it just stopped working one day .. He had tried to figure it out and in so doing he cut the U.F. about 5' above grade .. inside the garage the wire comes directly through the foundation wall . I did not see a sleeve and the wire is tight as tight can be.. I am told there is no splice and that most of the run is sleeved in PVC .. Why no all of the run ? who knows.. anyway I found a defective GFCI receptacle and thought bingo .. replaced it ,..turned the circuit on and ... Bam!.. tripped the breaker. removed the GFCI and separated both ends of U.F. I took my Fluke continuity meter and rang between all three conductors...every which way.. nothing .. crossed the ends out at the tree ... tone.. . Put a "distance to break meter" on it and it said 60ft .. measured the distance above ground with tape measure 57ft .. hmmmm Go back check continuity again between conductors .. nothing .. from the conductors to the foundation nothing ... cross them at the tree ? tone .. I then separated the wires at the tree and wire nutted to the feed at the house ... flipped it on .... Bam! tripped ... like a dead short only I can't read it on my Fluke ? anyone got any ideas ?.. Also the circuit only trips when this U.F. cable is connected ... I have never seen a dead short that would not tone on my meter ..I believe the short is between the hot and the grounding .. with no neutral connections it still tripped, I think,.. though I couldn't really remember and he did not want me to play anymore as time is
money .. I was just wondering if anyone has any Ideas or has had a similar bit of F.M. (#(@%!%& magic)

 

[kwired]

A megger test would have likely failed, your fault needs higher voltage then the output of your meter to push current through the fault.

I had about the same thing one time on some new NM cable. Happened to be 10-3 cable, I ran a circuit for a clothes dryer from a new coil of cable, it would throw breaker instantly. I did not have a megger at that time but even with a DMM it had some continuity in the meg ohm ranges where it shouldn't have. Luckily it was not in a place where it was roughed in and then enclosed only to find the problem later on.

The leftover portion of the new coil didn't test very well either, my supplier was kind enough to exchange that coil for a new one.

 

[M. D.]

The analogy I used to home owner was : Think of a plastic water line filled with water under little to no pressure and it has a tiny pin hole , you wouldn't even know it is there .. until you put 120 lbs of pressure on the same water line then the leak becomes obvious.
Thanks for the response .

 

[K8MHZ]

I love problems like this. I wish I was in your area, I would be happy to take a look at it.

When I get problems that just don't make sense, I start sketching. I make a simple drawing and only include everything I can see along with the measurements.

To sum things up, if I understand correctly you seem to have a section of U.F cable that reads open, yet trips a breaker when it is connected, correct? Or is it tripping a GFCI?

 

[growler]

The other day I was asked to check a 12/2 U.F. cable between a house and a yard light mounted on a tree ..The owner said it just stopped working one day .. He had tried to figure it out and in so doing he cut the U.F. about 5' above grade .. inside the garage the wire comes directly through the foundation wall . I did not see a sleeve and the wire is tight as tight can be.. I am told there is no splice and that most of the run is sleeved in PVC .. Why no all of the run ? who knows.. anyway I found a defective GFCI receptacle and thought bingo .. replaced it ,..turned the circuit on and ... Bam!.. tripped the breaker. removed the GFCI and separated both ends of U.F. I took my Fluke continuity meter and rang between all three conductors...every which way.. nothing .. crossed the ends out at the tree ... tone.. . Put a "distance to break meter" on it and it said 60ft .. measured the distance above ground with tape measure 57ft .. hmmmm Go back check continuity again between conductors .. nothing .. from the conductors to the foundation nothing ... cross them at the tree ? tone .. I then separated the wires at the tree and wire nutted to the feed at the house ... flipped it on .... Bam! tripped ... like a dead short only I can't read it on my Fluke ? anyone got any ideas ?.. Also the circuit only trips when this U.F. cable is connected ... I have never seen a dead short that would not tone on my meter ..I believe the short is between the hot and the grounding .. with no neutral connections it still tripped, I think,.. though I couldn't really remember and he did not want me to play anymore as time is
money .. I was just wondering if anyone has any Ideas or has had a similar bit of F.M. (#(@%!%& magic)

Did you turn the circuit on with the light disconnected?

If you turned power on with the light connected maybe it's the light that's bad and not the UF cable.

Normally with a dead short you should be able to read with your fluke meter.

 

[starbolin]

Metal salts in the wire insulation. Didn't even read all the post. As soon as you said UF and yard light. I don't bother to troubleshoot them anymore. Wasted too much of my fleeting youth chasing that stuff.

Metal salts can become a semiconductor with an unpredictable knee voltage. Think zeners and MOVs. The VOM readings will vary depending on meter range and lead polarity. They will deliver phantom voltages and vary throughout the day. As you found they may read open at low voltage but have quite considerable ampacity at higher voltages.

The incorrectly installed, partial conduit did more harm than good. Collecting moisture and not allowing salts to percolate down through the soil. I've seen conduits fully packed with salts. Bad environment for a cable. Buried in concrete is another bad environment for cable.

I love distance-to-fault meters. I used to build them for 3M. 600 ohm systems. But it can be very challenging to get a good reading in these cases as the fault may be distributed over a distance of several inches to several feet. The insulation has borken and thus the impedance of the cable is different in the area of the fault. This throws off the distance calibration.

The take away from these ramblings is that, if I see any indication of a fault on buried wire, I'm going straight to pull-it-out. If the HO is uncooperative then I may just cut it close to the ground a put a box on it. I don't like to do that as I have come back and found the HO has gotten too-clever-by-half and is using the white for hot and the bare for neutral:slaphead::slaphead:

 

[M. D.]

Metal salts in the wire insulation. Didn't even read all the post. As soon as you said UF and yard light. I don't bother to troubleshoot them anymore. Wasted too much of my fleeting youth chasing that stuff.

Thanks for being honest about the not reading .. and for the metal salts hypothesis . I did not spend too much time trouble shooting and he does now have functioning GFCI receptacle in his garage .. Dig and replace was the advice and I doubt he's the type who'll get that creative.

 

[M. D.]

I love problems like this. I wish I was in your area, I would be happy to take a look at it.

Right the wire in the ground nothing attached .conductors separate no continuity between any of them .. apply 120 volts ...Bam,. breaker trips just like a dead short ..just... Bam!...not much to draw really .

 

[kwired]

The take away from these ramblings is that, if I see any indication of a fault on buried wire, I'm going straight to pull-it-out. If the HO is uncooperative then I may just cut it close to the ground a put a box on it. I don't like to do that as I have come back and found the HO has gotten too-clever-by-half and is using the white for hot and the bare for neutral:slaphead::slaphead:

May be fine for a situation like the OP has, but I have found and repaired many underground faults, some on 1000 foot plus runs, cost much less to repair then to replace. (I do have underground fault locator which makes it much easier to know where to dig.)

 

[M. D.]

May be fine for a situation like the OP has, but I have found and repaired many underground faults, some on 1000 foot plus runs, cost much less to repair then to replace. (I do have underground fault locator which makes it much easier to know where to dig.)

Would that fault locator work in this case .. how much voltage does it apply .. I put a distance meter on it and worked to find the end of the cable .. but by the nature of this fault it could not see a short current path .. would your meter have seen it ? and if so what is it. mine is a fluke ts 100 and could not see it as a short .

 

[kwired]

Would that fault locator work in this case .. how much voltage does it apply .. I put a distance meter on it and worked to find the end of the cable .. but by the nature of this fault it could not see a short current path .. would your meter have seen it ? and if so what is it. mine is a fluke ts 100 and could not see it as a short .

Actually it probably wouldn't work on that cable, unless it also was faulted to ground. It is designed to locate where return current through earth is leaving the conductor being tested. Not certain how much voltage it applies but pretty sure it is much higher then the test voltage of a typical DMM. I guess it depends if you have a cable that had been in operation and then was somehow damaged or if you had something like I described earlier with the defective NM cable I once had.