Underground Short Circuit Locating

[George Stolz]

I had a call a few days ago where I have a short circuit buried somewhere underground. It's a UF cable from a service disconnect to a sprinkler clock.

Anybody got a good solution for finding the short?

As I understand it, an underground fault locator wouldn't do me any good, as the conductors aren't leaking into the earth, they're pinched together inside the cable, presumably.

 

[ibew441dc]

You could have some luck with a toner. Depending on how deep it is some toners can pick up a signal.

 

[480sparky]

Try a regular locator. When the signal suddenly dies, you're close. Repeat from the other end and see if you're in the same area.

 

[kbsparky]

We would attempt to locate it with a spit-fire type unit. Pulse locator. Unless the "short" is inside of a PVC conduit, it should pinpoint the fault location.

 

[480sparky]

Would a TDR give you the length of cable to the short, then use a standard locator to map out the route and measure along that to approximate where the short is?

 

[George Stolz]

We would attempt to locate it with a spit-fire type unit. Pulse locator. Unless the "short" is inside of a PVC conduit, it should pinpoint the fault location.

What's that?

Would a TDR give you the length of cable to the short...

What's that?

 

[TOOL_5150]

What's that?


What's that?

TDR- Time Domain Reflectometer. I only know that because I know what an OTDR is, where o= Optical. Learned about it in a fiber splicing class.

It basically sends out a signal on the wire, and tells you the length to the fault.


~Matt

 

[480sparky]

What's that?


What's that?

Is there an echo in here?

I think the Split fire is a standard faul locator, using a connection to earth to find it.

TDR.

 

[hurk27]

Is there an echo in here?

I think the Split fire is a standard faul locator, using a connection to earth to find it.

TDR.

I believe a spit fire, is a thumper, it transmits a short duration pulse at a given high voltage/current to create a audio thump in the ground at the location of the fault.

now with that aside, George, most UF cables if the inter hot conductor is exposed to the earth by a damaged outer jacket and inner wire covering, it will start electrolysis, after a few days of this it starts to heat up to the point other conductors are involved, this in most cases will lead to an over all short circuit between the conductors, and once this happens it will in most cases trip the OCPD.
480 in Post #3 give one of the best ways to find such a short, in these cases, I isolate both ends of the circuit including the EGC, this gives a more accurate signal to follow.

 

[quogueelectric]

Get a good underground toner like a ditchwitch or equivalent and READ THE INSTRUCTIONS THOUROUGHLY!! Then procede to find the fault. Each situation is unique.

 

[quogueelectric]

manytimes on a fault of a long nature you can use a sensitive ohmeter and read the short from both directions. Use a simple math proportion over the distance of the run.

 

[480sparky]

I believe a spit fire, is a thumper, it transmits a short duration pulse at a given high voltage/current to create a audio thump in the ground at the location of the fault...........

But doesn't a thumper require the conductor to have some sort of connection (fault) to earth? If so, then neither a thumper or other traditional fault locator will work in this case. The fault, according to George, is within the cable itself and not to ground.

FWIW, George, you say the short is 'presumably' within the cable itself. I can't picture what kind of action you could inflict on UF that would create damage to only the insulation inside and not on the sheath itself. Have you actually checked (meter or megger) that there's no fault to ground? If there's a ground fault, a normal fault locator will make you golden.

 

[quogueelectric]

But doesn't a thumper require the conductor to have some sort of connection (fault) to earth? If so, then neither a thumper or other traditional fault locator will work in this case. The fault, according to George, is within the cable itself and not to ground.

FWIW, George, you say the short is 'presumably' within the cable itself. I can't picture what kind of action you could inflict on UF that would create damage to only the insulation inside and not on the sheath itself. Have you actually checked (meter or megger) that there's no fault to ground? If there's a ground fault, a normal fault locator will make you golden.

The thumper is just a large high voltage capacitor which allows the cap to charge with high voltage allowing large amperage and destructive current to release on a cable it does not neccesarily have to be to ground yet often is.

 

[ptonsparky]

I take it by the post that you have not been there yet. Use your megger first and make sure it isn't going to earth. Then use locater if possible. My Fluke megger has a really good ohms setting and using that as suggested by quo is a good option, if you don't have the other equipment sugggested.

Pinched wires usually seem to be found at terminations not mid run.

Reread OP, you have been there. Sort my comments as warranted.

 

[George Stolz]

No, for all intents and purposes, consider that I wasn't there. I just got moved into my new van, and had handtools and a toothpick to work with. I didn't bother with a continuity test with the Vol-Con, because I expected it to not tell me much.

I will wrestle the megger from the guy using it at the moment, and really test it, it could very well be that there's leakage to earth as well as the short.

Thanks for the replies.

 

[480sparky]

The thumper is just a large high voltage capacitor which allows the cap to charge with high voltage allowing large amperage and destructive current to release on a cable it does not neccesarily have to be to ground yet often is.

So this high-votage / high amperage 'surge' is pushed into the conductor. It's shorted to another conductor. How does the surge know where the short is so it can create a sound or make the ground shake? In other words, how can it tell a short circuit from a normal splice?

 

[M. D.]

sounds like a job for the Amprobe AT 500,. never used one but I saw a thread on here where a guy had a very similar issue and had great success with this

http://www.amprobe.com/manuals/AT2005.pdf