Homebrew underground wire break locator.

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My father has a underground wire to a panel in his garage from his house. One lead of the 220V service has apparently "opened" somewhere underground. It is probably about 100 feet long.
I'd love to trace the break so I can dig it up next time I visit. We suspect that a splice was made in the wire someplace that may have failed.
Can anyone offer any good ideas for a way to locate the break without digging the whole thing up? Maybe something using a transister radio as a locator and a relay buzzer or something as a signal source.
Bob
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Bob F wrote:

There are cheap voltage proximity detectors (I've got one about the size of a pen); they cost about $10.
You can do a binary sort. Dig down to the cable at the 50' point. Any voltage? If not, the break is between the 50' mark and the source. Move toward the source to the 25' mark and dig down to the cable again.
Rinse. Lather. Repeat.
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A good idea but may not work if the break is only in the neutral wire. It might be possible to locate a neutral break by temporarily reversing the wires so the neutral is hot.
Don Young
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I can get to the wires at either end. for connections or re-connections.
Bob
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Binary search is the best way to physically minimize your effort. If you have any skill with electronics, you can try this: http://www.uoguelph.ca/~antoon/circ/foxhound.html Or, just get the equipment from Triplett.
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That's a helpful reference. I might even have the parts to breadboard that.
OR, I have a signal generator that starts at 100KHz. I'll have to experiment with that.
Bob
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Bob F wrote:

Probably most reliable is to rent a cable detector from the local rental place. I'm fortunate a friend who used to do sprinkler systems has one I can borrow on occasion. I suspect ones like his aren't that expensive if it's something you might use more than once, but I really don't know.
It was reasonably successful in isolating a break in the feeder to one of the waterers in the corrals last year -- we got within a couple of feet at 3-ft depth on the one end. I was never able to find the other end, though, even starting from that known point. In this case, it was Al cable and the sheath had failed. Being in the feedlot, even though that deep, it had been failed long enough a couple of feet had been dissolved entirely by the strong urea solution in the water, even at that depth.
But, it did find the starting point which saved quite a bit of time...
--
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If the open is "clean", that is, NOT grounded or crossed, you can determine the distance-to-open from either direction and use that information to determine where to dig. Of course, such testing generally requires the use of a TDR (time domain reflectometer), not something found in the average DIYer's toolkit.
If the open is "dirty" (grounded), the use of a ground fault locator is in order. Of course, that is also something not found in the average DIYer's bag.
I expect that neither of these testers is available at even the more widely-stocked rental place. Even if they were, successfully using such sophisticated test equipment is probably beyond the capability of a first/only-time user.
You would well to consider REPLACING the line.
Any effort made and expense incurred in repairing the existing line, if directed instead at replacing the line, would effectively reduce the cost of replacement. ...and that is IF you are successful in repairing the old line - certainly not a "given". Good luck.
--
:)
JR

Climb poles and dig holes
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wrote:

Any easy way to fake a TDR with a signal generator and an oscilloscope?
Bob
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wrote:

Yes...
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wrote:

Thanks!
Bob
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wrote:

You're welcome...
Do a search of TDR with a 555 timer, etc..then find or measure the characteristic impedance and velocity factor of the cable. That might get you close, assuming you know how the cable is run. If not, an RF generator and radio can trace the location.
Or just dig like others suggest.
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Possibly, but that is beyond MY capabilities. I'm just a telco grunt that USES the equipment. I know little about HOW it works. It took all I could muster to remember that I want to recommend a TDR and, even more, what the acronymn stands for. <grin> Please let us know how it ends up.
--
:)
JR

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Before digging I'd try something along these lines. Disconnect the 3 wires (Assuming it is North America? two hot and one neutral???) wires at both ends. Leave any ground connected. Using a multimeter that has a 'capacitance measurement' measure the capacitance of each wire with reference to the ground wire from both ends. If you find that say wire A has 0.2 microfarads and wire B has 0.1 micro farads and is roughly the same ratio from both ends B is broken somewhere around the middle. Etc. We have an electricians quality DMM (Digital multimeter) that measures capacity. You may be able to borrow? Have never done this, but many years ago did so using a 'telephone Test board' meter and DC supply reversing switch keys, to find open circuits in telephone lines. Wet leakage to ground and stray currents often made it difficult though. Any help?
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wrote:

I had a meter that measured capacitance, but it died. But this is definately worth checking out. Thanks.
Bob
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Actually, for TDR, if you have a good o'scope, you don't even need the generator. Some scopes have a "calibrate out" jack on the side. That can be used to provide a nice clean square wave signal to send down the wire. Then you measure it and the reflection coming back and calculate the distance. If you have 3 wires underground, assuming 2 will be good, you can use that as the 100 foot reference. Don't forget to include the extra wire inside the building, not just the part that is underground in your calculations.
John
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Is the reflection going to be obvious? This is something I've never tried to see.
I will have to do some experimentation before I visit again.
Bob
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Bob F wrote: ...

...
Maybe, maybe not... (How's that for precision? :) )
Depends on many factors including the cable characteristics itself, the type of fault, where the fault is located, the rise time and width of the pulse, etc., etc., etc., ... It takes some skill and experience to interpret the reflected waveform even w/ a "for purpose" instrument.
Here are a couple of links that have some decent description of some of the basics. I've never tried TDR on anything except very high impedance signal cable, so don't really have any idea of what it would look like for ordinary wiring. My old Tek scope doesn't have fast-enough rise time in the internal generator for anything of reasonable length so don't have a way to see how it might look as compared to what I have seen in the past, unfortunately.
http://www.tscm.com/riprcop.html http://www.tscm.com/riwavef.html
If you've got some good quality test gear and a sample roll of romex you could always give it a shot and see if you can figure out the length of a couple of test sections.
Overall, though, I think either the "binary section" or "rent a locater" methods will turn out to be more successful than homebrew TDR based on the level of sophistication it takes in commercial instruments to make a successful instrument. You might just get lucky though, so can't hurt to try...
--
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Hey! This is getting fun.
Thanks for the references. It looks like I might need to throw together a pulse generator for very short pulses.
Bob
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This should get you started...
http://home.tir.com/~artemus/Growler /
Try Johns suggestion of using the other 2 wires as your reference instead of wading through calculations. Impedance is listed for some romex on the 'net..
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