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| Locating a failed buried wire using a TDR |
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| cosmicray:
I am attempting to troubleshoot a problem, that is of my own making. About 25 years back, I buried an electric branch circuit (220V 60Hz) to supply an out building and a 1-HP well motor. The wires were direct buried in a trench, and were of insulation type USE-2. Three of the wires were 4-ga AL and one was 6-ga AL. Wires were color coded for the two hot sides, one neutral and one ground. The length of the run is 550-600 feet, buried at code required depth of 24-inches. Branch circuit breaker is a paired 50A type. In addition to the power wires, there was a telecom 3-pair tar-filled cable in the same trench, but it was never placed into service. About 4-5 years ago, one of the two hot wires failed, and behaved as tho it had developed an open circuit. At that time I swapped wires around, and brought the 4-ga neutral over to that hot position, and moved the failed wire into the neutral bus (on both ends). A quick fix, which seemed to solve the immediate problem, but did not reveal why one wire failed. A couple of months ago, once again, one of the hot wires failed, with similar failure symptoms. Now I have nothing to fall back on, so it's time to sort out what happened and why, and how to go about resolving this. My best hope, is that I can identify where in that long run is the failure point, dig that up, and see what happened and why, then (if possible) splice around the problem. I am trying to avoid retrenching the entire run. My best guess as to the reason for the failed wires is either a tree root, or possibly an induced lightning surge. The tools I have available are a very basic DVM, and a 100MHz SIGENT O-scope. Checking the wires (disconnected at both ends) have yielded some very odd resistance readings to ground, suggesting that I have open insulation (and possibly moisture) somewhere along the cable runs. At least one wire was indicating ra esistance reading that mimicked a capacitor charging up. Calling around to the usual sources, I have not been able to locate a suitable TDR, so I have ordered the TDR kit from farcircuits. Not as good as a professional unit, but at least I can gleam some information about the reflected signals. Does anyone have good ideas about how to troubleshoot this ? I am completely unclear about what the impedance of the buried wires would be, nor the velocity of propagation. |
| JXL:
This may work to find the break: https://www.amazon.com/F02-Underground-Locator-Tracker-Earphone/dp/B01GDZLZOU |
| Brumby:
Not sure how good your TDR solution will be - but I'd check from both ends of each wire to improve the accuracy ... or drive you nuts. |
| Gregg:
My guess is that there was water ingress into the faulty direct burial wire that caused corrosion of the aluminum and eventual failure under load. If that is the case, a TDR may give lot of false reflections and a lot of frustration as the breach may not be a clean break and the ground around it may be conductive enough to mask the reflection. An underground conductor locator would probably work much better, but a cheap one may not be much better than the TDR. Maybe using two methods and digging where they both indicate would help. You might be better to employ a professional underground locator service that has the good equipment and lots of experience. There is as much art to underground locating as there is science. The good locating devices are over $500 from what a friend of mine who locates for a large telecom company told me. |
| notsob:
Here's an old way to find it - may or may not work depending on your situation. Isolate the cable run. hook it up to the spark plug on a combustion engine (motor bike or whatever). Walk over the area with an AM radio as your detector. Cheap method, maybe worth a try |
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