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.