Electronics > Beginners
Locating a failed buried wire using a TDR
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james_s:
While locating the fault sounds like a fun excercise, I don't see how you will ultimately repair faulty direct burial cable without replacing the whole run anyway. I would run new wire in conduit if faced with this situation.
Gyro:
I agree, the cable is already buried, it just doesn't know that it's dead yet!   :D

However, one very basic idea that might yield some sort of result...

On the assumption that moisture has got into the cable at some point along its length, it must have got in though some defect in the outer covering. Assuming that the defect is still damp, there should be some sort of conductive path to ground at that point.

Try isolating and commoning the conductors at both ends and taking resistance measurements between them and a ground spike, stuck into the ground at intervals along the path of the cable. Assuming that you can get some sort of resistance reading, the lowest resistance point should be in the vicinity of the damage. [EDIT: It could equally indicate a damper area of ground of course!]

It's only going to be helpful in salvaging the cable if the problem is actual mechanical damage at a particular point. If so, you'll need to take a decent chunk out to eliminate moisture creep and make really good waterproof splices.

If the ground is dry, you may need to use more voltage than the DMM resistance range provides and use a low current range but it could be worth a try. All you've got to loose at this stage is a 300ft reel of thin insulated wire (test from both ends).
Yansi:
Why would you even want to search where the problem is, after 25 years of rot?  Replace the POS cable right away. Otherwise you will be looking for short/open next year again.
tecman:
A TDR is the right tool.  Even if you cannot determine the velocity factor, you should be able to determine the fault distance as a percentage of the overall length.  Sometimes this means measuring separately from each end, and determining the ratio of the fault to the overall distance.  Since you know (I assume) the overall length and location, knowing the percent of the distance to the fault will get you quite close. 

My guess is either lightning or water intrusion has caused your fault.

Paul
Gyro:

--- Quote from: coppercone2 on April 02, 2019, 03:17:53 am ---im very curious to know what kind of results a TDR would deliver in this situation

--- End quote ---

Me too, presumably with a short impedance of several k \$\Omega\$ the reflection would be very low-Q?
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