Locating a failed buried wire using a TDR

[TERRA Operative]

If there is a break in the insulation, you'll find that there will have been water ingress that will track down the cable by capilliary action.
You might find the break in one location, but the cable will be damaged for meters or more either side, and you'll be digging it up again before long.

In my opinion as an electrician, now is the time to lay a new cable inside a generously sized conduit.

[cosmicray]

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.

If for no other reason, I would like to know what the actual failure mode is/was. Today I tried to learn what the normative life expectancy of this type of wire in a direct burial situation is. Depending on the source, and particular variables, the numbers range from 20 to 50 years. If this wire failed due to a tree root getting into it, that is one thing. If it failed due to multiple insulation breaches caused by lightning, that is quite another. Induced lightning could very well be the cause. It may also cause me to reconsider burying the wire at all, and opt towards an overhead feed. Each has it's strengths and weaknesses. 25 years ago it never occurred to me that buried wire might be this vulnerable. Finding out what happened is as important as fixing it.

[tautech]

25 years ago it never occurred to me that buried wire might be this vulnerable. Finding out what happened is as important as fixing it.

Unless the cable is specified for direct burial I'd say you did OK for 25 years using ordinary mains cable but you would have got longer and probably permanent if it was in conduit or even PCV water pipe.

I lost a 2.5mm² sub-main to my garage some years back and ~25 years since the previous owner installed it even though there was only 4m of length underground.
Unless the sheathing is specified for marine or direct underground placement, it's always gunna fail.

Bite the bullet and do it properly this time.

[james_s]

What makes you think the cable was not rated for direct burial?

I've seen direct burial (UF) cable fail on multiple occasions, we have a lot of rocks in the soil out here.

[TheNewLab]

im very curious to know what kind of results a TDR would deliver in this situation

Precise with the right tool.

Google: DTF measurement

Still, my first port of call would be a sparky with a mains cable locator and follow the line of the hit until it stops.

LOL! DTF--old school touchtone  telephone?

[TheNewLab]

A TDR cannot display where a cable fault or junction is located only the distance to it based on certain parameters and as previously mentioned where a cable incorporates junctions or is saturated at multiple points determining the exact location of faults will be a tricky process. Half the battle for these types of jobs is determining the location and direction of the original cable trench and any branch feeds.

What Muttley said, it's not likely to be an easy job for a TDR because your cable has failed once already and it's possibly damaged in several places as there's no guarantee it failed in the same place, it's going to be further complicated due to water ingress and mechanical factors, sharp bends, direction changes, even the moisture level in the ground can make a difference to how the TDR 'sees' the cable if the insulation is compromised.

Personally, I think I'd bite the bullet and replace the lot, if I could afford to I'd also run a conduit so the cable can be pulled and replaced with relative ease next time.

My own typos aside..
wouldn't be easy to just test each witre with a TDR if different locations? if insulated there should not be much crossover.

Mr. Calson's lab built a DIY TDR reflectometer that was accurate within inches...

[Muttley Snickers]

LOL! DTF--old school touchtone  telephone?

DTF - Distance to fault, not to be confused with DTMF - Distance to my fridge.   

[Brumby]

... not to be confused with DTMF - Distance to my fridge.   

 


(But I did laugh)