Tracing an underground cable fault?

I'm surprised that the estate agents' subbed out "for sale/rent" guys don't to more damage or leave a "legacy" fault from their use of wooden stakes....


Regards


BOD

How about a Varley Loop Test followed by a Murray Loop Test for a more precise fault location?


Z.

No experience in this subject whatsoever, but you mentioned that the cable appears to run to its source across a garden.  


Can you see any signs of severe gardening having taken place, not only along the "obvious" route, but also in the vicinity or close by elsewhere?

Clive

These things can be a pain, and you have my sympathy - unknown route, unknown depth and unknown fault distance is sadly a common starting point.

If all cores are continuous, but there is a high resistance fault between 2 cores, or armour and core, it suggests that rather than the cable being cut right through just the insulation is damaged.

It could be pure water, but 1600 ohm is a bit low for a point fault - equally it could involve cooked insulation at some local hotspot. Not only verify core to armour, but assuming you now have all ends available, also check armour to terra-firma  via an electrode - if there is a break in the jacket, this may actually make it easier to find.

I'd be a bit wary of saying that TDR will always be able to find a 1600 ohm shunt fault easily - it is much better on a dead short or a total open. If the results are inconclusive, in certain circles it is not unknown to apply enough volts to make the fault heat up and become a proper open or short you can find. (well it is coming out anyway, so if it goes bang it goes bang.. ?) If you do this, another weapon that we sometimes reach for to trace shallow buried cables is the thermal camera, as hot spots are also a clue.


However, before getting stuck into all that, I'd agree on hiring a decent cable tracer - the sort where you inject a low frequency RF signal at one end and walk along with a receiver and  spray can or some chalk, and dot out the route on the ground  - even if you still have to divide and conquer a bit when you get close,  it will save a lot of random holes.

Hi Alan,


My first suggestion is a cat and genny, but you need the ability to connect onto the cores with the signal generator. Ideally, connect to the two good cores, with a short at the far end. This will put a good radio signal down the cable. You can then use the cat to mark the cable route, and come up with a measurement of how far to the end. Look for new fence posts on the route! The cat should give you a reasonably good depth. Whilst I never tell people the depth as I suspect they will use a jcb to get down to that level before digging by hand, mine is accurate to a few cm. 


Use the cat and genny again, but with the genny connected to the bad cores, or one bad core and armour, and no connections at the far end. If the cat is good enough, you will start with a good signal, which will drop dramatically after you pass the fault. 


A TDR is also a good tool. With good cores, you have the opportunity to measure to the end of the cable as well as the fault. You can use this ratio with your known route and length as more evidence to decide where to dig. 


Unfortunately faults with a reasonable resistance aren’t the easiest to find - a good short or a good open circuit are the favourites. 


Regards,


Alan.

An initial question, do you have any good cores not involved in the fault?


Yes, at least one, possibly 2, though that may not help, as the Boss doesnt want it left as is, as there is clearly a

fault in the cable, and he doesnt want to leave it with that fault.

The thinking is, if it is a core to core fault, then we use the armour as CPC, however, if 2 cores are damaged, the 3rd may be on the way out too.

If it is a armour to core fault, then just use that core as CPC, but, there will obviously be a break in the outer sleeve, and he doesnt want it to corrode any more, which it obviously will.

Thanks.

TDR will find it, as would a basic bridge across a few "good" cores


However, that will tell you "how far" but not which direction - start with a CAT and Genny to ascertain the route - from there, it might be obvious where the damage is - or give you an opportunity to dig a hole and cut the cable to reduce the search area


Regards


OMS

Hi Alan, 


An initial question, do you have any good cores not involved in the fault? 


Regards,


Alan.