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Tracing an underground cable fault?

alanblaby
Has anyone any experience of tracing an underground cable fault?

The cable supplies a private estates street lighting.

It's a 3 core 4mm SWA, approx. 50 metres in length, it has a Line to Earth fault.

I haven't separated the armour and internal earth core, so it could be a armour to Line cable fault, or a CPC to LIne fault. Either way the cable needs to be repaired.

We do not have drawings of where it runs, and from a site survey, it isnt obvious how it has been run either, logic would suggest it runs under the pavement, but a test digging yesterday at the first lamppost suggests it runs a direct route to its source across a garden.

I tested at both ends yesterday, it gave a wandering ~1600 ohms L-E reading, I was hoping the reading would be steady, then I could estimate where the fault was with the differing resisitance values from each end, but the reading was not steady at all - water ingress?

I did zap it a few times at 1000V insulation test, this made no difference, and, of course, gave a 0.00 Mohm reading.


Are there Companies with the equipment that is capable of tracing these faults?

We really don't want to dig up 50 metres of tarmaced pavement.
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    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.
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  • 0
    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.
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