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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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  • 0
    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.
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  • 0
    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.
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