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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.
  • 0
    https://www.youtube.com/watch?v=tjz4CFrrMuE


    Z.
  • Former Community Member
    0 Former Community Member

    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. 




    You could diagnose the fault by thumping the cable with a time domain reflectometer (TDR) such as the Megger TDR900 which will tell you how far along the cable (distance) to the fault. Or you could use a more advanced model such as Megger TDR2000 which can tell you what type of fault you have, i.e. it can detect water ingress,  shorted conductor, etc

  • 0
    Bangs head against wall.

    Yes, we know what section of cable has the fault.

    Dripping water on the cut out?  You really think we would not disconnect the cable to check it properly?


    "Underground fault finding equipment is available but I am not an expert on the types available or how to use it in practice."


    Clearly.

  • 0
    I would initially disconnect at the lighting columns and sectionalise the sections of S.W.A. to eliminate the possibility of a fault at a column base, and to reduce the lengths to be investigated.  The problem may be due to dripping water inside a column getting onto the cut out at the base. It sounds like water ingress, perhaps at an underground joint. Underground fault finding equipment is available but I am not an expert on the types available or how to use it in practice.


    An example from across The Pond.

    https://www.youtube.com/watch?v=vJeJvk_0Exc


    Z.

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