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. 

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.