Fault on underground Sub Mains cable

[kibble19]

Hi,

I am currently in the middle of completing a new installation in time for a Halloween event at a local activity centre. Over the weekend, while I was not there, it seems that they have lost power to the sub mains distribution panel.
Unfortunately I didn't have my Megger to hand yesterday as it is being calibrated, I will get it back later today, so I couldn't test the cable.
I have 400V across all three phases and neutral at the Sub station end, protected by 3 x 300A BS88 fuses. The 120mm 4Core SWA is buried direct in to the ground and runs 530m. At the other end it is connected in to a PowerPact panel. I have no voltage at the panel end. The fuse chamber was still in the on position at the sub station, but upon testing to see if I have voltage at the distribution panel end I only have continuity beep between all three phases and neutral.
I did notice that L1 fuse has blown, which I replaced yesterday but still no voltage.

I feel that the cable must be damaged underground, but I don't see how it can stay on at the fuse chamber? Any ideas?
I will be able to test the cable later today.

Also if it is damaged underground, does any one know of any tester out there to find the section of damaged cable, as its a 530m run and digging that back up will be costly!!

 

[kibble19]

How deep is the cable buried?

Averaging 18inches

 

[ruston]

I know they have had telehandler and a lot of small good lorry's going past the buried route.

Good chance a sharp rock has travelled down and pierced the cable, the backfill will have been loose and settled , the rest with a bit of help from the traffic.

 

[kibble19]

Good chance a sharp rock has travelled down and pierced the cable, the backfill will have been loose and settled , the rest with a bit of help from the traffic.

Obviously I will be able to tell later when I complete R1+R2 and IR tests
But it will still stay in the On position, do you think clean cut? Because I should have a least have 1 or 2 phases if partly damaged?

 

[ruston]

I would say that. Blown the fault clear. maybe someone other than you has replaced the fuses .
Good luck, off for the medical mot now lol.

 

[kibble19]

I would say that. Blown the fault clear. maybe someone other than you has replaced the fuses .
Good luck, off for the medical mot now lol.

Thanks
Good luck to you!

 

[Paul.M]

Had the same fault as above but on a smaller scale (only 30m-35m run).

BS88 L1 blowing taking out a third of a small office block. No obvious reason why until I looked at the newly laid cable run. It was across the entrance of a private car park and the entrance was being used for access to a new building site next door. An empty excavation truck went over the new tarmac and nothing happened. 5 minuets later a loaded truck drove over it and BANG!

Dug up the trench and they may have well used sharpen granite for back fill! It had a "dusting" of sand and the rest was just big chunks of rock. Narrow down your search, hire one of the testers above and check wherever a road goes over the cable run.

 

[Pete999]

Averaging 18inches

https://library.ukpowernetworks.co....ation+of+Underground+Cables+-+LV+to+132kV.pdf

 

[Pete999]

Averaging 18inches

Any sand bed under and over the cable, or cable tiles covering the cable, or has it just been chucked in Willy Nilly and back filled with whatever spoil was left from digging the trench?

 

[kibble19]

Any sand bed under and over the cable, or cable tiles covering the cable, or has it just been chucked in Willy Nilly and back filled with whatever spoil was left from digging the trench?

I know sand bed under and over of 100mm I was there for that, as I know they wouldn't if I didn't make that clear. Back filled with excavated spoil, I understand

 

[Pete999]

I know sand bed under and over of 100mm I was there for that, as I know they wouldn't if I didn't make that clear. Back filled with excavated spoil, I understand

As already mentioned then possible high traffic area might be the site of the fault, you could hire those testers I posted info on, but it's a specialist operation, best bet maybe would be to get a cable tracing company in, the one I used way back was THOMAS CLARKE, don't know if they are still in existence, but a quick google search would come up with option. Good Luck.

 

[static zap]

....... check wherever a road goes over the cable run.

or a emergency passing place ..if single track !

 

[ruston]

or a emergency passing place ..if single track ![/QUOTE

Even a tele handler at 18 inches in a narrow trench will devestate a cable with only four inches of sand over it and backfilled with quarry waste.

 

[Lucien Nunes]

If you can access it you can trace it. However if there are areas that are difficult to access, but you know the route accurately, you could TDR it to find out how far away the fault is from 0ne end. With a reasonable fix from that, a likely cause such as recent groundworks may present itself near the expected fault position.

 

[kibble19]

If you can access it you can trace it. However if there are areas that are difficult to access, but you know the route accurately, you could TDR it to find out how far away the fault is from 0ne end. With a reasonable fix from that, a likely cause such as recent groundworks may present itself near the expected fault position.

Sorry whats TDR?

 

[westward10]

Sorry whats TDR?

Time - Domain Reflectometer.