MV cable sheath testing

[petamm]

Hi, do you have any experience with the after lying cable sheath testing in case the cable is placed in a protective plastic tube/pipe? I mean the whole lenght. Nice for protection, bad for testing and for fault location. The sheath has no direct contact to the ground so you can not guarantee the sheath is fault free. Any experiences/tips?
I have one negative experience - we did the sheath test on a new MV XLPE cable some way (sometimes it is possible even with the tubes)- Result ok as well as the other acceptance tests(VLF 3Uo 60min+PD testing). After 1 or 2 years in operation the fault occured. The problem was in a cable insulation but the manufacturer asked for the after lying test results, and of course, they were claiming it could be a mounting/laying problem. The service company said the test were OK so it could not be mounting/laying problem. The manufacturer replied: "how did you make a sheath test in case it was placed in in a protective plastic tube/pipe? It is almost impossible". (what I agree too). It was very difficult to find out what was the problem - the cable was still under warranty and we were asking for the compensation.

 

[7anoter4]

IEC 60502-2 ch. 20 Electrical tests after installation 20.1 D.C. voltage test of the oversheath
Here it is stated:
"For the test to be effective, it is necessary that the ground makes good contact with the entire outer surface of the oversheath. A conductive layer on the oversheath can assist in this respect."
A conductive layer could be moist backfill or a graphite layer. See:

http://www.cbi-electric.com/afcabweb/content/tb11.html

 

[ausphil]

Yes, we've found the oversheath test to be useless if it is in a plastic conduit.

We did some tests to find out how good the graphite coating was. We found that after about 5-10 metres, this was useless to rely on if the cable wasn't laid in a nice earthed situation.

The graphite layer is great for the spark test during extrusion, and giving a continuous connection whilst the cable is wound on the drum, however once it comes off, it doesn't do a great job of end to end earth potential if it is laid in an insulated conduit.

Ausphil