Maintaining healthy sheath insulation is extremely important for the safe operation and lifespan of an MV cable. If there are faults in the outer sheath, you can expect an ingress of moisture, debris, and the premature degradation of the cables structure. In turn, it is difficult to pinpoint the location of the fault as it can occur anywhere along the run of the cable (which is usually buried underground). There are, however, methods to accurately determine the location of the fault, which apply a surge energy of around 500J that tests for breakdown and insulation resistance of the outer sheath.

When Eccentricity comes across a sheath fault, we are able to utilize specific test equipment which enables us to locate the fault very easily and accurately. Once the location of the fault has been determined, the cable can be exposed by means of vacuum digging which prevents further damage. It is at this point you can physically see the damage to the outer sheath of the cable. Damage usually occurs due to poor cable laying methods or not enough care taken during installation.

Rectifying the damage can sometimes be as simple as installing some resin filled zip tube heat shrink, which re-seals the outer sheath and re-establishes the insulation resistance. In some circumstances, further damage to the under layers of the MV cable can occur and extra repairs such as rectifying termite protection or other protective layers within the cable must be completed. Once the integrity of the cable has been re-established from the repairs, it is important to re-test the cable prior to backfill to prove the rectification works. Once verified, the cable can be backfilled and tested once again in its final resting position.

Sheath Faults
Image courtesy of EE Publishers.

Have you come across sheath faults on your installation and unsure how to proceed? Give us a call to discuss solutions suitable to your installation.