As a cable jointer, most of the time I am called in to jobs at specific times during construction to complete high voltage cable terminations. I usually only see the end stages of most projects, and the cables have already been installed by other contractors. As the cable jointer and electrician certifying the cable installation, it is important to verify the cable has been installed appropriately. As the cables are generally buried direct underground or in conduit systems, it can be difficult to inspect the cable for physical damage that may have occurred during installation.
This is when a sheath integrity test should be performed. Eccentricity recommends testing the sheath integrity prior to installation (on the drum) and post installation (in the ground). This gives you a good baseline to compare to after the cable has been installed and whether damage has occurred during installation.
Potential hazards that can damage the MV cable sheath include:
- Binding machinery that wraps single core cables to tightly into trefoil
- Rocks / debris present in the underground trench
- Sharp edges along the route of installation
- Not using suitable aggregate material or sand when backfilling around cables
The sheath integrity test consists of a DC voltage injection on the screen wires of generally 1000V on MV cables to the general mass of earth. By proving the insulation resistance of the outer sheath is acceptable, it can be expected no damage to the outer sheath has occurred during installation. Before Eccentricity completes any terminations for our clients, we complete this sheath integrity test to further ensure the baseline of the cable installed is adequate and the termination has not compromised the sheath in any way. If you would like to know more about what happens when a sheath fault is detected, read our article on How to Find and Rectify Sheath Faults on MV Cables.