MEDIUM AND HIGH VOLTAGE CABLE TESTING BASIC INFORMATION AND TUTORIALS

Most cables that are rated for use at voltage levels above 600 V are shielded cables. A shielded cable has a conductor in the center, a semiconducting layer over the strands that is surrounded by insulation, a semiconducting layer, and then a metal foil or wire mesh that surrounds the whole assembly.

There is usually another layer over the shield that makes up the outer jacket of the cable. It is a common practice to hi-pot test the cables on initial installation in order to verify that the cables were not damaged when they were pulled into place and that all the splices and/or terminations were installed properly.

The voltage level that is selected usually is lower than factory test levels, frequently 80% of the dc equivalent of the factory test level.

There are normally two considerations that are given to hi-pot testing of cables as a routine maintenance practice. One is a function of the chosen maintenance philosophy [i.e., breakdown maintenance, preventive maintenance, predictive maintenance, or reliability-centered maintenance (RCM)].

The other depends upon the type of operation and how critical it is to have continuous power without interruption.

The debate on whether or not to perform maintenance hi-pot testing centers around the fact that a cable in marginal condition can be caused to fail by the hi-pot test itself. A cable that is in good condition should not be harmed.

People who subscribe to maintenance testing feel that it is much better to have the cable fail under test. Cable maintenance testing frequently is performed at 50-65% of the factory test voltage.

Problems can then be corrected while the circuit is intentionally shut down, thus avoiding an in-service failure that could interrupt production.

It is important to remember that the necessary material, such as splice kits or cable terminations, should be available to facilitate repairs should the cable fail during testing.