For the past several years, the maintenance department has conducted insulation resistance (IR) tests on feeder cables and on conductors of critical branch circuits. Where a test indicates a problem cable, the test is repeated and if the results are consistent the cable is replaced. This program was instituted after a series of cable failure related production shutdowns. While those shutdowns happen less often, they still happen.
The plant engineer asked you to review the program, specifically looking at the pass/fail thresholds for the IR testing and whether there are tests that can identify the causes of cable damage once it’s identified for a given cable. Is the plant engineer on the right track, or should another approach be taken?
The plant engineer is confusing a PdM technology with a PM technology. Using pass/fail for IR testing of cables may identify gross failures, but it won’t catch acceleration in insulation deterioration. The program needs to be redesigned such that the IR readings are trended over time for each cable. If you can get the previous test data and trend those now, that would allow you to identify cables that should be replaced during the upcoming outage.
Cables fail for a variety of reasons, including installation errors, age, voltage spikes, and temperature. Rather than try figure out why a particular cable failed, identify controllable factors (e.g. transients) and start addressing them (e.g., transient protection).