The fault is not always in the equipment used, but in the environment it's being used in.
An oil pumping station utilized a long overhead feeder to transfer prime power generation from diesel generators to a pumping station. This line had a series of mysterious faults and subsequent trips with no apparent cause.
To investigate this issue, instrumentation was set to capture faults at one end of the line. After capturing the first fault (see figure below), data was analyzed to determine the fault location on the feeder. That location was inspected and again yielded no conclusion of the cause. After capturing 4 to 5 faults, evaluating the fault location and performing multiple inspections, still no root cause had surfaced.
Figure 1. Voltage and current waveforms for captured line-to-ground fault on overhead line.
This led to recommending hiring locals to literally stand in several locations along the line and visually monitor the line until another fault occurred. From this action, one of the acute locals noticed insect activity on the line where wasps were building mud nests on the insulators. As shown in the figure below, this mud contamination covered significant insulator creep. It would have been just a matter of time until this mud nest would flashover on the insulator. After any flashover and fault for this issue, it is clear that the mud nest would have been vaporized leaving no indication of a problem. There is no solution that can control the nest building activities of these insects. Any material added to the insulator would compromise its insulating capability. The best and only option was replacing the overhead line with an underground cable.
Figure 2. Insulator contaminated with mud wasp nest.