Aircraft EWIS high voltage testing considerations

There are two factors that need to be considered when using aerospace wires/cables for higher voltage systems:

1. Determining the long-term insulation life impacts of high voltage utilization
2. The impact that high voltage testers may have through repeat high potential testing

In this article we will focus on the second factor. For this factor, a common concern is the damage that high voltage testers could do to wire insulation. In situations where hundreds or thousands of short high voltage pulses (500V to 1500V) are placed on a wire, there is a concern that each application, however short, may degrade insulation performance and accumulate damage with each use.

If you are interested in more information on high voltage testing, you may want to read the article Life Predictions of EWIS Components.

Consider the following scenario: You are set to examine an aircraft that has been in service for the last 20 years. You know that the wires in that zone are aged and there are plans to replace them in the near future. The equipment you will be using to examine the wire harness, will place a short, high voltage pulse down the wire to detect opens and/or shorts. Should you be concerned about potentially damaging the wire insulation (e.g. pop a hole in the degraded insulation)?

To illustrate this, consider the following figure showing a generalized representation of wire insulation degradation. As the wire ages, the insulation withstand voltage (the voltage carrying capacity) slowly declines until it fails based on one of three reasons:

Generalized wire insulation breakdown voltage as a function of age

1. Random failure event causes damage to the insulation reducing the withstand voltage. This could be caused by a cut (due to maintenance error) or via abrasion (rubbing against structure or a worn clamp) and can occur at any time.
2. Normal operation voltage causes an insulation breakdown. This would be the natural end of service life of the wire/cable (Point #3 in the figure).
3. Harnesses tester/megger places a high voltage on the wire that causes a breakdown of the insulation (points #1 and #2 on the above figure).

Is it possible that the high voltage from a tester could create damage to wire insulation that was not there prior to the test? Yes. However, in order to get to that point, there had to have been some existing damage or wire insulation degradation (assuming the voltage is not too high) in the first place. Would it not be better that this be found on the ground under controlled conditions rather than result in a potential failure in flight?

There is a tradeoff between the voltage used for testing and the detection of faults. As the voltage increases, more failure locations can be found, but this will shorten the service wire life (causing an increase in the time gap between tester failure and normal operations failure).

Lastly, it is important to keep in mind that the high potential testers are not a “one-size-fits-all” solution. Degradation assessment may be necessary to understand the true conditions of the wiring system.