High-potential testing of aircraft equipment electric insulation

The purpose of equipment dielectric test is to detect insulation faults, not to stress solid insulation. An investigation of air-gap breakdown through holes in aircraft insulation materials simulating faults in aircraft equipment demonstrated the following: 1. the 500-volt high-potential tests will not detect common faults in aircraft insulation; 2. the common concern over insulation damage from repeated high-potential tests is not justified; 3. dielectric breakdown through small diameter faults in insulation is more likely to produce immediate failure than similar discharges over surface paths. A single circuit interruption transient discharging through a 0.003-inch-diameter hole may completely fault the insulation, while a discharge of similar energy across surface gaps may require hundreds of repetitions before failure; and 4. when small-hole discharges through insulation occur between copper and iron, insulation may fail immediately even with supposedly tracking resistant materials such as mica. Insulation puncturing and tracking circuit interruption high-voltage transients reaching values of several thousand volts are discussed. The value of these transients is shown to be substantially the same for 28-volt systems as for higher voltage systems and the critical circuit factors determining the value of these transients are analyzed.