Alternating and Direct Voltage Endurance Studies on Mica Insulation for Electric Machinery

A program studying voltage endurance of various mica insulations is reported. This includes dielectric breakdown data with both alternating and direct current on numerous similar samples. Effects of repetitive testing, prolonged a-c (60-cycle) stress and mechanical damage to mica insulation are reported. Detection of insulation faults is described and conclusions are drawn that d-c overpotential maintenance tests can be equally searching but less damaging than a-c overpotential tests for large high-voltage rotating machinery.