Polymeric Insulation Damage Due to AC Surface Flashover

Damage to various insulators used in telephone equipment, by a 60 Hz flashover between electrodes on the surface in air at atmospheric pressure, has been examined using a scanning electron microscope. Breakdown voltages between electrodes mounted on the insulating material and separated by approximately 1 mm were obtained. Insulation resistance before and after breakdown was measured. Each material examined showed a different damage pattern, although of those tested, only the mica-filled phenolic showed low insulation resistances after flashover. These resistances fell into three Gaussian distributions centered at 300, 1000, and 22000 Ω. Damage in all cases appeared to be due to the bombardment and thermal effects of the discharge. In most cases, melting of the surface, followed by resolidification occurred; under these circumstances the insulation resistance did not appear to have been degraded when measured shortly afterwards. However, the long term stability of the insulation after exposure to surface discharges has not been measured. Asperities on electrode edges that could lead to enhanced field emission were also observed.