Weak Chain Link Breakdown Phenomena of Multigrade Insulation Materials

Several failures of multigrade insulation systems have been experienced in metal clad, metal enclosed switchgear, and miniature isolated phase busduct systems. The failure sequence of the series insulation system appeared to originate from a point farthest away from the ground plane and not from the insulation in contact with the ground plane. However, in actuality, the ground plane was periodically elevated in relationship to the insulated busbar due to the presence of moisture created by load and ambient temperature cycling and adverse weather conditions. The insulation material in contact with the ground plane, being a high quality track resistant material, did not exhibit permanent deterioration. However the weakest insulation material degraded with time, weakening the overall insulation path to a point that an elevated ground plane condition resulted in failures of the series insulation system. The phenomena experienced in a specific application and the alternatives considered to correct the situation are described.