Disharges

The electric strength of solid insulation may decrease greatly with time of voltage application if discharges occur 1 within gaseous inclusions, 2 at sharp-edged conductors embedded in the solid, 3 on the surface from adjacent conductors, or 4 as a result of surface contamination. Mechanisms of breakdown and factors which affect the electric strength of gases are surveyed, as a basis for understanding the complex behavior of internal and surface discharges: particularly temporal changes in their magnitude and recurrence frequency. Mechanisms by which discharges cause progressive degradation and ultimate failure are considered, also techniques for assessing the relative resistance of materials to surface or internal discharges or "treeing". Finally the review surveys the merits of different methods for ensuring against failure by discharges. Reliance on short-term over-voltage tests reduces the cost of testing but there is considerable risk of trees or tracking being initiated, rendering the insulation more likely to fail in service. Discharge measurements reduce this risk, but it is important to discriminate between discharges in the test object and external interference. There is as yet inadequate information for specifying acceptable discharge levels for the many types of insulation and service conditions. insulation; indeed, chemically active products generated in them by ionization may promote progressive electrochemical deterioration and eventual breakdown by surface tracking or thermal instability.