The insulating capacity of an insulation system is guaranteed provided no breakdown-producing discharge processes occur in it. When considering actual technical situations, the insulating capacity has to be described by appropriately selected voltages. The characteristic voltage for the insulating capacity of an insulation arrangement - the so-called withstand voltage - is assigned to the event 'non-breakdown' (highest voltage at which 'non breakdown' is still observed), but it has to be derived from the complemen tary event 'breakdown' since only this event is capable of being measured. This is carried out using the statistical withstand voltage. This is a quantile of order p of the breakdown voltage. Insulation arrangements employed at the same insulation level on a system need to have co-ordinated rated withstand voltages. The rated withstand voltages are agreed (standardised) voltage values which the insula tion is capable of withstanding in a standardised test method. Such test methods are used worldwide as type tests and routine tests on insulation. They are inherently only meaningful when considered from a statistical standpoint. We shall now try to establish concisely a relationship between what has been stated in relation to the variate 'breakdown voltage and the standardised test methods, as well as in relation to insulation co-ordination.

Inspec keywords: discharges (electric); electric breakdown; insulation co-ordination; statistical analysis

Other keywords: insulating capacity; insulation coordination; withstand voltage; breakdown-producing discharge process; statistical evaluation; standardised test method; characteristic voltage; breakdown voltage; insulation arrangement

Subjects: Probability and statistics; Power system protection; Insulation and insulating coatings; Dielectric breakdown and discharges