Explosion of Air-Blast Circuit Breakers Subsequent to Thermal Degradation of Auxiliary Resistor Wire Insulation

The explosions of two of Hydro-Quebec's 230-kV air-blast circuit breakers were investigated. They resulted from the thermal degradation of the silicone rubber wire insulation that led to the formation of an explosive gas mixture (methane-air) inside one of the pressurized opening resistor chambers. Gas evolution rate versus current was measured during thermal degradation of the wire insulation in a laboratory experiment simulating real service conditions. The tolerance limits of the resistor insulation to thermal shock were evaluated. Results showed that the degradation process occurs with a very high speed. More than 300 1 of methane could be generated in the resistor chamber in less than 1 s.