Pulse breakdown tests on individual specimens with self‐healing breakdowns resulted in information on both the thermal and the electric breakdown properties. Thermal breakdown due to Joule heat was found to occur nearly uniformly over the whole specimen at voltages generally lower than the electric breakdown. voltages. Calculations with relations derived for the thermal breakdown field agreed well with observations showing increase of breakdown field with decreasing pulse duration. Electric breakdown was found to be a chance event producing breakdown holes tens of microns in diameter. The process was interpreted to consist of three consecutive stages: (1) the field produces a very small charge pulse causing a temperature rise of a few hundred degrees centigrade in a breakdown channel; (2) this induces thermal runaway in the channel; and (3) the energy stored in the specimen discharges through the channel causing destruction. The magnitude of the charge pulse, duration of the process, and the size of the breakdown hole were calculated.