Electrical-conductivity measurements are reported for the material As30Te48Si12Ge10 used in glass switching diodes. The conductivity at 1000 K is more than three orders of magnitude greater than is predicted by simple extrapolation of the room-temperature behaviour. The measured parameters are adequate to account for the observed ‘on’-state resistance and subnanosecond switching times of the diodes according to a model in which thermal runaway initiates discharge of the self capacitance.