Thermal runaway of metal oxide elements at high AC stress is caused by a positive
feedback process due to the temperature dependence of the resistive component of
leakage current. Moreover, the power–voltage characteristic is highly non-linear so
that a small increase in voltage will result in a considerably higher power. The effect
of this is to shift curve P of Figure 5.4 upwards. This displacement will be of similar
effect as that described for the ageing effect.