Since the current is thermally activated in the low conduction region, there is a strong dependence of the V –I characteristic on the temperature, so that high temperaturescause high currents to flow, and this process is known to accelerate the ageing of the sample. As an example [81], continuous operation under 135 C for one day degrades the ZnO arrester by a decrease of 12 per cent in the voltage level, V, whichproduces a current of 1 mA through the arrester.