The name of the next characteristic point is somewhat misleading. The rated voltage
(the symbol: Ur) of a metal-oxide arrester is not, as one might at first assume, a voltage,
which can be limitlessly applied (that one is the continuous operating voltage mentioned
earlier). Instead it characterizes the capability of the arrester to deal with temporary
overvoltages in the system. It can only be applied temporarily – the standards stipulate a
time period of 10 seconds. Some manufacturers permit a time period of 100 seconds.
The characteristic shows that under these conditions there is a leakage current (more
precisely, its resistive component) of ca. 300 μA. In extended operation, this would lead
to an increased operating temperature of the arrester, but not within a time period of ten
or even one hundred seconds. The actual cause of the temporary time limit is the sudden
great increase in the temperature and the frequent rise in leakage current due to its positive
temperature coefficient (the temperature-dependence of the U-I-characteristic is not
shown in the simplified depiction in Figure 2), after, for example, the arrester has diverted
a current impulse to the ground (that is, after it had to "operate"). In this case an
extensive application of the rated voltage could render the arrester incapable of recooling;
instead it would become thermally unstable and would continually heat up until it
reached self-destruction (so-called thermal runaway).