The definition of the line discharge class is based on the assumption that a long
transmission line, charged to a certain overvoltage during a switching operation, will
discharge into a connected arrester in the form of a traveling wave process. Assuming
the equivalent circuit diagram of a line is an iterative network of π-elements, formed by
inductances and capacitances, the current will flow at a value which is determined by
the voltage value and the surge impedance of the line, for a duration given by the length
of the line and the propagation speed of an electro-magnetic wave. Ideally, it adjusts to
a rectangular-shaped current impulse. This process must be simulated in a laboratory in
a line discharge test. In this case the current impulse is normally generated with the help
of a distributed constant impulse generator, which is nothing more than the line simulation
made up of a series connection of a finite number – about 10 to 30 – of π-elements.
The IEC standard 60099-4 now defines five different line discharge classes. Increasing
demands are made on the arrester from class one to class five, in which the electrical
parameters of the impulse generator are established for the test: