The nominal discharge current In serves to classify an MO arrester. From a technical
point of view it is calculated from a typical maximum lightning current amplitude that
can be expected in the substation, for which the insulation coordination is performed via
the arrester’s lightning protection level. This amplitude is calculated from the flashover
voltage Ufo of the line insulators, the lightning protection level Upl of the arresters and
the surge impedance Z of the line for Imax = (2·Ufo – Upl)/Z. In this equation, the factor 2
reflects the fact that according to the rules of traveling wave processes, the lightning
current reaching the substation doubles in the arrester because the arrester is approximately
equivalent to a short-circuited line termination (with a constant voltage source in
series). Here is a worked example for a 420-kV-system: Ufo = 2.1 MV, Upl = 806 kV, Z
= 350 Ω Imax = 9.7 kA.