These parameters are derived from typical characteristic values of high-voltage transmission lines2. No direct conclusions about the energy stress which is imposed on the arrester during a test can be drawn from this table. For that reason the IEC standard 60099-4 provides an additional diagram which represents the converted energy in a test object, with reference to its rated voltage1, which occurs during a single line discharge2. This energy is not a fixed value, but instead depends on the arrester protective level, or more precisely, on the switching impulse residual voltage Ures with the smaller switching current impulses from Table 5. The higher the residual voltage, the less energy the arrester absorbs during the line discharge, since the line will discharge less intensely when the residual voltage is higher. The diagram referred to is depicted in Figure 21. It is now possible to easily identify the problem when the energy handling capability is specified with the help of the line discharge class. If MO resistors are applied with a given amount of specific energy handling capability, then the arrester can, depending on the residual voltage it has, be assigned to different line discharge classes. The following example proves this (the red dashed lines in Figure 21): when using MO resistors,which can absorb 2 kJ/kV of energy per line discharge (i.e., double the value, namely 4 kJ/kV, during the operating duty test – performed with two successive line discharges without becoming thermally unstable), the arrester has a line discharge class of two at a ratio of Ures/Ur = 2. However, with the same MO resistors it already can be assigned to
line discharge class three at the ratio of Ures/Ur = 2.35. But the seemingly "better" arrester with the line discharge class of three might possibly be worse for the planned application, since its protective level is higher! In order to reach the line discharge class of three while maintaining a ratio of Ures/Ur = 2, MO resistors must be used with an energy handling capability of almost 6 kJ/kV (about 3 kJ/kV per discharge: the blue dotted lines in Figure 21), that means those with greater diameters.