Figure 7.68 shows cross-sectional areas and backup protection for lightning current arresters and combined lightning current and surge arresters class I. Figure 7.69 shows examples of backup protection for SPDs class II. Figure 7.70 shows the same for SPDs class III.
The specific influence of impulse currents must be taken into consideration when rating the backup fuses for SPDs (Figure 7.71). There is a noticeable difference in the way fuses disconnect short-circuit currents compared to the way they disconnect loads with impulse currents, particularly with lightning impulse currents, waveform 10/350 ms. The performance of fuses was determined as a function of the rated current of the lightning impulse current.
Section 1 (Figure 7.71): No melting
The energy brought into a fuse by the lightning impulse current is so too low to cause a melting of the fuse.