The fuses are therefore not selective with respect to the disconnection of lightning impulse currents. It must therefore be ensured that, because of the behaviour of the impulse current, the maximum permissible backup fuse according to the data sheet and/or installation instructions of the protective device is always used.
From Figure 7.72 it can also be seen that, during the melting process, a voltage drop builds up across the fuse that in part can be significantly above 1 kV. For appli- cations as illustrated in Figure 7.73 a melting of the fuse can also result in a voltage protection level of the installation being significantly higher than the voltage protection level of the SPD itself.