The system availability required by IEC 60439-1, even in the event of responding
SPDs, can be fulfilled by the aforementioned ‘follow current suppression’ characteristic
of the device. For SPDs with low sparkover voltage, in particular, designed to not
only take on the function of the lightning equipotential bonding but also that of surge
protection in the installation, the performance of the follow current limitation is more
important than ever for the availability of the electrical installation (Figure 7.75).