When SPD for power is working, there might be a short circuit fault of SPD, for example, when SPD suffers a lightning current which exceeds its withstanding capacity or transient overtage from the grid . In such cases, the backup fuse will act and disconnect SPD from the line so as not to cause fire.

SPD backup fuse(“F2” in the figure) should meet the requirements below.

1. Should not act in case of surge. 

2. Should act in time and reliably in case of short circuit fault of SPD; 

3. Should Cooperate with main circuit over-current protection device F1.

The nominal current of F2 shall not be too small (unable to meet requirements 1) nor too large (unable to meet requirements 2 and 3). It is likely that the situation can not be satisfied at the same time. Then it  needs to be decided by the designer according to the actual situation. If continuous power supply is required, F2 should act and F1 should not act. If continuous SPD protection is required, F2 should meet the discharge capacity of SPD.

Assuming that the recommended backup protection fuse is X ampere, F1 and F2 are selected as follows according to different situations (see the table below) 

X value can be selected from the table below（IEC 61643-12，annex P，e.g. In=10kA，X =40；Iimp=15kA，X=200）.