For selection of power SPD, generally 3 points need to be considered.
1. The combination of SPD, which is determined by the power supply system.
There are 5 normally used power supplysystems, TN-S，TN-C，TN-C-S，TT and IT.
Correspondence between power system and SPD combination
Single phase TN： 2P，1P+1
Single phase TT：1P+1
3 phase TN-S：4P，3P+1
3 phase TN-C、IT：3P
3 phase TT：3P+1
TN-C-S, former half is TN-C and the latter half is TN-S.
2. Max. continuous operating voltage Uc
Uc is determined by the nominal opeating voltage of the power system Un. Uc must be higer than Un(Un is the voltage between line and neutual or between line and PE). Considering grid voltage fluctuation, Uc is generally required to be at least 1.15 times greater than Un, according to the local gird quality.
3. Discharging capacity of SPD, which is determined by lightning protection level（importance） and lightning protection zone（location）.