Selection of SPD in TN system, TT system and IT system

      According to IEC61643.11, SPD installation position and protection mode selection for TN system, TT system and IT system are shown as follows

Figure 1: TT system SPD installed in RCD using 4P wiring mode

Figure 4: The IT system SPD is installed in the RCD and adopts the 3P connection form SPD

       Appendix J of GB 50057 above applies to models where the SPD front-end overcurrent protection device is a conventional fuse or microbreaker. At present, the mainstream backup protector SCB for SPD front-end overcurrent protection device can realize full current protection of SCB+SPD because its low short-circuit operation current is 3A, and the selection is easier for the user. Regardless of RCD or not, SPD with 4P protection mode can be used in TT system and TN-S system. SPD with 3P protection mode can be used in TN-C system or IT system, which can effectively cut off fault current, lower residual voltage, and better insulation protection effect for equipment.

    The surge protector in Figure 1 is mounted under the RCD(Residual Current Action Protection device). If the surge protector at this time will short-circuit failure under the impact of high energy surge or temporary overvoltage, the fault current needs to return to the power supply through the ground resistance RA, RB. When the fault current is small, RCD can be used to cut off the power supply. Using the installation mode in Figure 2, the surge protector is installed above the RCD(residual current action protection device) and adopts the mode of 3P+N. Through the barrier of the discharge gap, when the surge protection device fails in short circuit, it will not cause an electric shock accident due to the ground fault. The fault current returns through the neutral line, and the fault current is large, which will make the fuse operate and effectively cut off.