The ZGG40-275 (3+1) Modular Power Supply SPD is made up of three varistor power supply modules and an air discharge tube power supply module for secondary protection of L-N and N-PE respectively. It is applied in the low voltage AC power distribution system, and connected between AC power supply and system equipment in parallel. It can prevent equipment from the impulse surge and transient overvoltage caused by the outside environment (such as lightning, EMI, etc.) or by the system (such as switching effects of the system, startup or shutoff of the inductance and capacitance load, etc.).