Why Is A Fuse Needed Before A Surge Protector?

A surge protector is essentially a varistor, exhibiting high impedance at low voltage and low impedance at high voltage. When the power grid operates below the maximum continuous operating voltage, the two electrodes are in a high-impedance state. If the voltage between the two electrodes exceeds the ignition voltage due to lightning strikes or operating overvoltage, the gap is broken down, and the overvoltage energy is released through arc discharge. After the surge, the arc is extinguished by an arc extinguishing system composed of arc splitting plates and an arc extinguishing chamber, restoring the high-impedance state to protect the system. If the surge protector itself malfunctions, it will remain in a continuously conducting state, causing a short circuit in the power supply/system. In this case, the upstream drop out fuse needs to immediately disconnect the grounding circuit to ensure the normal operation of the circuit (the function of the fuse). So how does this circuit breaker or fuse distinguish whether the short circuit is caused by a lightning strike (referred to as A) or by damage to the surge protector itself (referred to as B)? Because if A is misidentified as B, the circuit breaker will trip, and the main circuit will burn out. Conversely, if B is misidentified as A, the main circuit will continue to short circuit, also leading to circuit burnout. All your questions can be answered if you understand the principle of adding a fuse before the surge protector. The lightning that our surge protectors protect against is actually induced lightning with high voltage peaks, high current, and very short duration. A fuse needs to meet certain conditions to blow, namely energy accumulation. A transient lightning strike obviously won't blow the fuse while the surge protector is working. Because of the fuse, neither your so-called A nor B will burn out the circuit; it will only cause the fuse to blow, thus safely disconnecting the circuit.

The purpose of adding a fuse to the surge protector:
1. To prevent damage to the SPD and its lines due to power frequency follow current caused by lightning strikes (for discharge gap type devices).
2. To facilitate maintenance and replacement of the SPD.
3. To prevent line faults caused by SPD aging (such as increased leakage current of MOV devices). The fuse at the front end of the SPD should be installed according to the parameters provided by the lightning arrester manufacturer. If the manufacturer does not specify, the general selection principle is as follows: Determine the breaking current (C) of the switch or fuse based on (the maximum fuse rating of the surge protector A) and (the maximum supply current of the connected power distribution line B).
Determination method: When B > A, C is less than or equal to A. When B = A, C is less than A or C is not installed. When B < A, C is less than or equal to B.