Why is there a fuse in front of the surge protector?

The surge protector is actually a varistor, which has the characteristics of high pass and low resistance. When the grid is running under the condition of not exceeding the maximum continuous operating voltage, there is a high resistance state between the two electrodes. If the voltage between the two electrodes exceeds the ignition voltage due to lightning strikes or operating overvoltages, the gap will be broken down and the overvoltage energy will be released through arc discharge. After the shock wave, the arc will be extinguished by the arc extinguishing system composed of the arc splitter and the arc extinguishing chamber, and it will be restored to a high resistance state to protect the system. If the surge protector itself fails, it will be turned on for a long time, causing a short circuit in the power supply/system. At this time, the front-end fuse is required to cut off the ground loop in time to ensure the normal operation of the loop. (The role of the fuse) Then how does this circuit breaker or fuse distinguish whether it is a short circuit caused by a lightning strike (referred to as A) or a short circuit caused by the damage of the surge protector itself (referred to as B), because if A is distinguished as B If the circuit breaker is disconnected, the main circuit will be burned. On the contrary, if B is identified as A, the main circuit will continue to be short-circuited and the circuit will be burned. All your questions can be summed up as long as you understand the principle of installing a fuse in the front of the lightning arrester. The lightning protection device we use is actually an induced lightning with high peak voltage, high current and very short time. The fuse must meet certain conditions to be blown, that is, energy accumulation. It is obvious that a transient lightning strike will not blow the fuse when the lightning arrester is working. Because of the fuse, no matter what you call A or B, you will not burn the circuit, but just let the fuse open, so that the circuit is safely disconnected.

 BUSSMANN fuse

The purpose of surge protector and fuse:

1. Prevent power frequency freewheeling (for discharge gap type devices) caused by lightning strikes from damaging SPD and its circuits.

2. It is convenient to maintain and replace SPD.

3. Prevent line failure due to SPD aging (such as increased leakage current of mov devices). SPD front-end fuse should be installed according to the parameters of the arrester manufacturer. If not specified by the manufacturer, the general selection principle: According to (the maximum fuse strength A of the surge protector) and (the maximum power supply current B of the connected distribution line) to determine (the breaking current C of the switch or fuse).

How to determine: When: B>A, C is less than or equal to A When: B=A, C is less than A or not installed C When: B<A, C is less than B or not installed