Why is a fuse required upstream of a surge protective device (SPD)?

A surge protective device is essentially a varistor featuring high‑throughput and low‑resistance characteristics. Under normal grid operating conditions where the voltage does not exceed its maximum continuous operating voltage, high‑resistance status is maintained between its two electrodes. When lightning strikes or switching over‑voltages push the grid voltage above its firing voltage, the internal gap breaks down, and over‑voltage energy is dissipated via arc discharge. Once the surge subsides, the arc is quenched by the arc‑splitting and arc‑extinguishing chamber assembly. The SPD then reverts to a high‑resistance state to protect the system.

If the SPD itself malfunctions, it may lock into a permanent conductive state, resulting in short‑circuiting of the power supply system. The upstream fuse must then trip promptly to isolate the earthing circuit and guarantee normal system operation. This is the core function of the upstream fuse.

One practical challenge lies in distinguishing between two fault scenarios:

Condition A: Short‑circuit triggered by lightning surges;

Condition B: Short‑circuit caused by internal failure of the SPD itself.

Mis‑identification carries risks. If Condition A is misjudged as Condition B, circuit breaker tripping may damage the main circuit. Conversely, if Condition B is misread as Condition A, the system will keep recognising a persistent short‑circuit fault.

These issues can be well understood once the working principle of the upstream fuse for SPDs is clarified. Induced lightning handled by SPDs delivers high‑voltage peaks, high impulse current, yet extremely short duration. Fuses operate based on accumulated thermal energy. Transient lightning surges will not blow the fuse while the SPD is conducting surge current. Thanks to the upstream fuse, neither scenario A nor B will burn down the main circuit; instead, only the fuse will blow to achieve safe circuit disconnection.

Purposes of installing a fuse upstream of the SPD

1. To prevent power‑frequency follow‑current (for gap‑type SPDs) from damaging the SPD and associated wiring after lightning events.

2. To facilitate maintenance and replacement of the SPD.

3. The upstream fuse shall be selected in accordance with the SPD manufacturer’s specifications, to avoid circuit faults arising from SPD ageing (e.g. increased leakage current of MOV‑type SPDs).

Where no manufacturer‑specified fuse rating is available, follow the general selection principles based on:

A = Maximum rated fuse current of the SPD

B = Maximum service current of the connected distribution circuit

C = Breaking capacity of the switch / upstream fuse

Selection rules:

- If B ＞ A: C ≤ A

- If B ＝ A: C ＜ A, or no fuse is fitted