Surge‑protection devices mostly rely on varistor elements, which combine high energy‑handling capacity with low‑resistance conduction. As long as grid voltage stays below the maximum continuous operating voltage, the SPD keeps a high‑impedance state across its two terminals. When lightning surges or switching transients drive the system voltage past its breakdown threshold, the internal spark gap triggers. Excess surge energy is discharged through an electric arc. After the transient event fades, the integrated arc‑splitting and arc‑quenching assembly extinguishes the arc. The SPD returns to its high‑impedance mode to safeguard downstream equipment.

Should the SPD suffer internal failure, it may remain continuously conductive and create a solid short‑circuit across the power network. It is the job of the upstream fuse to open the grounding path without delay, so the rest of the installation can keep functioning normally.

A real‑world operational difficulty arises when telling two distinct short‑circuit events apart:

Case A: Short‑circuit resulting from lightning impulse activity

Case B: Short‑circuit brought on by intrinsic SPD failure

Misclassification leads to serious hazards. Treating a lightning‑induced short‑circuit (Case A) as an SPD hardware fault (Case B) may cause circuit‑breaker tripping and main‑circuit burnout. On the flip side, mistaking a defective SPD (Case B) for a lightning‑related disturbance (Case A) leaves an unresolved short‑circuit condition on the system.

All these points become clear once you grasp the operating logic of SPD‑fronted fuses. Lightning‑induced surges feature steep voltage peaks, large impulse currents, but extremely brief duration. Fuses operate on thermal energy accumulation. Short‑lived lightning impulses are insufficient to blow the fuse while the SPD diverts surge energy. With properly sized upstream fuses, neither Case A nor Case B will destroy main‑circuit components. Only the fuse will melt, safely isolating the faulty branch.

Key Reasons for Fuse Installation Ahead of SPDs

1. Protect the SPD and its cabling against damage caused by power‑frequency follow‑current (applicable to gap‑type SPD units) following lightning strikes.

2. Simplify field servicing and SPD replacement work.

3. The upstream fuse rating must comply with SPD manufacturer data. This prevents circuit faults triggered by SPD degradation, such as rising leakage current within MOV‑type components.

If the manufacturer provides no recommended fuse parameters, apply these general selection criteria:

A = Maximum allowable fuse rating for the SPD

B = Maximum operating current of the connected distribution circuit

C = Breaking capacity of the switching device or fuse

Selection guidelines:

‑ Where B ＞ A: C ≤ A

‑ Where B ＝ A: C ＜ A, or omit the fuse entirely