Why Does a Surge Protective Device (SPD) Require a Fuse? Purpose and Selection Criteria Fully Explained

When installing a Surge Protective Device (SPD), it is generally required to equip a backup protection fuse at its front end. Some customers may

have doubts about this, considering that it increases both equipment

procurement costs and installation complexity. So, why is it necessary

to configure a fuse? What are its functions and selection standards? This article will elaborate on the following aspects:

Main Purposes of Installing a Fuse in Front of an SPD

Preventing Damage from Power Frequency Follow Current: When a lightning

strike occurs, it may cause a power frequency follow current. Without

effective protection, this can seriously damage the SPD and related

circuits. Installing a fuse helps promptly cut off short-circuit faults, enhancing system safety.

Addressing the Risk of SPD Failure and Short Circuit: Surge protective

devices are self-healing over-voltage protection devices. However, if exposed to excessively high

voltage, excessive current, or prolonged duration, they may fail to

self-heal, leading to a phase-to-ground short circuit. In such cases, a fuse or circuit breaker is needed for backup protection.

Achieving Effective Isolation Between Load and Power Supply: The working principle of an SPD is to break down its nonlinear components upon

detecting an over-voltage, thereby triggering the load switch in the circuit to open,

disconnecting the power supply from the load equipment and protecting end devices from over-voltage damage.

Preventing SPD Overheating and Aging Due to Leakage Current: Each

lightning strike may gradually degrade the SPD’s performance. If leakage current persists, it will cause the SPD’s temperature to rise and

accelerate aging. The fuse employs a thermal protection mechanism to cut off the circuit before the SPD exceeds its tolerable heat limit.

Facilitating Replacement and Maintenance: Installing an independent fuse also simplifies the process of removing and replacing the SPD, improving maintenance efficiency.

Requirements for Fuse Selection with SPDs

The fuse must be capable of withstanding 20 standard impulse current

tests (8/20μs and 1.2/50μs) without tripping at rated current.

The fuse should provide inverse time-delay characteristics for both

long-delay and instantaneous current protection, used for overload and

short-circuit protection respectively. The fuse element must be replaced

after operation.

If the SPD’s Imax > 40kA, it is recommended to select a fuse rated between 63–100A; if Imax ≤ 40kA, a circuit breaker or fuse rated between 20–60A is recommended.

Fuse Selection Method

Let:
A = Maximum fuse strength of the SPD (A)
B = Maximum supply current of the distribution circuit (A)
C = Rated current of the selected fuse or circuit breaker (A)

Selection principles are as follows:

If B > A, then C ≤ A;

If B = A, then C < A, or C may be omitted;

If B < A, then C ≤ B, or C may be omitted.

Inspection Content for Fuses in Front of SPDs

Routine inspections should include the following items:

Check for cracks, flashover, damage, or contamination on porcelain parts;

Observe whether the fuse tube is bent or deformed;

Ensure contacts are in good condition, without signs of overheating,

burning, or melting;

Verify all assembled components are intact and not loose or detached;

Inspect lead connections for reliability and ensure clearances to surrounding components meet requirements;

Confirm installation is secure, with phase spacing and tilt angle

complying with specifications;

Check the operating mechanism for flexibility and signs of corrosion.