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Why Does Your SPD Fail?

2026-06-25

A Surge Protective Device(SPD) is designed to protect sensitive electronic equipment from transient overvoltages. However, like any protective component, it has a finite lifespan and can fail. Understanding why it fails and how to check its condition is essential for maintaining protection.

Why Does Your SPD Fail?

1. Gradual degradation of internal components

SPD failure is most often caused by gradual degradation of its core component — the metal oxide varistor (MOV), or under certian configuration, the gas discharge tube.

Each surge discharge causes microscopic changes in the SPD's internal material structure.
Repeated surges over time can shift the MOV's clamping voltage and gradually increase its leakage current.
Eventually, the component reaches the state where it can no longer respond effectively.

Note: The degradation process is expedited in harsh environments characterized by frequent lightning strikes or grid instability.

2. Overload caused by extreme surge events

An SPD is rated for a specific surge current capacity (e.g., 20kA, 40kA, etc.). When a surge exceeds this capacity — such as from direct lightning strike — the SPD may:

Thermal damage: The excessive energy causes internal heating beyond the component's limit, causing melting or cracking.
Short circuit: MOV may be completely breakdown, resulting in a permanent short circuit.
Disconnection: Under certain conditions, the internal thermal disconnector will activate to prevent fire hazards, causing a permanent open-circuit condition and complete loss of SPD functionality.

3. Temporary overvoltage

Temporary overvoltage (TOV) refers to a voltage rise caused by power grid faults, with a relatively long duration ranging from several milliseconds to several seconds, such as:

Neutral line disconnection
Phase-to-ground fault
Grid switching operations

Unlike fast surges, TOV events carry high energy and have a long duration. If the overvoltage substantially exceeds the SPD's MCOV (Maximum Continuous Operating Voltage), most standard SPDs cannot withstand the TOV and will fail.

4. Environmental factors

High temperature: Excessively high ambient temperatures will accelerate the aging of semiconductor junctions.
Moisture and condensation: Moisture can corrode internal connections and reduce insulation resistance.
Dust accumulation: Dust accumulation can reduce heat dissipation efficiency, causing the operating temperature to rise.

How Identify SPD Failure?

Method 1: Check the status indicator light

Most SPDs are equipped with a built-in status indicator:

Indicator Light Status	Explanation
Green/Normal	The SPD is functioning normally and providing protection.
Red/Replace	The SPD has failed and must be replaced immediately.
OFF	Possible causes include power supply issues or SPD failure. Verify the power supply before considering replacement.

Note: Check the status indicator at least every 6 months. In harsh environments, monthly inspection is advised.

Method 2: External inspection

With the power supply disconnected and proper safety measures in place, visually check the SPD for any of the following indications:

Blackening marks on the enclosure	Indicates internal arcing or burn damage
Cracks or bulges on the enclosure	Possible internal gas pressure buildup
Burning smell	Evidence of overheating
Loose or melted terminals	Indicate poor connections or thermal stress

Note: The presence of any of the above conditions indicates that the SPD is almost damaged and requires immediate replacement.

Method 3: Measure the leakage current (recommended for qualified personnel only)

Leakage current refers to the minor current that passes through the SPD under rated continuous operating voltage. As the SPD ages, its leakage current gradually rises.

For a healthy MOV-based SPD, the leakage current at rated nominal voltage is normally below 20 µA.
Once the leakage current exceeds 20 µA, the SPD is considered degraded and replacement is recommended.

Procedure:

1. Ensure the system is powered at nominal voltage.

2. Using a true RMS clamp meter or a specialized leakage current tester, take measurements on the grounding conductor of the SPD.

3. Compare the measured value with the manufacturer's specifications.

Note: This test must only be performed by a qualified electrician using appropriate personal protective equipment (PPE) and tools.

Method 4: Functional test performed with a surge simulator (laboratory setting)

For critical applications, controlled surge testing can be conducted as follows:

Apply standardized surges using a combination wave generator (e.g., 8/20 µs or 10/350 µs waveforms).
Monitor the SPD's clamping voltage and response time using an oscilloscope.
If the clamping voltage exceeds the rated VPR (Voltage Protection Rating) under test conditions, the SPD has failed.

Note: This method is seldom used in routine field maintenance due to high equipment costs and complex operation.

Method 5: Remote monitoring (for smart SPDs)

Certain advanced SPD are equipped with remote signaling functionality (via dry contacts or communication modules), enabling status data to be sent to a Building Management System (BMS) or SCADA system. In the event of an alarm condition, the system will automatically alert the operator, eliminating the need for on-site manual checks.

What to Do When the SPD Has Failed?

1. Disconnect power to the circuit, provided that it is safe to do so.

2. Replace the failed SPD with a new unit meeting equivalent or upgraded specifications.

3. Check the rest of the system – a failed SPD may indicate an upstream issue that needs to be addressed.

4. Record the failure event for future maintenance planning.

Conclusion

Failure Cause Analysis	Early Warning Signs	Verification Method
Aging/Repeated Surges	Indicator turns red; leakage current rises over time	Leakage current > 20 µA
Extreme Surge Overload	Enclosure deformation, burn marks, and immediate failure	External inspection
Temporary Overvoltage (TOV)	The SPD is open-circuit or shows signs of thermal damage	External inspection + failure indicator
Environmental Stress	Gradual change in indicator status; physical corrosion	External inspection + leakage test

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