Why Did The SPD Fail Root-Cause Decision Tree

Why this matters

A surge protective device (SPD) that has reached end of life shows a red/failed indicator or no longer protects. The SPD is a sacrificial device: its metal-oxide varistors (MOVs) clamp transient overvoltage by diverting energy to ground, and they degrade a little with each event until they reach end of life or fail open or short. A "failed" SPD usually means it did its job and absorbed surges, but the root cause analysis matters because the failure mode tells you what the system experienced. The split is between normal end-of-life degradation from accumulated surges, a single large transient (lightning-class or a utility swell), or a sustained overvoltage condition (an open neutral or a wiring fault) that drove the MOVs past their rating in seconds. Replacing the SPD without finding a sustained-overvoltage root cause means the new one fails the same way.

Symptom presentation

The SPD's status indicator shows failed (red, or the green "protected" light is out), an audible alarm sounds, or the unit is visibly damaged, discolored, or has a tripped internal disconnect. The customer may report a recent storm, a utility event, flickering, or that downstream electronics were also damaged. The SPD may have failed silently, only discovered on inspection. On a panel-mounted SPD, its dedicated breaker or fuse may have opened, which is the device's thermal disconnect operating to take the failed MOVs offline safely. A bulged case, a burnt-plastic smell, or a single phase indicator out while the other stays green all narrow the failure mode and the root cause behind it.

Quick checks

• Read the status indicator and any alarm; note whether one phase/MOV bank failed or all.
• Ask about recent events: lightning, a utility outage/restoration, generator transfer, or downstream equipment damage.
• Measure line-to-neutral and neutral-to-ground voltages on both legs; an abnormally high line-to-neutral or a high neutral-to-ground reading signals a sustained overvoltage or open-neutral root cause.
• Check the SPD's overcurrent protection (its dedicated breaker/fuse) and whether it opened.
• Inspect the SPD's connection leads for length and tightness; long or loose leads reduce protection and stress the device.

Isolation tree

1. SPD failed after a storm or known surge event, system voltage now normal? Normal sacrificial operation or a single large transient consumed the MOVs. Root cause is the external surge; the SPD performed as designed.
2. SPD failed with no surge event and line-to-neutral reads abnormally high? Sustained overvoltage drove the MOVs past rating. Find the source: an open or high-resistance utility/service neutral lets one leg float high, cooking the SPD on that phase.
3. Only one leg's MOV bank failed, that leg reads high to neutral? Single-leg overvoltage, classic open-neutral signature; the floating leg sees over-nominal voltage continuously.
4. SPD's dedicated breaker/fuse opened with the device shorted? The MOVs failed short (typical end-of-life or over-stress mode) and the overcurrent device cleared them safely, as intended.
5. New SPD also fails quickly after replacement? A sustained overvoltage or wiring fault is still present; do not keep replacing the SPD, fix the upstream condition.

Confirming diagnosis

Measure both legs line-to-neutral and line-to-line, and neutral-to-ground, with the SPD disconnected. On a normal 120/240V single-phase service each leg should read near nominal to neutral and the two legs near 240V line-to-line, with neutral-to-ground low (a volt or two at most). Balanced readings near nominal point to a past transient or simple end of life as the cause. A leg reading high to neutral while the other reads low, with neutral-to-ground elevated, confirms an open or high-resistance neutral upstream as the root cause, a utility or service-neutral problem that must be corrected before a new SPD goes in. Wiggle-test the service neutral and watch for the readings to swing, which localizes a loose neutral. Inspect the SPD leads: they should be short and straight, since every inch of lead and every bend adds impedance that raises the let-through voltage the protected equipment sees during a surge. Verify the SPD's overcurrent protection is sized and intact per the manufacturer, and that the SPD type and short-circuit current rating match the point of installation. If voltages are balanced and normal, the leads are correct, and the unit simply aged out after years of clamping, an end-of-life replacement is the conclusion.

Remediation

If the cause was a normal surge or accumulated end of life, replace the SPD with a unit of the correct type for its location (Type 1 ahead of or Type 2 behind the service disconnect, versus a Type 3 point-of-use device) and the correct voltage rating, using short, straight leads and the manufacturer's overcurrent protection. Keep the connection leads as short as the installation allows and avoid sharp bends so the let-through stays low. If a sustained overvoltage or open neutral was found, repair the neutral fault first (re-terminate a loose service neutral, or have the utility correct a primary-side neutral problem) and confirm balanced line-to-neutral voltage before installing the replacement, or it will fail again within minutes. Coordinate SPD ratings between cascaded devices (service plus subpanel) so the upstream device handles the bulk energy and the downstream device clamps the residual. Verify the new unit's indicators read protected on both phases after energizing. Document the root cause so the customer understands whether they experienced a one-time surge event or have an ongoing voltage problem that needs the utility involved.

References

• National Electrical Code (NFPA 70), Article 242, Overvoltage Protection (SPDs).
• NEC Article 230.71/408, Service and panelboard provisions for SPD installation.
• NEC Article 250, Grounding and Bonding (neutral integrity for overvoltage cause).
• UL 1449, Standard for Surge Protective Devices.
• IEEE C62.41/C62.45, Surge environment and SPD application guidance.