Surge Protection Device Showing a Fault Indicator? What It Means & What to Do
A surge protection device (SPD) showing a fault or "end-of-life" indicator has almost always done exactly the job it's designed to do: it absorbed one or more transient overvoltage events until its internal protection components reached the end of their service life, and it's now telling you it needs replacing. This isn't a device malfunctioning — it's a consumable safety component that's used up its capacity, similar in spirit to a fuse that's blown to protect what's downstream of it.
Here's what the fault indicator actually means, why it happens, and what to check before simply swapping the cartridge or unit.
Never leave a faulted SPD in place "because the board still works." Once an SPD has reached end-of-life, it can no longer protect the circuit from a surge — and on some types, a failed unit that isn't disconnected by its internal thermal fuse can become a fire risk. Replace it promptly.
Why an SPD Wears Out by Design
Most low-voltage SPDs use a metal oxide varistor (MOV) as their core protection component. Under normal supply voltage, the MOV presents extremely high resistance and effectively sits invisible in the circuit. When a transient overvoltage arrives — from a lightning strike on the network, a nearby switching event, or a large load switching off elsewhere in the building — the MOV's resistance collapses almost instantly, diverting the surge current safely to earth and clamping the voltage the rest of the installation sees. Every time it does this, the MOV degrades slightly. Enough transient events, or one severe one, and its leakage current in normal operation rises to the point where the SPD's internal thermal disconnector isolates it before it can overheat — which is exactly what triggers the fault/end-of-life indicator.
SPD Types by Installation Point
| Type | Installed At |
|---|---|
| Type 1 | Main incomer — handles direct/partial lightning current |
| Type 2 | Distribution boards — handles induced/switching transients |
| Type 3 | Point of use, near sensitive equipment — fine residual protection |
5 Reasons an SPD Faults Early or Repeatedly
A Genuine Severe Surge Event
A single large transient — a nearby lightning strike, a major switching event on the network — can consume most or all of an SPD's remaining service life in one event. If the fault appeared right after a storm or a known network disturbance, this is almost certainly the explanation, and it's the SPD doing exactly its job.
Undersized SPD for the Site's Exposure
A site with overhead supply cabling, a tall structure, or a location with a known high isokeraunic (lightning) level puts far more stress on its SPDs than a typical urban site fed underground. If a Type 2 device is failing repeatedly and quickly, the surge energy it's actually experiencing may exceed what its rated discharge current was ever designed to absorb — a higher-rated device, or an upstream Type 1, may be the real fix.
Sustained Overvoltage Rather Than a Transient
SPDs are designed to clamp brief transients, not to sit conducting continuously. A sustained overvoltage condition — a lost neutral on a TT/TN system, a generator or UPS output out of tolerance — can keep the MOV conducting far longer than a normal surge event, ageing or destroying it rapidly. If SPDs on a board are failing unusually often with no storm activity to explain it, check the supply voltage stability first.
Natural End-of-Life From Cumulative Wear
Even without one dramatic event, ordinary background transients accumulate wear on the MOV over the years. This is completely normal ageing rather than a defect, and it's why SPDs are treated as a maintenance-replaceable consumable rather than a permanent fit-and-forget component.
Genuine Manufacturing Defect
The least common cause, but worth ruling in if a brand-new unit faults almost immediately with no surge activity or supply issue to explain it. A new replacement from a different batch, alongside checking installation and earthing were correct, will usually confirm or rule this out.
Quick Reference: What to Do When the Indicator Shows Fault
| Situation | Likely Cause | Action |
|---|---|---|
| Faulted after a storm or known switching event | Genuine surge event absorbed as designed | Replace the cartridge/unit — normal operation |
| Faults repeatedly, site has overhead supply or high exposure | Undersized for site exposure | Review discharge current rating; consider Type 1 upstream |
| Multiple SPDs failing with no storm activity | Sustained overvoltage on the supply | Check supply voltage stability and neutral integrity |
| Old unit, gradual fault, no specific trigger event | Normal cumulative end-of-life | Replace as routine maintenance |
| Brand-new unit faults immediately | Possible manufacturing defect or installation fault | Check installation/earthing; replace from a different batch |
Frequently Asked Questions
Is it safe to keep the board running with a faulted SPD in place?
The rest of the board's circuits keep working normally, since a faulted SPD's thermal disconnector isolates it from the circuit rather than causing a wider outage — but the installation no longer has surge protection at that point, and any equipment downstream is exposed to the next transient event. Replace it as soon as reasonably practicable, not on a "when convenient" basis.
Can I just replace the cartridge instead of the whole SPD?
Many SPD ranges use a plug-in replaceable cartridge design specifically so the base unit and wiring don't need disturbing — check whether the installed device is a cartridge type before assuming the whole unit needs replacing.
How do I know if I need a Type 1, Type 2, or Type 3 SPD?
Type 1 is required where there's a risk of direct lightning current entering the installation, typically via an external lightning protection system or overhead supply — check this against a site risk assessment. Type 2 is the standard distribution board protection for most installations. Type 3 is added close to particularly sensitive equipment as additional fine protection, on top of — not instead of — Type 1/2 upstream.
Does an SPD protect against every kind of electrical fault?
No — an SPD only protects against transient overvoltage events. It does nothing for overcurrent, earth leakage, or sustained overvoltage, all of which need their own dedicated protection (MCB/MCCB, RCD, or a voltage-monitoring relay respectively).
How often should a healthy SPD be inspected?
Most SPDs have a visible status indicator (green/red flag) that should be checked as part of routine electrical maintenance visits — annually at minimum, or more often on sites with high surge exposure — rather than waiting to discover a fault only when equipment downstream is damaged by an unprotected transient.
Related Reading
Surge protection sits alongside the rest of a distribution board's protective devices — these guides cover the others:
Shop the Parts
| Product | Use | Link |
|---|---|---|
| ABB OVR Surge Protection Device, 275V, 40kA | Standard Type 2 distribution board replacement | View product → |
| ABB OVR Surge Protection Device, 400V ac, 40kA | Three-phase distribution board protection | View product → |
| ABB Surge Arrester, 20kA | Higher-exposure sites needing greater discharge capacity | View product → |
| Full surge protection device range | Browse by type (1/2/3), discharge current and voltage rating | Browse surge protection devices → |
| Residual current devices | Earth fault protection to pair alongside SPDs on the same board | Browse RCDs → |
| All genuine ABB products | Surge protection, RCDs and switchgear from the same manufacturer | View all ABB products → |
Disclaimer: ABB, and any other manufacturer or brand names referenced in this article, are used solely for identification and compatibility purposes. Z&F Corporation is not affiliated with, authorised by, or an official representative of these manufacturers; all trademarks are the property of their respective owners.
