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. Talk to Our Expert Team 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. How a metal oxide varistor based surge protection device diverts a transient overvoltage to earth and eventually reaches end of life Under normal voltage the MOV blocks current. During a surge, its resistance collapses and it diverts the surge to earth. Repeated surges degrade the MOV until a thermal disconnector isolates it and triggers the fault indicator. Normal operation MOV Load High resistance — no current to earth During a surge MOV Load (protected) Resistance collapses — surge diverted to earth SPD Types by Installation Point Type Installed At Type 1Main incomer — handles direct/partial lightning current Type 2Distribution boards — handles induced/switching transients Type 3Point of use, near sensitive equipment — fine residual protection 5 Reasons an SPD Faults Early or Repeatedly 1 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. 2 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. 3 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. 4 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. 5 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 eventGenuine surge event absorbed as designedReplace the cartridge/unit — normal operation Faults repeatedly, site has overhead supply or high exposureUndersized for site exposureReview discharge current rating; consider Type 1 upstream Multiple SPDs failing…