The MOV Is Doing All the Work
A voltage surge is a brief, sharp spike above the normal 120V on a circuit — triggered by a lightning strike nearby, a utility switching event, or even a large motor (like a refrigerator compressor) cycling off. The spike lasts milliseconds or less, but that's long enough to degrade or destroy sensitive electronics.
Every surge protector worth the name contains one or more metal-oxide varistors, or MOVs. An MOV is a semiconductor component with a useful trick: under normal voltage it passes electricity without resistance, but the moment voltage climbs above a set threshold — called the clamping voltage — it diverts the excess energy sideways, into the ground wire and away from your devices. The lower the clamping voltage, the tighter the protection; look for a rating of 400V or below for a quality unit.
Response time matters too, though most MOV-based protectors react in a few nanoseconds — fast enough to catch the sharpest transients before they reach your equipment.
Why Surge Protectors Wear Out
Here's what most homeowners don't realize: every surge the MOV absorbs physically degrades it. The component is rated in joules — a measure of how much total energy it can absorb over its lifetime. A 1,000-joule protector can handle ten 100-joule hits, or one 1,000-joule hit, and then it's effectively done. After that it still passes electricity to your devices — it just no longer protects them.
That's why reputable surge protectors include an indicator light showing the MOV is still functional. If the light goes out or turns red, replace the unit immediately. A protector with no such indicator is a gamble after any significant surge.
A nearby lightning strike can vaporize an MOV in a single event. Whole-house surge suppressors at the panel — which handle the biggest spikes before they reach branch circuits — add a meaningful first layer of defense, and point-of-use protectors at individual outlets catch whatever residual surge slips through.
A surge protector is not a permanent installation. Treat it as a consumable, check the indicator light after any major electrical event, and replace it on any reasonable doubt. The MOV inside it is the only thing standing between a power spike and whatever's plugged in.