A surge protector is a component that does nothing at all until the voltage on your wiring goes somewhere it should not, at which point it briefly becomes a conductor and dumps the excess to ground. It is a sacrificial part, and understanding that explains most of what is confusing about them.
What a surge is
A voltage spike lasting microseconds. Lightning is the dramatic source and the rarest; far more common are the utility switching its grid, a transformer nearby, and the motors in your own house — an air conditioner or a fridge compressor cutting out throws a spike back onto the wiring every time.
That last category is why surge damage usually looks like gradual degradation rather than a single dead appliance. Electronics absorb small hits for years until something fails and nobody connects it to anything.
How it works
The active component is typically a metal oxide varistor, which has a very high resistance at normal voltage and a very low one above a threshold. Below the threshold it is invisible to the circuit. Above it, it conducts hard and clamps the voltage, sending the energy to ground.
It wears out by doing its job
Every event degrades the component slightly. After enough of them it stops clamping effectively, and — this is the important part — it goes on looking exactly the same. Most panel-mounted units have an indicator light for precisely this reason, and a protector whose light has gone out is no longer protecting anything.
A protector that has taken a very large hit may fail immediately, which is the intended outcome. It is the cheapest component in the chain and it is meant to be the one that dies.
Panel-mounted versus power bar
A power bar protects what is plugged into it. A panel-mounted device sits at the service entrance and clamps everything downstream, which includes all the things you cannot put a power bar on: the furnace, the air conditioner, the built-in oven, the garage door opener.
They are complementary rather than competing. The panel unit takes the large incoming hits; a good bar at the desk handles what gets past it and what your own house generates.
It needs a ground to work
The whole mechanism is diverting energy to ground. On a circuit with no ground connection there is nowhere for that energy to go, which is why a surge protector plugged into an ungrounded outlet with a three-prong adapter is decorative.
How to spot it in your house
- A panel-mounted unit is a small box beside or inside the panel, usually with one or two indicator lights.
- If the indicator is off or red, the protection has been used up even though the device is still fitted.
- A three-prong outlet in an older house is worth testing before relying on a plug-in protector — an inexpensive plug-in tester will tell you whether the ground pin connects to anything.
The next question
Surge protector types — Type 1, 2, and 3 explainedRelated terms