A device designed to protect electrical equipment from random spikes (sudden, sharp increases) in voltage, line surges and other temporary extreme power conditions. These devices work by detecting sudden changes in current conditions and either absorbing brief shocks or rapidly rerouting excess current to ground. The faster the device can react, the more likely it can protect an attached device from any damage, and typical reaction times for surge suppressors are measured in millionths of a second.

Most consumer-grade surge suppressors can protect against faults in the power system such as line surges and most types of transient spikes, but will not necessarily protect devices from direct lightning strikes on power lines which occur in close proximity to the device they are installed to protect.

Until the late 1990s, most consumer-grade surge suppressors used electronic components called thyristors as the protective devices in the circuit. These devices have since been replaced by faster, more accurate, and much longer-lasting rapid-switching circuits. Thyristors tend to lose their efficiency very soon after they are manufactured, and provide little or no protection after about two years. This is a function of aging, not wear and tear, so a thyristor will become ineffective over time even if the device in which it is used has never been plugged into an electrical outlet. Newer solid-state circuits that perform the same task do not suffer from this shortcoming and are likely to last as long as the devices in which they are installed.

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