See also Uninterruptible power supply .
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A surge suppressor (sometimes optimistically called a "surge protector") is a device inserted in the alternating current ( AC ) utility line and/or telephone line to prevent damage to electronic equipment from voltage "spikes" called transients. A more accurate term for this type of device is "transient suppressor." A typical surge suppressor is a small box with several utility outlets, a power switch, and a 3-wire cord for plugging into a wall outlet.
In most countries where electronic equipment is used, the effective AC utility voltage is 110 to 120 volts; the peak voltage is on the order of plus-or-minus 160 to 170 volts at a frequency of 60 hertz . But transients, which arise from various causes, commonly reach peak levels of several hundred volts. These pulses are of short duration, measured in microseconds (units of 10 -6 second), but in that time, they can cause hardware to malfunction. The worst type of transient occurs when lightning strikes in the vicinity (it is not necessary for a power line to be directly hit). Such a "spike" can peak at thousands of volts and cause permanent damage to equipment.
A surge suppressor prevents the peak AC voltage from going above a certain threshold such as plus-or-minus 200 volts. Semiconductor devices are used for this purpose. The power line is effectively short-circuited to electrical ground for transient pulses exceeding the threshold, while the flow of normal 60-Hz current is unaffected. For the suppressor to work, a 3-wire AC power connection must be used. "Cheater" adapters, which allow 3-wire appliances to be used with 2-wire outlets or extension cords, defeat the electrical ground connection and render most surge suppressors ineffective.
Surge suppressors should be used as a matter of habit with all semiconductor-based electronic and computer hardware, including peripherals such as printers, monitors, external disk drives, and modem s. But the suppressor should not be relied upon to provide protection against lightning-induced transients. The safest procedure, inconvenient though it be, is to ensure that all susceptible hardware is plugged into the suppressor box, and to unplug the suppressor's main power cord when the equipment is not in use if you live in a thunderstorm-prone area.