Surge Protection Devices

By Laura Reynolds

The surge protector has become a permanent feature of homes, offices and anywhere else electronic equipment is used. It may be a strip unit, as illustrated by the image, or be built into individual large electronics. The purpose of a surge protection device is to to ground or dissipate voltage from too much electricity, as in a lightning storm or transmission surge.

The Facts

  • Most surge protectors use MOVs–metal oxide varistors (or variable resistors). The wire in them, like the filament in a light bulb, is engineered to carry a certain load and will simply overheat and melt when the load becomes too great (or they wear out). These resistors are either lined up or arranged on a circuit board to stand between the protected electronics and the power supply. Surge protectors recognize transient voltage gradually and begin working gradually, draining off more voltage as the surge builds. Most units used for home electronics will protect against surges of three to four times regular 120-volt power. Once they have been “tripped” by a surge, they will cut the power to the protected equipment and the light will go out.


  • Most strip-type surge protectors can be identified by the switch and operating light. Since metal oxide varistors work only until they wear out, these strip surge protectors must be replaced periodically. Some surge protectors are incorporated into uninterruptible power supply, or UPS power modules, to protect electronics against surges (or transient voltage). These protection devices use the battery of the power module to absorb extra voltage.

Risk Factors

  • Metal oxide varistors surge protectors will not protect connected electronics from lengthy (or sustained) surges, which could create a fire hazard. They also do not “catch” sudden surges due to “spikes” (surges of less than two nanoseconds), short circuits or under-voltage (brown-outs) that may damage equipment. Equipment that is connected to another piece of equipment that is not surge-protected can be damaged by a surge passed through the unprotected equipment.


  • Surge protectors that are most commonly used for home equipment vary in size from the foot-long strip type to the shoebox-sized UPS modules. Other types of surge suppressors are engineered into electronic or mechanical equipment. Their size varies with the load and application for which they are designed. The communications industry uses suppression diodes and telephones use an older technology using carbon-arcs. Gas discharge tubes and selenium voltage suppressors are used by the power industry.


  • A power strip is not a surge suppressor. A surge suppressor will be labeled as a “transient voltage surge suppressor.” It also will have an on-off switch and red light to signal operation. It will have a plate giving a maximum voltage load (330, 400, 500 or more) certified by Underwriters Laboratory.Surge suppressors, whatever their design, will not protect against catastrophic power surges or (depending on the type) against certain, more frequent hazards. The only way to protect your equipment against all risks associated with power fluctuations is to disconnect it from the power–unplug it.