Surge Protection

By Mike Holt for EC&M Magazine

Protection against voltage surges can take several forms, such as preventing the surge at its origin (impossible for lightning and difficult for surges associated with the power system), diverting the surge to ground before it enters the building, and finally, clamping the surge by a Surge Protective Device at the equipment. The industry trend is to call them Surge Protective Devices (SPDs), but they are also called Transient Voltage Surge Suppressors (TVSSs).

Of these approaches, only a SPD option is available to the end-user and the installation must be done by a professional in accordance with the National Electrical Code, Articles 280 for Surge Arresters and 285 for Transient Voltage Surge Suppressors: TVSSs.

Transient Voltage Surges

Transient voltage surges, sometimes called “spikes,” are short-term deviations from a desired voltage level or signal which can cause equipment malfunction or damage. A transient voltage surge on the 60 Hz AC sine wave is very brief in duration, and the surge voltage is much higher than the AC line voltage. Equipment driven by microprocessors is especially vulnerable to transient voltage surges.

Sources of Transients

Transients occur through inductive magnetic coupling and whenever line current is interrupted. Transients outside the facility are often caused by lightning or utility grid switching, but the majority of surges occur within the facility by turning loads on or off, like a variable-speed driven motor.

What Are Surge Protective Devices?

SPDs are designed to reduce potentially damaging short-duration transients present on utility power lines, data networks, telephone lines, closed circuit and cable TV feeds, and any other power or control lines connected to electronic equipment.

A SPD reduces the magnitude of surges to protect equipment by reducing the surge to a level that can safely be passed through to the load. SPDs can’t” reduce swells in the AC power, they can’t reduce the harmonic conditions produced by non-linear loads, nor can they provide utility bill savings.

How Surge Protective Devices Work

In the simplest terms, SPDs prevent damaging transient voltage surge levels from reaching the devices they protect by providing a shunt path for transients away from the load before they can enter the equipment.

A following analogy should be helpful to understand the action of a SPD. Consider a water mill protected by a pressure relief valve. The pressure relief valve does nothing until an over-pressure pulse occurs in the water supply. When that happens, the valve opens and shunts the extra pressure aside, so that it won’t reach the water wheel. If the relief valve was not present, excessive pressure could damage the water wheel. Even though the relief valve is in place and working properly, some remnant of the pressure pulse will still reach the wheel. But pressure will have been reduced enough not to damage the water wheel or disrupt its operation.

Proper Grounding

For a SPD to function properly, the electrical system and the communications systems must be grounded in accordance with the NEC. Of particular concern is reducing the difference in ground reference between port connections for multi-port appliances. This is accomplished by grounding all communications systems that enter a facility be grounded to the buildings or structure grounding electrode system. The worst possible mistake, and a violation of the NEC, is to provide separate grounds for the power system and the communications system.

Author’s Comments: Most of the material contain in this article was extracted from Leviton’ s Applications Manual and Reference Guide for Surge Protection and Line Conditioning Products and EPRIs Developing a Consumer-Oriented Guide on Surge Protection both available free, visit

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