Basics of Surge Protection

Causes of Surges

Surges can be caused by sources outside a home, such as when large electrical loads are turned on and off. Disturbances can result from the routine operation of electrical machinery at a nearby factory or large commercial facility, but they can also be caused by the electric utilities protective devices.

The most damaging source of electrical surges, however, is lightning. Lightning is caused by the attraction of positive and negative charges in the atmosphere. This results in a buildup and discharge of electrical energy. Lightning can occur within a cloud, from cloud to cloud, or from cloud to earth. According to the National Oceanic and Atmospheric Administration (NOAA), there are an estimated 2000 thunderstorms at any given moment in the world, resulting in 1 00 lightning strikes every second. In the United States alone, there are over 22 million lightning strikes in an average year. A typical lightning strike might range from 20,000 to 1 00,000 amps at a potential of up to 30 million volts.

Electrical Equipment Damage

Lightning does not have to strike a home, or near a home to cause electrical damage. A lightning strike on a power line several miles away still has the potential to cause extensive electrical damage in a home. In addition to causing surges on the power line, lightning can also cause damaging surges on telephone lines and TV cables.

Thunderstorm Locations

Thunderstorms occur everywhere in the United States. The following map shows the approximate mean annual number of days with thunderstorms in the United States.

Surge Protection Terminology

There are a variety of SPDs available to protect sensitive electronic equipment from surges. In order to understand these devices, it is necessary to understand some basic terminology.

Joule Rating

One of the more common ratings for an SPD is the amount of electrical energy the device can absorb in a designated time without failing. This rating is sometimes called the joule rating because the joule (J) is a basic unit of measurement for energy. However, this rating is often referred to by other names such as transient energy rating or single pulse energy dissipation rating.

In theory, the higher the joule rating, the more energy an SPD can channel away from the protected circuit. However, procedures for testing the amount of single pulse energy that an SPD can dissipate without failing vary, so the joule rating should be considered in the context of other ratings provided by an SPD supplier.

One way to think about the concept of a joule of electrical energy is to relate it to another more familiar electrical unit, a watt. One watt is the basic unit of measurement for power, and power is the rate at which energy is used. More specifically, for every watt of power, one joule of energy is used every second (joules = watts x seconds). This means that a common 7 5 watt light bulb uses 7 5 joules of energy per second.