Electrical Safety

How Surge Protectors Work?

Surge Protective Devices (SPDs) are designed to limit damage from electrical surges. Surges happen extremely fast and may pass through electrical breakers before the breaker can trip. This is because breakers are designed for over current protection, and not surges. Electrical surges can become powerful enough to damage or kill downline products that include electronics. Even if downline products continue working, their product lifespan may be reduced.

Surge protection has become even more important these days as many more products include electronic chips. Even LED lighting can be affected by surge activity. While commercial/industrial surge applications have been around for years, the NEC 2020 code was updated to mandate surge protection for residential dwellings as well.

Normal SPD Operation

Surge protective devices remain passive until a line voltage exceeds the turn on voltage of the SPD’s internal components, as shown in Figure 1. When that occurs, SPD components become conductive and shunt excess current away from the line, typically to a grounding conductor, as shown in Figure 2. The remaining Let-Through Voltage passes downstream along the circuit. These transient overvoltage events occur over fractions of a cycle, thus lasting only for microseconds.

Current Ratings for SPDs

The current ratings that are typically associated with SPDs are the Surge Current Rating, the Short Circuit Current Rating, and the Nominal Current Rating.

The amount of current that an SPD can safely shunt is its Surge Current Rating, which is also known as Surge Capacity. This can be presented as a per mode or per phase rating and the resulting value is specified in thousands of amperes (kA). Alternatively, this rating may be published as a total surge current rating for the SPD, which may be estimated by summing the current capacity of its surge components or verified by testing the SPD as a complete assembly. The higher the surge current rating, the more protection the SPD supplies.

UL 1449 defines short circuit current ratings (SCCRs) as “The suitability of an SPD for use on an AC power circuit that is capable of delivering not more than a declared rms symmetrical current at a declared voltage during a short circuit condition.” This rating indicates the level in which the SPD can safely remove itself from the electrical system during a short circuit condition.

Nominal Discharge Currents, often marked as Inominal or In, indicate the amount of current to which sample SPDs have been subject during UL 1449-prescribed tests. Inominal ratings are required to designate the SPD Type, which indicates the environments in which an SPD may be installed. For example, to receive a 20kA Inominal rating, the SPD must survive fifteen 8×20 microsecond transients at 20kA as well as other safety tests.


Surge Protective Devices are designed to protect equipment electrical surge damage, by diverting excess current to ground. Proper selection of an SPD requires an understanding of its Surge Current Rating, Short Circuit Current Rating, and Nominal Discharge Current Rating. It also requires that the intended application, installation environment, and operating characteristics are also understood. Properly coordinating these elements helps ensure that well-designed surge protection strategies are applied to electrical distribution systems. Reputable SPD manufacturers can provide technical support to assist in selecting suitable devices for specific applications.

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