How SPDs Work

NEMA Surge Protection Institute

In the simplest terms, surge protective devices (SPDs) prevent damaging transient voltage surge levels from reaching the devices they protect. 

A useful analogy makes this clearer. Consider a
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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, or perhaps the linkage for the saw. 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. This describes the action of surge protective devices. They reduce transients to levels that will not damage or disrupt the operation of sensitive electronic equipment.

Fault and Surge Current

While often confusing, fault currents and surge currents are two distinctly different events. When selecting a surge protective device, it is critical that its intended use is clearly understood and that the product is properly sized for the surge environment in which it will be installed. Additionally, the device should provide, or be provided with, a method to safely remove itself from the electrical distribution system in the event of a failure. An understanding of surge currents and fault currents, and their differences will provide a foundation on which proper surge protection product selection can begin.

Three Basic Operating Modes

Most surge protective devices (SPDs) have three basic operating modes:

In each mode, current flows through the suppresser. What may not be understood, however, is that a different type of current can exist in each mode.

The Awaiting Mode
Under normal power situations when "clean power" is supplied within an electrical distribution system, the SPD performs minimal function. In the awaiting mode, the SPD is waiting for something (i.e., a transient) to occur and is consuming little or no ac power; primarily that used by the monitoring circuits. 

- Figure 1 -


The Diverting Mode
Upon sensing a transient event, the SPD changes into the Diverting Mode (Fig. 2). The purpose of an SPD is to divert the damaging impulse current away from critical loads, while simultaneously reducing its resulting voltage magnitude to a low, harmless level. 

As defined by ANSI/IEEE C62.41-1-2002, a typical current transient lasts only a fraction of a cycle (microseconds), a fragment of time when compared with the continuous flow of a 60Hz, sinusoidal signal.

- Figure 2 -

The magnitude of the surge current is dependent on its source. Lightning strikes, for example, can contain current magnitudes exceeding several hundred thousand amps. Within a facility, though, internally generated transient events will produce lower current magnitudes.

Since SPDs are designed to handle large surge currents, one performance benchmark is the product’s tested single-pulse surge current capacity.1 Often confused with fault current, but unrelated, this large current magnitude is an indication of the product’s tested maximum-withstand capacity. 

1 (Source: NEMA LS1-1992/R2000)

For example, an SPD with a single-pulse surge current capacity of 100,000 amps (8/20) per mode means the product should be able to safely withstand a transient current magnitude of 100,000 amps (8/20) in each mode that offers protection. (A mode is an electrical connection path between two points of current flow, i.e., line-to-neutral, line-to-ground, line-to-line, neutral-to-ground.) An impulse that exceeds the product’s surge current rating is most likely to cause failure or degradation to the suppressor.

The Failure Mode
As is the case with most electrical products, SPDs have known limits and will very likely fail when operated beyond their performance parameter. As mentioned above, a transient that exceeds the SPDs maximum surge current capacity can cause the product to fail. 

Other causes of TVSS failure include: 

  • Installation errors

  • Misapplication of a product for its voltage rating

  • Sustained over-voltage events

When a suppression component fails, it most often does so as a short, causing current to begin flowing through the failed component. The amount of current available to flow through this failed component is a function of the available fault current and is driven by the power system. 

Fault Currents: When and Why

Fault current conditions occur during short-circuit episodes in a distribution system. For example, suppose the insulation on two, phase conductors is failing. If the conductors come in contact with each other, a large amount of current will flow through this touching connection or short. During this "phase fault," or "phase short," large levels of current – possibly over 100 kA– flow through the conductors, fuses, circuit breakers and any other devices connected in the fault path. Unlike surge currents that occur in microseconds, faults can last a quarter cycle or even longer.

- Figure 3 - 

AIC Ratings

Electrical distribution system components such as circuit breakers, panelboards and fuses are assigned fault amperage interrupting capacities, or AIC ratings. These are mechanical ratings that assess the device’s ability to maintain integrity if a fault condition occurs downstream of the protection device. For example, a 10 kAIC-rated circuit breaker can safely interrupt 10,000 amps of fault current without blowing apart or internally short circuiting. A 65 kAIC switchboard must mechanically sustain 65,000 amps of fault current flowing through the switchboard and remain undamaged. These fault current ratings can be determined by consulting the manufacturer’s data sheet and are most often listed on the protective device.

AIC & SCCR Ratings and an SPD

While no credible manufacturer would deliberately design a product to fail, the potential for malfunction exists in all electrical devices. It is critical that failures, regardless of how infrequently they occur, minimize damage and present no risk of personal injury. The purpose of coordinated overcurrent protection in an SPD is to ensure that in the event of a fault, the device can safely and promptly remove itself from the electrical distribution system. The SPD is concerned only with faults produced within the suppressor and not those that may occur elsewhere in the distribution system.

While some SPD manufacturers incorporate fault current protection within their devices (e.g., fuses), care should be taken to confirm these important factors:

  • proper coordination of the SPD with the available fault current of the SPD installation location

  • proper product performance testing

  • compliance with all applicable UL requirements

For products manufactured without inherent fault current protection, such protection should still be provided. This can be accomplished by installing the devices via an external fusing system or circuit breaker. By incorporating coordinated fault current protection, the SPD will be removed from the rest of the distribution system if the suppressor experiences a fault condition or failure. 

The Importance of Overcurrent Protection

If no overcurrent protection is in place to take the failed protection device off-line, the fault will seek another upstream overcurrent device to clear. Often the alternate is the main breaker of a branch panelboard or service entrance switchboard, depending upon the location of the surge protective device. Properly coordinated overcurrent protection allows the SPD to remove itself from the distribution system under all fault conditions without catastrophic consequences to the device or other connected loads.

While often times confusing, the selection of a surge protective device must include a basic understanding of the differences between surge current and fault current. Also essential to the selection process is knowledge about the application of an SPD, the intended installation environment and the device’s operating characteristics. Understanding these informational building blocks will help ensure a well-designed protection system.

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