When selecting a surge protective device (SPD), it is critical you understand its intended purpose and take note of the environmental conditions in which it will operate. In addition, you must size it properly so it can safely remove itself from the electrical distribution system in the event of a failure. You should understand surge currents, fault currents, and their differences before selecting a surge protection product.

Most SPDs have three basic operating modes: awaiting, diverting, and occasionally, failing. In each mode, current flows through the suppressor. However, some people fail to realize that a different type of current can exist in each mode.

The awaiting mode

Under normal power situations (when you have clean power within an electrical distribution system), the SPD performs few functions. In the awaiting mode, the suppressor waits for something (like a transient) to occur and draws only a small amount of leakage current. In this mode, the current passing through the suppressor creates a standard 60 Hz sinusoidal waveform (Fig. 1, above right).

The diverting mode

Upon sensing a transient event, the SPD enters the diverting mode. The SPD diverts the damaging impulse current away from critical loads, while reducing the resulting voltage magnitude to a harmless level. As defined by IEEE C62.41, a typical current transient lasts only a fraction of a cycle (usually microseconds) — an almost negligible amount of time when compared with the continuous flow of a 60 Hz sinusoidal waveform.

The magnitude of the surge current depends 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.

Because SPDs are designed to handle large surge currents, one performance benchmark is the product's tested single-pulse surge current capacity. Often confused with fault current, this large current magnitude is an indication of the product's tested maximum-withstand capacity. An impulse that exceeds the product's surge current rating will likely cause the suppressor to fail or degrade. NEMA LS1-1992 presents a more detailed examination of this concept.

The failure mode

As is the case with most electrical products, SPDs have known limits and will very likely fail when conditions cause them to operate beyond their performance parameters. As mentioned above, a transient that exceeds the SPD's maximum surge current capacity can cause the product to fail. The following are other causes of failure:

  • Installation errors

  • Misapplication of products for their voltage-rating

  • Sustained overvoltage events

When a suppression component fails, it does so as a short, which causes current to flow through the failed component. The amount of current available to flow through this failed component is a function of the available fault current, and it's driven by the power system.

Fail-safe considerations

While no credible manufacturer would deliberately design a product to fail, the potential for malfunction exists in all electrical devices. Therefore, it is critical that failures occur in a manner that minimizes damage and presents no risk of personal injury. Coordinated overcurrent protection in an SPD ensures 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.

While some SPD manufacturers incorporate fault current protection within their devices (such as 200 kAIC fuses), you should still confirm the following factors:

  • Proper coordination of the SPD with the available fault current of the SPD installation location, and

  • Proper product performance testing and compliance with all applicable UL requirements.

What if you have products manufactured without inherent fault-current protection? You'll need to add that protection into the system. You can do this by installing the devices via an external fusing system or circuit breaker. By incorporating coordinated fault current protection, the SPD will remove itself from the rest of the distribution system if the suppressor experiences a fault condition or failure.

While often confusing, the selection of an SPD must include a basic understanding of the differences between surge current and fault current and knowledge of the intended installation environment and the device's operating characteristics. Understanding these informational building blocks will help ensure a well-designed protection system. And that means a safer facility with enhanced uptime.

Carroll is an engineer with Current Technology Inc., Irving, Texas.

Sidebar: 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 10kAIC-rated circuit breaker can safely interrupt 10,000A of fault current without blowing apart of internally shorting. You can determine fault current ratings by consulting the manufacturer's data sheet; they are also often listed on the protection devices.