Surge protection basics are the foundation for a solid protection strategy.

The infamous “voltage spike” lasts for microseconds, yet its damage is often permanent — and sometimes catastrophic. Where do these spikes — transient surges in voltage or current — come from, and what should you do about them?


Causes of transients are usually represented by lightning bolts because they make for dramatic images, but they're not the only source of such events. Transients also occur on the utility grid for various reasons, but most transients originate from inside a given facility. A common source is switching loads on or off.

Transient surge protection can take several forms. For example, you can prevent the surge at its origin by using a soft-start motor. However, with sources like lightning, prevention is impossible. In these cases, you must turn from preventing the surge to coping with it. For surges originating outside the facility, you can divert the surge to ground at the service entrance with a surge protective device (SPD). For those originating inside your facility, you must also use SPDs to reduce the surge level at the inside distribution panels and at key points of use. By protecting the distribution SPDs from outside surges, you can specify smaller SPDs for a significant cost savings when the number of distribution panels is high.

SPDs operate in one of two modes:

  • Surge arresting: Limits surge voltages on the primary side of the meter by discharging or bypassing surge current (Art. 280).

  • Transient voltage surge suppression (TVSS): Limits transient voltages on the secondary side of the meter by diverting or limiting surge current (Art. 285).

In either case, the device prevents continued flow of follow current while remaining capable of repeating these functions. How long it remains capable depends on the number of times it carries out these functions as well as the severity of the transients.

SPDs can't reduce swells in the power, nor can they reduce harmonic conditions. And SPDs alone can't provide energy savings or reduce your utility bill. SPDs can absorb or divert surge energy, but most use the diversion method and provide a shunt path for transients before those transients can enter the equipment. Consider the following analogy: The pressure relief valve that protects a water mill does nothing until a surge occurs in the water supply. When that happens, the valve opens and allows the extra pressure out, so it won't reach the water wheel. Even if the relief valve is in place and working properly, a remnant of the surge will still reach the wheel, but it will have been reduced enough to prevent damage to the water wheel.

An SPD won't function properly unless the system is grounded according to Art. 250. Many often forget to reduce the difference in ground reference between port connections for multi-port appliances. You must ground all communications systems to the same grounding electrode system as the power. It's an NEC violation to provide separate grounds for the power system and communications system — doing so is just inviting a flashover from one system to the other.