In the U.S—with the exception of utility, telecommunications and rail industries—all AC power connected surge protective devices (SPDs) must be installed in accordance with National Electrical Code (NEC). 

The 2017 revision of the NEC includes three articles that address SPD application and expand its use outside for emergency power systems:

• Article 620.51(E), Surge Protection for Elevators, Dumbwaiters, Escalators, Moving Walks, Platform Lifts and Stairway Chairlifts
• Article 645.18, Surge Protection for Critical Operations Data Systems
• Article 670.6, Surge Protection for Industrial Machinery

The NEC requires that a SPD be “listed” or recognized for such purpose and approved by a Nationally Recognized Testing Laboratory (NRTL). One example of a NRTL listing service is Underwriters Laboratories Inc. (UL).

The broadening application of the NEC furthers the responsibility for NRTLs, like UL, to prioritize safety for facilities, electrical professionals and patrons. Read on to learn about the main UL standard that assesses SPD products for safety and provides basic ratings for determining applications.

A surge protective device (SPD) is designed to protect electrical systems and equipment from surge events by limiting transient voltages and diverting surge currents. 

Surges can originate externally, most intensely by lightning, or internally by the switching of electrical loads. The sources of these internal surges, which account for 65% of all transients, can include loads turning on and off, relays and/or breakers operating, heating systems, motors and office equipment. 

Without the appropriate SPD, transient events can harm electronic equipment and cause costly downtime. The importance of these devices in electrical protection is undeniable, but how do these devices actually work? And what components and factors are central to their performance?

Lightning is known to be the most significant source of surges—bolts have been recorded to have a million to a billion volts and between 10,000 to 200,000 amps. However, lightning only makes up a portion of all transient events in a facility.

Because transients can originate from both external sources (like lightning) and internal sources, facilities ought to have both a lightning protection system and surge protection installed.

This begs the question: what is the difference between these two systems, and how do they work together?  

Grounding, bonding and lightning protection systems protect against electrical events that have potential for significant damage or downtime. These events are hopefully few and far between in the lifetime of a facility. However, lower energy transient events can occur more frequently that may go unnoticed, until one day a piece of electronic equipment stops working.

Significant damage can occur to the more robust systems as a result of lightning-induced surges resulting within a radius of several kilometers, or from repetitive switching-induced surges. Costs can range from degradation of electrical or electronic systems, to data loss and equipment destruction. Some of these costs can appear relatively minor, but the loss of an essential service or revenue associated with a facility or plant shut down can be enormous.