Knowing industry guidelines can save you money and enhance performance.

The vast array of surge protection devices, and the ratings that characterize them, can be confusing. How do you sort through all the choices to determine the design strategy and product choices that will provide the protection you need at a cost you can afford? Let’s look at three common approaches.

One approach is to provide surge protection devices (SPDs) only at the service entrance. You may think you’re solving all of your surge problems in one stroke, but this approach doesn’t work—as we’ll show you later.

A second approach is to provide point-of-use protection with individual SPDs at critical equipment. This approach works well for low-level surges, but fails to protect equipment from higher-energy surges.


A third approach is to combine complementary SPDs at the entrance and at the equipment. This isn’t simply a matter of adding more devices, as we’ll explain shortly. To complete the protection provided by this third approach, you must install lightning protection. And, to ensure your surge protection devices can work, you must address the grounding practices throughout the facility—starting with the entrance neutral-to-ground bond.

SPD ratings. To understand why the first two approaches work—and how to implement the third approach—you need to know a few things about SPD ratings. The easiest and best guide to follow is the NEMA LS-1 Standard on Low-Voltage Surge Protection Devices. The LS-1 document outlines the critical elements for a technically sound surge protector specification without getting bogged down by insignificant ratings. Another excellent source recently published is IEEE C62.62, Standard Test Specification for Surge Protective Devices for Low-Voltage AC Power Circuits, which complements the NEMA standard.

Because tests for ratings like response time and energy dissipation (joules) are not uniform, these ratings don’t add much value to a specification. Response time figures, typically based on component data, disregard the installation parameters of the surge protector. This fails to account for the fact that conductor length will affect this number and there is no standard conductor length. However, it’s safe to assume that the surge protector will respond “instantaneously” when it sees a surge.

Joules is also a number typically taken from calculated component data. As a rule of thumb, a surge protector designed with 20mm diameter metal-oxide varistors (MOVs) will have a joule rating of 80J per MOV, which correlates to about 6,500A of surge current withstand capability. Therefore, a device with 4 MOVs, for example, would have a joule rating of 320J, or 26,000A, of surge current.

Because testing can easily and reliably confirm maximum surge current ratings, these ratings are meaningful in describing how robust a surge protector is. Manufacturers subject the device to a high surge current test based on criteria outlined in IEEE C62 test standards.

Another critical rating is the suppressed voltage rating (SVR) provided by Underwriters Laboratories. The lowest assigned UL1449 rating is 330V peak. The rating known as maximum continuous operating voltage (MCOV) will affect the UL1449 SVR. In general, the higher the MCOV, the higher the SVR. The MCOV should be at least 115% of the nominal operating voltage of the system the protector will be in. This will provide enough tolerance for temporary system over voltages based on the steady-state tolerances provided in ANSI C84.1, Electric Power Systems and Equipment-Voltage Ratings (to which utilities in the United States adhere). Remember that a surge protector should conduct, or turn on, only when transient voltages last on the order of microseconds. Steady-state system over-voltage conditions can degrade or destroy a surge protector. In fact, UL1449 now requires a failsafe test to confirm “loss of neutral scenarios” by subjecting the surge protector to twice its nominal rating.

Other ratings such as noise filtering may be of importance in attenuating electrical noise generated by motor-operated equipment. A rule of thumb is 20 db provides 10:1 noise attenuation, while 40db provides 100:1 and 60db provides 1000:1. In general, 35db up to 100MHz provides excellent filtering.

Art. 285 is new to the 2002 NEC. It covers SPDs installed on the load side of the service disconnect (UL1449 Listed). Previously, the NEC covered only surge arresters installed on the line side of the service (in Art. 280)—these are not Listed under UL1449 and not used at a typical panelboard. Sec. 285.11 requires an AIC rating for the SPD. This has caused confusion, because a hardwired parallel-operated SPD panel is not assigned a nameplate ampacity rating. UL1449 never required such a nameplate or marking unless the SPD was series-operated (connected in-line and drawing continuous load current). UL is working with manufacturers to address an AIC marking requirement, and manufacturers are sizing surge panels to carry an AIC of a typical molded case circuit breaker for the application. If the main service breaker has a 100KAIC fault current, that doesn’t necessarily mean the surge panels would require this level. Also, be sure not to confuse the maximum surge current ratings (8 x 20us waveform) with the AIC rating. The AIC rating is based on steady-state available fault current from the power source of the facility using a time duration of three and a half cycles, as opposed to the surge current withstand time duration associated with an induced-lightning stroke in the domain of a few microseconds.

Implementation strategy. Several standards apply to SPD implementation. One in particular, IEEE C62.41, Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits, provides location categories for a generic facility in three distinct locations: Service entrance (Cat. C), major feeders & branch circuits (Cat. B), and outlets (Cat. A). The standard assigns each category a surge magnitude based on these locations, as well as a waveform based on the source of the surge (lightning, induced, or equipment generated). This concept supports the approach of cascading or networking SPDs throughout a facility to anticipate surges originating from equipment internal to the building or from outside influences like lightning.

Developing your implementation strategy around this concept is a proven way to maximize protection while, in most cases, dramatically reducing cost. How would you do this? First, provide lightning protection per NFPA 780, Standard for the Installation of Lightning Protection Systems. Second, provide high-energy surge protection at the service entrance. This protection will reduce the surge to a level that downstream SPDs can handle. Remember, SPD protection is like a window—it can span only so far up or down. So you must cascade to get the full span of protection.

Your next step is to provide branch circuit panelboards with wired-in surge protective devices to divert surges generated by neighboring equipment internal to the building. Studies show about 80% of surges originate within the facilities themselves. Now, you can see cascading really helps you target against your main sources of surge without leaving you open to high-energy external surges. At this design level, you also want to provide surge protection for network cabling and similar infrastructure. Your last step is to provide dedicated point-of-use protection. Where you have equipment with power and communications ports, remember both are doors through which surges can enter, so protect accordingly.

Installation. When connecting a hardwired, parallel-operated surge protector, you must follow some basic installation guidelines. Typically, the surge panel will be connected to dedicated breakers within the panelboard. These can be any rating, even as low as 20A. Their purpose is to act as a safety disconnect for any future servicing. Use as short a conductor length as possible between the surge panel and the connection to the breakers in the panel board. The parasitic inductance of the excessive leads introduces a voltage loss in series with the shunt (or parallel) connected SPD. A rule of thumb is you’ll need an extra 100V on the SVR of each SPD in that panel for every foot of conductor.

The application of facility-wide surge protection is critical to maintaining the flow of production. Your goal isn’t simply to protect the investment in equipment—though that is a major cost-savings. Your goal is to prevent downtime so revenue can continue to flow. A single catastrophic power disturbance can easily wipe out any “savings” realized by not investing in the proper SPD devices, strategy, and installation.

Wakeham is director of marketing & product development for Leviton’s Power Quality Division.