Reliable surge protection depends on proper design and component selection.

You’ve installed a transient voltage surge suppression (TVSS) device, but how do you know it will protect your loads? An improperly connected unit will provide little or no surge protection. Unfortunately, misinformation about TVSS products is pervasive enough—and convincing enough—to lead many people to select and install TVSS devices incorrectly, resulting in unnecessary damage to equipment and costly downtime and repairs.


A transient voltage surge is a random, high-energy, short-duration electrical disturbance with a rise time of only 1 ms to 10 ms. It’s a sub-cycle event, and in that respect it differs from an overvoltage condition, which requires a different approach to solving.

About 80% of recorded transient voltage surges come from internal switching transients caused by turning motors, transformers, photocopiers, or other loads on and off. Externally generated surges, on the other hand, come from induced lightning, grid switching, or other facilities on the same distribution node. A surge suppression device (SPD) tries to divert most of the transient energy away from the load, via a low-impedance ground path.

Most SPDs use metal oxide varistors (MOVs) in the suppression circuit. Under normal conditions the MOV is a high-impedance component. When subjected to a voltage surge (typically voltage over 125% of the nominal system voltage), the MOV will quickly become a low impedance path to divert surges away from loads. The MOV reaction time is on the order of nanoseconds, which means it’s typically 1,000 times faster than the incoming surge.

System design. Most SPD manufacturers and TVSS experts promote applying a two-stage, or "cascaded," approach for surge suppression (Fig. 1). This involves installing the main SPD at the service entrance panel and applying a secondary SPD at each subpanel. This is the best method for providing surge suppression that allows the lowest let-through voltage to your electrical system and its connected devices. Let-through voltage is the surge remnant that passes through suppression devices and into your distribution system. The lower the let-through voltage, the better the protection.

The trend today is to supply SPDs mounted inside panelboards, switchboards, or other distribution equipment. This arrangement is referred to as an “integrated application,” and it makes up more than 70% of new applications. The integrated design eliminates many of the issues associated with standalone devices and provides significant benefits.

Using SPDs as part of an integrated system is an effective approach, but it isn’t always the best way to go. For example, using standalone SPDs for existing or retrofit applications is a cost-effective option if an electrical system is already in place. But before installing a retrofit, you must determine the impact of a standalone TVSS relative to the whole electrical system—particularly in relation to overcurrent protection. You typically connect standalone SPDs to an available circuit breaker via a cable and mount them next to a switchboard or panelboard. The installation is usually easy, but you must be aware of some potential hazards to ensure proper performance and safety.

Selecting a surge protection device. With their history of high reliability, the idea that SPDs may fail is difficult to believe for many people. Yet they can and do fail. The main cause of SPD failure is improper wiring at installation. Another mode of failure occurs when a sustained overvoltage causes the MOV to heat to failure. Thus, safe failure should be a key selection criterion for SPDs.

Although no SPD can withstand sustained temporary overvoltage, a properly manufactured SPD with internal fusing will safely disconnect itself from the electrical system without damaging it or injuring people. This feature protects the integrity of the system and ensures an upstream breaker won’t trip and shut down facility operations. If an SPD doesn’t have this feature, it’s probably a poor choice.

When it comes to surge current ratings, more isn’t necessarily better. Some SPD products have fuses with excessive surge current ratings. These designs may provide the proper system coordination, but they can sacrifice low-level fault protection and fail to disconnect during low current faults. Ironically, this will not only result in catastrophic failure, but it will also trip the upstream breaker or fuse.

Yet people will often specify an excessive surge current (kA) rating, with seeming disregard for the let-through number because they are trying to protect against a high-voltage surge occurrence, such as a lightning strike. The problem with this logic is short circuit interrupt ratings (kAIC) and surge current ratings (kA) aren’t the same thing. The short circuit interrupting rating of a breaker refers to its mechanical ability to break or clear a fault current. A surge current rating refers to the product’s ability to withstand a surge, not its ability to provide protection.

In any case, the assumption that an SPD can handle the kind of power it would encounter in a lightning strike is stretching physics a bit far. Fortunately, most of the energy from a lightning strike flashes to ground—or utility surge arresters divert it to ground. The remaining energy on the AC system is surge current. The surge current diverted by an SPD is a small fraction of the lightning strike current.

A service entrance suppressor will experience thousands of surges of various magnitudes. So what surge current ratings should you use? From statistical data, it’s possible to project a life expectancy of more than 25 years for a 250kA/phase SPD in a high exposure area (Fig. 2). A 400kA/phase SPD would provide more than 200 years of service—much longer than the electrical system itself. That obvious overkill doesn’t provide a good value for the extra money spent. Thus, many experts recommend using a 250kA-rated SPD on the main service entrance panel and 120kA on the downstream distribution panels.

Look for a system that provides continuous monitoring of all internal surge components and, if applicable, maintenance-free operation. These systems will ensure the investment in surge protection works as intended. They also allow you a way to safely remove the SPD from the electrical distribution system, in the event of a failure.

Let’s recap the design considerations. For an effective installation, first ensure your SPD:

  • Has a fusing system that doesn’t fail when subjected to either a high- or low-impedance fault.

  • Has a kAIC rating coordinated with the electrical system.

  • Can safely disconnect the SPD from the electrical distribution system under low-current fault conditions.

Before buying a TVSS, determine your specific TVSS requirements. Those including the voltage levels at your service entrance and critical subpanels, the number of critical subpanels, and whether this involves new or existing panelboards. A one-line diagram would be helpful, as would a copy of your most recent overcurrent protection coordination study. Then, discuss your particular situation with the vendor. If you get straightforward answers based on sound engineering principles, add that vendor to your short list. Look for a reputable manufacturer that uses proven technology and offers SPDs with low let-through voltages.

One claim that roils TVSS experts is that a single device provides complete protection. This is especially inflammatory when the sales presentation makes misleading statements or deliberate omissions about proper bonding and grounding. The engineering consensus is you must apply SPDs in a two-staged protection plan, and your grounding system must comply with IEEE Standard 142 and NEC Art. 250. When a vendor tries to tell you otherwise, keep looking. A quality vendor also won’t hesitate to answer questions about the limitations of its TVSS products because its reputation rides on performance. And performance depends on proper selection and application.

Chiste is a product manager with Eaton/Cutler-Hammer, Calgary, Alberta, Canada.