Surge protective devices (SPDs) have become an integral part of residential, commercial, and industrial power quality applications. The best SPDs provide enough safety so that, in the unlikely event of their failure, nearby equipment will not be damaged, processes will not be shut down (by tripping of upstream circuit breakers or fuses), or personnel will not be harmed as a result of smoke, fire, or flying debris. Fortunately, market demand and manufacturers' innovations have led to improved SPD design and features, based on new fusing techniques and advancements in metal-oxide varistors (MOVs).

Let's discuss the causes of SPD failure, proper SPD testing, and recent design improvements.

The cause of SPD failures

SPD failures rarely occur — and when they do — they are typically not due to a surge event. The two most common reasons for SPD failure are incorrect application, such as installation of a wye-configured SPD into a delta system, and sustained temporary overvoltage (TOV) on an MOV.

A TOV up to 200% of normal voltage can result from: utility faults; loss of the neutral on 3-phase, 4-wire systems; or an improperly wired device. A TOV, which is not a surge event, accounts for approximately 95% of all SPD failures, according to in-house experience and analysis. These occurrences degrade the MOV gradually, changing its resistance from megohms to milliohms.

SPDs with high surge ratings (i.e., kA) usually have multiple MOVs connected in parallel to each other so they can share the surge current. However, laboratory tests show that if MOVs are connected in parallel circuits, the MOVs will fail one at a time. This occurs because the remaining MOVs have much higher impedance and will not conduct a significant amount of current after the first MOV fails. Therefore, the current will flow through the failed, low-impedance MOV, creating upstream and/or downstream interruption and unsafe conditions on the system.

Figure 1 illustrates how an MOV reacts to overvoltage conditions. The typical maximum continuous operating voltage (MCOV) of a 120/208V, 3-phase, 4-wire system is 150V. The MOV could likely handle a 50% increase to the nominal system voltage, which is 180V (1.5 × 120V), for a period of days. However, prolonged or frequent overvoltage occurrences will reduce the reliable life expectancy of the MOV.

Balance between safety and performance

The addition of the limiting currents of 10A, 50A, 100A, and 500A to the UL 1449 standard increased the number of tests and expanded the number of failure modes that were tested, increasing the safety of the SPD (See SPD Testing.)

A new edition of UL 1449, UL 1449 3rd Edition, was published in September of 2006. Compliance to this new edition of UL 1449 becomes mandatory in September of 2009. This new edition adds a Nominal Discharge Current test, which stresses the SPD with 15 transients at a level that the SPD might be subjected to. In order to pass this test, all overcurrent and overload components must remain intact.

Thermal protection

MOV degradation happens gradually. The increase in leakage current (caused by TOVs) through the failing MOV progresses at the same rate as the MOV's degradation. This heats up the MOV and adjacent thermal disconnector, causing the disconnector to trip. When an SPD fails, the MOV short-circuits and must be disconnected from the system immediately.

SPD manufacturers may recommend using a circuit breaker or an internal or external overcurrent fuse in front of an SPD. To disconnect properly designed SPDs, individual MOV fusing is required. In general, the characteristics of the materials that make up overcurrent fuses don't allow for a simple design. The same overcurrent fuse cannot conduct high-surge current and perform low-fault current disconnection. Because MOVs and fuses share the current well during a surge event, you can improve an SPD's surge-current rating by having one individual fuse per MOV.

On the other hand, you must use overcurrent fuses in combination with a thermal disconnector for low-fault current disconnection. The thermal disconnector will open for sustained low-fault currents but will remain closed for fast high-fault current conditions. Therefore, the combination of thermal disconnectors with overcurrent fuses on individual MOVs is the best solution.

Another configuration uses multiple MOVs, each accompanied by a thermal fuse spring (TFS) and a fuse trace (FT) connected in series (Fig. 2). The TFS and FT should withstand a surge current equal to the surge-current rating of the MOV with which they are associated. An overheated MOV produces sufficient heat to prompt a thermal disconnection. For small fault currents — or if the occurrence lasts a longer period of time — the TFS will disconnect first (Fig. 3).

SPDs designed with a TFS will allow disconnection of the shorted MOV at the overheating stage. However, in incorrect installations or highly abnormal overvoltage conditions, an FT will help in the disconnecting process. At very high fault current levels, the FT will open faster than the TFS. In these instances, the FT improves the SPD's SCCR rating.

A further improvement on the thermal fuse spring and fuse trace is the thermally protected MOV. This concept uses the thermal fuse spring as discussed above but it is mounted directly on the MOV's surface, enabling better heat transfer and reliable operation. In addition, these advanced thermal fuses use a spring-loaded shutter that assists in arc extinguishing during an event when the thermal fuse spring opens. This design concept also allows for the MOV, TFS, and shutter to be packaged in the same housing, reducing space requirements.

For an SPD to protect the electronic components of a system from damage or degradation, it must function with a high degree of safety and reliability. Typical well-designed SPDs are maintenance-free, dependable, and last the lifetime of the facility.

At the manufacturing stage, proper testing helps ensure the reliability of SPDs. In a facility application, adding integral thermal protection in series with individually fused MOVs provides protection against low-fault currents (high-impedance faults) by preserving high-surge current protection. A cascaded approach for coordinated, facility-wide protection, as recommended by the IEEE, offers the best solution for a safe and reliable surge-suppression system.

SPD Testing

Surge protective device (SPD) manufacturers must adhere to numerous tests, which should undergo independent verification. UL 1449 is the most commonly applied standard for SPDs in North America. Tests in UL 1449 are designed to address the most common failure modes of SPDs: sustained TOVs. This is accomplished by two major sets of tests, the abnormal overvoltage limited current tests and the abnormal overvoltage unlimited current test. To pass the tests, the device must not produce a fire or shock hazard during or after the tests.

The abnormal overvoltage limited current test is conducted by applying an abnormal overvoltage (e.g., up to twice the rated voltage) to the SPD via a conductor pair, which is connected to two of the following SPD terminals: phase-to-neutral, phase-to ground, and neutral-to-ground. Prior to February 2005, the requirement of the test was to limit the current to 0.125A, 0.5A, 2.5A, and 5A by means of the power-source impedance. In February of 2005, UL 1449 was expanded to include abnormal overvoltages with currents limited to 10A, 100A, 500A, and 1,000A. Compliance to these tests was required by February 2007. This had a major impact on the SPD market as a number of SPD manufacturers stopped making SPDs completely, dropped models of SPDs that could not pass the tests, or redesigned their SPDs.

The abnormal overvoltage unlimited current test is conducted in the same manner, except the impedance must allow for a current of at least 5,000A. The manufacturer chooses what level of available current it wants to subject the SPD to. This value is called the short-circuit current rating (SCCR). The end-user must note the difference when comparing the SPD SCCR with the interrupting or withstand ratings of other system components, such as circuit breakers, contactors, and relays. The SPD SCCR is tested at abnormal overvoltage, and other components are usually tested at nominal system voltage. For example, on a 120/208V SPD, the test will apply a line-to-line voltage (208VAC) on a phase-to-neutral SPD terminal (120VAC). These are important tests because they relate directly to equipment and personnel safety.

Mossop is product line manager for Surge Protection and Power Factor Correction for Eaton's electrical business in Pittsburgh. He can be reached at Chiste is product sales manager for power quality products for Eaton's electrical business in Seattle. He can be reached at

Editor's Note: The original version of this article was previously published in Power Quality magazine in 2001 and is updated to reflect recent changes included in UL 1449, 3rd edition.