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Design Advances Improve SPD Reliability and Safety

Design Advances Improve SPD Reliability and Safety

Proper selection of surge protective devices can increases uptime and improve personnel safety. Facility downtime costs industrial and commercial facilities nearly $26 billion annually in the United States alone. Identifying and preventing everything that can shut down your facility is next to impossible. However, one source that can be pinpointed and dealt with is transient voltage, and an increasingly

Proper selection of surge protective devices can increases uptime and improve personnel safety.

Facility downtime costs industrial and commercial facilities nearly $26 billion annually in the United States alone. Identifying and preventing everything that can shut down your facility is next to impossible. However, one source that can be pinpointed and dealt with is transient voltage, and an increasingly accepted way to combat transient voltage is by installing surge protective devices (SPDs).

It’s impossible to prevent voltage surges from entering, or occurring in, a building, so SPDs must divert surges to ground. By design, an SPD is "self-sacrificing"—it removes harmful voltage spikes even under conditions that can cause its ultimate failure. The key is to ensure that in the unlikely event of its failure the SPD won’t trip the upstream protective device, damage equipment, or harm people. Well-designed SPDs are generally maintenance free and should last the lifetime of the facility.

Why do SPDs fail? Although failures are rare, they can occur if used the SPD is used incorrectly, for example, if a wye-configured SPD is installed into a delta system. They can also occur when the primary component—the MOV—is forced to sustain a temporary overvoltage (TOV). A TOV (up to 200% of normal voltage) can result from a utility fault, loss of the neutral on a 3-phase, 4-wire system, or an improperly wired device. Though not a surge event, improper wiring accounts for the majority of SPD failures. These occurrences gradually degrade the MOV, changing its resistance from megohms to milliohms.

Fig. 1 (right)depicts how an MOV reacts to over-voltage conditions. For example, on a 120/208V 3-phase 4-wire system, the maximum continuous operating voltage is typically 150V. An MOV could likely handle a 50% increase to the nominal system voltage (1.5 x 120 = 180V) for a period of days. However, prolonged or frequent overvoltage occurrences will reduce its life expectancy.

Proper SPD testing promotes safety and reliability. SPD products that promote fuses with excessive surge current ratings don’t provide the proper system coordination. They sacrifice low-level fault protection and fail to disconnect during low-current faults. This can result in catastrophic failure—which can result in a fire—and tripping of the upstream breaker or fuse.

To ensure an SPD will operate at peak safety, efficiency, and reliability, reputable SPD manufacturers will perform independently verified tests. The two UL1449 tests you should be familiar with are the limited current test and the AIC rating test, both of which an SPD must pass to receive the UL 1449 listing. The device must not produce a flame or allow exposure of the internal parts while under test.

The limited current test can be conducted by applying an abnormal overvoltage, such as a line-to-line voltage, to the SPD via a conductor pair connected to two of the following SPD terminals: phase-to-neutral, phase-to-ground, and neutral-to-ground. The test circuit limits current to 0.125A, 0.5A, 2.5A and 5A by means of the power source impedance.

The AIC rating test can be conducted in the same manner as the limited current test, except the impedance must allow for a current of at least 5,000A. As an end user, you must note the difference when comparing the SPD AIC rating with the AIC ratings of other components in the system, such as the circuit breakers, contactors, and relays. Manufacturers test the SPD AIC rating at abnormal overvoltage and usually test other components at nominal system voltage. For example, on a 120/208V SPD, the test involves applying a line-to-line voltage (208V) on the phase-to-neutral SPD terminals (120V). These are important tests because they relate directly to equipment and personnel safety.

The NEMA LS-1 specification, published in 1992, designed test parameters around an SPD surge rating of 100kA per phase. The specification required "no more than 10% clamping voltage degradation after ultimate surge.” The ease and speed of performing this test allowed it to gain support and popularity. However, the clamping voltage, or let-through voltage, during ultimate surge is as much as 10 times higher than the nominal system voltage. Therefore, even if it were possible to receive a 200kA surge at the SPD, the clamping voltage would damage or destroy the downstream electronic components. The relevance of this test becomes questionable for two reasons: Your equipment is unprotected during such surges, and test labs are unable to generate peak surge currents greater than 240kA. The ultimate surge test may support claims of longer product life. However, testing for product longevity is not as important as testing for and ensuring system and user safety.

The importance of SPD thermal protection. MOV degradation happens very gradually. The increase in leakage current—due to TOV—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, so you must provide a means for immediately disconnecting it from the system. SPD manufacturers recommend using an overcurrent circuit protective device (OCPD)—circuit breaker or fuse—in front of an SPD. Properly designed SPDs require separate OCPDs for each MOV.

The characteristics of overcurrent fuses don’t allow for a simple design. The same overcurrent fuse can’t conduct high surge current, which requires a large cross-sectional area, and perform low fault-current disconnection, which requires a small cross-sectional area. You can improve the SPD surge-current rating by installing one fuse per MOV, because the MOVs and fuses share the current during the surge event.

However, for low fault-current disconnection, over-current fuses require using a thermal disconnector. The thermal disconnector can handle high surges without opening and performing low fault-current disconnection, but the thermal disconnector isn’t fast enough to respond to a high fault-current condition. Therefore, the combination of thermal disconnectors with the overcurrent fuses on individual MOVs solves this problem.

Thermal fusing. An overheated MOV produces sufficient heat to prompt a thermal disconnection. For small fault currents, or if the occurrence is over a longer period, the thermal fuse spring (TFS) will disconnect first. Some SPDs use a fuse trace (FT) and an MOV in series with a 30A or 60A circuit breaker. Those SPDs pass standard safety tests but may fail in real world applications. For example, if the fault current is less than 30A, the SPD might catch fire, but the circuit breaker won’t trip. SPDs designed with a TFS allow disconnection of the shorted MOV at the overheating stage.

However, in instances of incorrect installation, or when a highly abnormal over-voltage condition occurs, 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 AIC rating of the SPD.

Fuse trace technology. Silver fuse traces have been in use for years, but this design protects only to a certain level of surge current unless the cross-sectional area is large. New FT designs use copper in their circuitry to provide better surge ratings with less cross-sectional area, allowing for disconnection on low fault currents. Coordination of MOV/TFS and FT is important. Fig. 2 depicts time/current curves ('t / I' curves). The first curve, between points 1 and 2, is the MOV/TFS 't / I' curve. The second curve, between points 3 and 4, is the FT' t / I'-curve. The TFS is capable of disconnecting the MOV in the low fault condition range (between point 1 and curves intersection). The FT is capable of disconnecting the MOV in the high fault-condition range (between curves intersection and point 4).

Surge protection devices are becoming increasingly more important in preventing personnel hazards, equipment damage, and downtime. They remove voltage spikes from the electrical system even under potentially SPD-fatal conditions. How you select SPDs and match their specifications to your system will determine how well they protect you.

Chiste, C.E.T, is product manager, surge protection & power conditioning product line, Eaton/Cutler-Hammer, Calgary, Alberta, Canada.

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