Can Your SPD Go to Extremes?

Jul 1, 2008 12:00 PM, By Carey Mossop and Alan Chiste, Eaton Corp.

To protect electronics from damage or degradation, an SPD must function with a high degree of safety and reliability

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.

Fig. 1. Approximate time duration to MOV failure, based on the voltage increase in the percentage of the maximum continuous operation voltage (MCOV).

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.