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How and Why Surge Protection Devices Fail Chris Martin, Inside Sales Engineer Surge protection devices (SPDs) provide protection against the devastating effects of lightning and load switching to all equipment electrically downstream of where they are installed. Most SPDs typically include light-emitting diodes (LEDs) on the front of the unit, which provide some level of visual indication that informs the user if the SPD is on line and providing protection. When the LED is on, the unit is functioning properly and providing protection. When the LED is extinguished, protection has been lost and the downstream equipment is not protected. Some SPDs are equipped with dry relay contacts that change state when the surge protection has been lost and can be used as a remote alert to the end user of the SPD’s failure. A few SPD manufacturers offer products that are equipped with remote communications capabilities (RS-485 ModBus or TCP/IP Ethernet) that provide the user with a range of useful data, including the presence or loss of the surge protection device. All of these monitoring options are designed to tell the end user when the device has failed. When SPDs Fail Additionally, most lightning strikes are not single events, but consist of several strikes. For a surge protection device that is about to fail, it would be possible for a multi-strike lightning strike event to knock the SPD out of service while the remaining strokes of the same event are left to cause damage to the unprotected load. Methods of Surge Protection: MOVs MOVs offer good voltage clamping characteristics, low leakage currents, and high surge-current capacity, but have a finite lifespan. Other advantages include sub-nanosecond response times and widespread availability. How MOVs work At voltage levels well below the maximum continuous operating voltage (MCOV), the device will look like a classic resistor with values greater than 100 mega ohms. As the voltage approaches the MCOV, the device leakage current will be very small, but will rise slowly with voltage. The MCOV is the voltage that results in the maximum permissible leakage current for the application type (typically not more than 50 μA for AC circuits). MCOV is typically in the range of about 60 to 70 percent of the voltage at which turn-on will occur. Turn-on occurs when the voltage is high enough that the device is conducting 1 mA of current (this voltage varies with temperature). Above the turn-on voltage, increasing numbers of the ZnO grain boundaries begin to become conductive and the device passes current while converting a portion of the power into heat. In the normal operating region, small increases in voltage result in large increases in the number of conducting junctions. When the current flow and voltage have become large enough that all junctions have moved into conduction, the device enters the upturn region where its resistance is defined by the bulk resistance of the ZnO and the thickness of the device defines the equivalent linear resistor value. At the high end of the upturn region, the temperature of the ZnO grains rises sufficiently to cause the semiconductors at the grain boundaries to reconstitute and become permanently conductive. If enough semiconductors become permanent conductors, the device will fail. Minimal failure occurs when the device let through has changed by 10 percent. Major failure occurs when the device becomes an effective short circuit (0.3 ohms/cm of thickness). Catastrophic failure occurs when the device explodes. Other Methods of Surge Protection MOVs also can be used in parallel with each other to create multiple surge current paths within the device. By allowing the MOVs to divide and share the surge current, the stress felt by each individual component is reduced. This increases the lifespan of the MOVs and can help increase the surge capacity of the device. The End of Life for SPDs Under conditions where there is an unlimited short-circuit current available and an overvoltage event occurs, MOVs will conduct until they fail, short circuit and clear over-current fuses. If the availability of short-circuit current is limited, the MOVs will conduct until failure and may actually short circuit without clearing over-current fuses. When fuses do not have adequate time and energy to clear, the devices can ignite a fire and emit electrically conductive gases that can cause short-circuit paths. The greater the over-current, the less time it takes for the fuse to open, which is referred to as the clearing time for the fuse. Over-current fuses protect against these events that occur near the end of the component’s life. Selecting a fuse of the proper size ensures it will tolerate currents up to the full-rated surge current of the protector. The fuse will clear during an MOV failure, but not during a common surge event. The best way to maintain surge protection is to predict the SPD’s end of life accurately, allowing it to be repaired or replaced before it fails. This ensures that the device is at its optimum surge protection level, before being exposed to a large power-quality event. Some surge manufacturers incorporate a modular design that utilizes several surge modules per mode. These designs may have two to three modules per mode, depending on the surge rating of the product. The claimed benefit of this design is that if one of the modules fails, a replacement module can be installed to bring it back up to full capacity. The drawback is the SPD usually is reduced by as much as 50 percent of its rated capacity during the time of failure, and there is no way of knowing when one is about to fail. End of Life Prediction Conclusion For more information please visit: www.tnbpowersolutions.com
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