In the news: “Multi-outlet power surge suppressors cause two families on Bainbridge Island, Wa to lose their homes to fires.” “The U.S. Embassy in Grenada reports a 150k loss to fire caused by an overheated surge protector.” “A melted surge protector causes the fire destruction of a mobile office structure in Hanford, Ca.” - Clearly, these surge protectors failed. Or did they?
Although, design or material weaknesses cannot entirely be ruled out, the short answer to the above question is: No, the surge protectors probably did exactly what they were designed to do, divert electrical surges away from sensitive equipment. Why then did they cause these fires?
What all of the above-mentioned surge protectors have in common is that they utilize Metal Oxide Varistors (MOV) as surge suppressing technology. MOVs are made up of grains of zinc oxide in a matrix of other metal oxides sandwiched between two metal plates. These metal plates are the electrodes that join the MOV to the surge protector’s power lines. Wherever the zinc oxide grains touch each other, interfaces form that have many of the properties of a semiconductor, allowing current to flow above a certain voltage.
MOVs are designed to engage, or allow current to flow, when their turn-on or break-down voltages are exceeded. When the voltage applied is below the turn-on level, the varistor remains essentially non-conductive. Each time the voltage exceeds the turn on level, the MOV engages to divert this energy away from the protected equipment. When that happens, the current finds a route through the interfaces between the metal oxide grains, following the path of least resistance (lowest impedance). As the current passes through the interfaces, it changes the structure of the interface between the particles and makes this particular path non-useable in the future. That means that every time energy needs to be diverted through the MOV, a new path needs to be found until all paths are taken up. As fewer and fewer paths become available, the impedance level rises, increasing the internally generated temperature when a surge current flows. This internally generated heat may lead to a thermal runaway condition, which can eventually lead to a complete meltdown and the above-mentioned fires.
Therefore, MOVs that are doing their job correctly inherently have a limited life expectancy. Even the smallest surge that passes through the MOV degrades it a bit further. This characteristic may make you wonder, if there is not another technology out there that does not degrade like MOVs. There is - silicon avalanche suppression diodes (SASD). However, MOVs have two distinct advantages that make them the appropriate choice for many applications. As opposed to SASDs, MOVs can take higher surge currents for the same size of the component and are considerably less expensive. SASDs, on the other hand may be the better choice if lower but longer transients, such as utility grid switching surges are expected at your facility.
To avoid unnecessarily shortening the life expectancy of an MOV based surge protector, it is important to choose an appropriate clamping level. If it is too low and the MOV engages even at voltage levels that may not cause any harm to your equipment, it degrades much faster. Nowadays, many surge protectors that utilize MOV technology feature indicator lights that show if the suppression device is working. However, please note that the degradation of the MOV is a continual process, which the indicator cannot show, only the state of absolute failure. We therefore recommend replacing the surge protector every 1-3 years for commercial applications and 3-5 years for consumer applications. In any case, if the surge protector feels hot to the touch, has burn marks or signs of discoloration replace it immediately.
In its 3rd edition of the UL 1449 safety standard for surge protective devices, Underwriters Laboratories Inc. (UL), the industry’s leading product safety certification organization also acknowledged the safety hazard associated with MOVs subjected to sustained over-voltages. It now requires that MOV based suppressors be thermally protected. That means, if the protector starts to get hot, it will disconnect itself from the power source. While this will help prevent possible fires, it also removes the surge suppression capability from the equipment!
It is important to remember that MOV based surge protectors require regular maintenance. They are not your “buy it and forget about it” solution. Please contact our experts at 208.772.8515 to assist you in finding the right protection technology and the appropriate technical specifications for your application.
Technical Writer, Protection Technology Group