MIDWEST’S Engineering Department has had several customer’s ask to investigate some fires that have occurred in their offices. These fires were caused by commercial surge suppressor strips. Each strip had integral circuit breakers. In each case, neither the integral circuit breakers nor the distribution panel’s circuit breakers tripped.
National Fire Protection Association (NFPA) has researched and documented innumerable cases where the surge suppressors have caught fire. In many cases, large fires have resulted; thus the NFPA’s concern. The surge suppressor strips typically contain a circuit breaker and three or more metal oxide varistors. Metal oxide varistors are non-linear circuit components that conduct large currents when the applied voltage exceeds a certain value, usually 130 Volts AC RMS.
In the intended application, the varistors conduct in the presence of large voltage transients, such as from a lightening strike. But a large transient of Kilovolts is not the only situation that can cause the varistor to conduct. A small overvoltage can also cause conduction. Of course, small is relative. In this context, small might be a voltage increase such that the applied voltage is double or triple the normal 120 Volts, and can exist for hours. Such events can result from wiring failures in the power distribution system. In this case, the varistors will conduct small currents, perhaps just a few amperes. However, the double or triple voltage will still be dropped across the varistor. So, since power P = E * I, it is obvious that hundreds of watts will be dissipated as long as the overvoltage exists. Since varistors are typically only rated for about 1 watt continuous, severe overheating will occur. Such severe heat generation will result in destruction of the varistor and cooking of circuit boards. These elements are often reduced to just carbon, which continues to conduct, and continues the generation of heat inside the surge suppressor. The result can be a fire. Undiscovered, the building can burn down.
It is salient to note that if just a few amperes flow, then neither the internal circuit breaker nor external circuit breakers will trip, as these are probably rated at 15 or 20 amps.
Underwriter’s Laboratories has tried to address this issue in the following revisions of UL 1449:
- 2nd revision of UL 1449, titled ““UL Standard for Safety for Transient Voltage Surge Suppressors”. Compliance with this revision is mandated as of February 2007.
- 3rd revision of UL 1449, titled “UL Standard for Safety for Surge Protective Devices”. Compliance with this revision is mandated as of September 2009. This revision has also become an ANSII standard.