A customer called MIDWEST because some of their surge suppressor strips had caught fire underneath office workers’ desks.
The surge suppressors had integral circuit breakers in them for over current protection. However, despite the smoke and fire, the internal circuit breakers never tripped. The surge suppressors were still supplying power to the workers’ desktop computers.
Carefully taking apart the surge suppressors, MIDWEST discovered that extensive damage had been done inside the strip. The actual electronic component inside the strips that is meant to mitigate high voltage line spikes is called a metal oxide varistor. Most of these had burned up inside the strips. In addition, the heating caused massive damage to the printed circuit boards. The metal oxide varistors had greatly overheated, and both the varistors and the circuit boards had mostly been baked into carbon. This carbon was not a perfect short circuit, but had sufficient impedance to limit the current flow below the 15 amperes of the circuit breakers. But it had sufficient conductivity to generate large amounts of heat inside the strips.
From this and other evidence, MIDWEST determined the varistors had suffered an overvoltage condition, probably twice or three times the normal line voltage of 120 volts. This caused the varistors to conduct slightly, and dissipate power. It is very roughly estimated that the component dissipated between 5 and 20 times its nominal rated power dissipation of one watt, or 5 to 20 watts.
Experience has shown that power dissipations of less than roughly 5 times rated do not cause catastrophic damage. Above 5 times rated, and components begin to release the “magic smoke” that manufacturers seem to build into electrical components. Dissipations over about 20 times rated tend to cause violent disintegration of the component, accompanied by immediate flames, possible arc flash, and most noticeably, a very loud acoustic signature.
The varistors have an estimated 200 – 300 Volts RMS on them; therefore conduction currents were probably between 10 milliamperes to 100 milliamperes. Obviously, the 15 ampere circuit breakers would not trip from this conduction current. The overvoltage and the heating probably lasted several minutes.
It might seem strange to realize that the varistors never experienced any high voltage spikes, which is their usual reason for being. A sufficiently high voltage spike normally causes the varistors to explode with the sound of a gunshot, and vaporized zinc oxide and copper becomes spalled in the area around the varistor. That is, the internal bulk varistor material and copper electrodes vaporize in an arc flash, and deposit themselves in tiny balls on adjoining surfaces.
But no such spalling or deposited material was discovered upon disassembly of the melted plastic housings. Instead, the 5 to 20 watts dissipated in the 1 watt body of the varistor simply cooked itself, and everything around it into carbon. Then the carbon supported current flow.
In such a situation, surge suppressors have resulted in a number of fires throughout the US.