Babrauskas, V., Electric Arc Explosions, pp. 1283-1296 in Interflam 2010—Proc. 12 th Intl. Conf., Interscience Communications Ltd, London (2010). 1283 ELECTRIC ARC EXPLOSIONS Vytenis Babrauskas, Ph.D. Fire Science and Technology Inc. 9000 – 300 th Place S.E., Issaquah WA 98027, USA Abstract When an electric arc is created, a pressure event occurs. There can be two aspects to this: the shock and sound waves propagated from the expanding arc channel, and the bulk pressurization of the en- closure, if arcing is taking place within a closed volume. The present paper is the first systematic re- view of the research on both these pressure phenomena. Quantitative studies on electrical arc explo- sion pressures date back to the 1920s, although arc pressures generated by lightning, which is a type of electric arc discharge, have been studied since the 1700s, but understanding of the phenomena is still not complete or exhaustive. Experimental data are compared to theoretical predictions. It is shown that in an enclosed volume some extremely high pressures can be generated, if the arc current is sufficient. Such pressures can destroy buildings and mechanical equipment and cause injuries or death to nearby individuals. Even without enclosures, the shock waves produced from high energy arcs can cause injuries, although arc flash injury may be of greater concern. Injury potential generally requires that high currents be available, and serious damages or injuries are not associated with low- energy arcing occurrences. Keywords: arc pressure, electric arcs, electrical accidents, explosions, shock waves, switchgear. Introduction Electric arcing in circuits with sizable maximum short-circuit current capacity can be a highly ener- getic effect. In fact, buildings have collapsed due to arc pressure, since in an enclosed space some surprisingly large pressures can be built up. ‘Arc flash’ is the thermal radiation component associated with energetic electric arcs, and it has received a great deal of study in recent years since thermal ra- diation has been a cause of serious burn injuries to electricians 1 . Consequently, computational meth- ods and research have focused on the design of appropriate protective clothing. For arc pressures, however, no comparable activity has taken place. In fact, the literature is sparse and not systematic on this topic. It is the purpose of this paper to provide the first-ever review of electric arc explosions. The emphasis is placed on pressures developed and on the calculational methods available for these, along with experimental data that have been published. Some of the results are strikingly high. For instance, in one test explosion overpressures of 83 atm were obtained. The magnitude of this can best be appre- ciated by considering that a fuel-air deflagration will typically attain only around 7 – 8 atm, barring pressure-piling effects or other turbulence enhancements. During normal operation of a circuit break- er, arc pressures of roughly 3 atm magnitude can be expected 2 , but these devices are designed to sus- tain the pressures generated by the normal arcing associated with circuit opening. With regards to the energy supplying the arc, arcs can be of three types: (1) discharge of a fixed amount of stored energy (e.g., a capacitor; a current transformer); (2) DC power sources; or (3) AC power sources. Lightning strikes are the most important form of stored energy discharge, since ca- pacitive discharge tends to be confined to specialized situations and is uncommon as a source of in- dustrial accidents. Since most of the power transmission and distribution networks are AC, the bulk of the research available has focused on AC arcs. But heavy-power DC systems also exist and are im- portant in certain industries (e.g., electric train propulsion). DC arc explosions are fundamentally dif- ferent since there is no ‘zero-crossing’ in DC. In AC circuits, an arc will extinguish at 2× the power frequency (e.g., at 100 or 120 Hz), although it may reignite very shortly afterwards. In DC circuits, this extinguishing characteristic does not exist, and arcs will generally extinguish only due to external circuit interruption or due to excessive electrode consumption. Most aspects of arc behavior only de- pend on arc current and arc power, and not on the type of power supply, but where the type of power supply does matter, this will be considered. The discharge of a fixed amount of energy is termed a