014-11The arc flash phenomenon first became widely publicized primarily as a burn hazard. The radiant heat—“plasma cloud”—constituted the serious danger to the safety of personnel. That was because most arcs occurred while someone was working on the equipment, and in a position to suffer injury or death even if not the inadvertent cause of the arc itself.

Although recognized as long ago as 1985, the associated pressure wave or “arc blast” drew much less attention—again, probably because arcing was unlikely to occur unless someone was close by as the probable cause. “Arc resistant” switchgear has since been developed to mitigate both thermal and blast effects on the surroundings. Such construction does include added strength to resist explosive pressure, but its chief characteristic is a venting system to deflect arc products to a safe area.

One type of electrical enclosure that is also subject to arc blast, but with some important differences, is the motor terminal box. Whereas the arc in stationary apparatus may occur with a worker or operator present, directly exposed, an explosion in a terminal box is more likely to occur unexpectedly with the box closed and no one in attendance.

Furthermore, even in an “oversized” box, the enclosure volume is so much smaller than in the typical switchgear enclosure that the explosive pressure developed will be much higher. The “shrapnel” effect of flying parts can be a greater danger than in other types of enclosure. Finally, whereas all arc flash/blast occurrences in stationary apparatus result from an electrical fault, explosions in motor terminal boxes may be initiated by a non-electrical breakdown, such as a bearing failure.

European standards have been developed for terminal box construction capable of withstanding both electrically and mechanically the thermal and blast effects of internal arcing up to stated limits of available kVA in the supply circuit. Because of differences in circuit protection practices (fuses vs. circuit breakers), no such standards exist in the United States. Using established design criteria, and the lessons learned in development of arc-resistant switchgear, terminal boxes for large motors can be designed for arc blast safety, but design verification is possible only through testing in a high-power laboratory. Few such facilities are available in North America.

Share Button