CHAPTER NINE - Safe Electrical Equipment for Hazardous Areas

Abstract

This chapter discusses the safe electrical equipment for hazardous areas. There are no special measures taken in ordinary electrical equipment to encapsulate contacts that arc as they close or open. Thus, light switches, torches, sockets and plugs, pushbutton contacts, relays, electric bells, starters, circuit breakers, and open fuses are all potential sources of ignition should there be flammable gas, vapor, or dust in the atmosphere. The danger from arcing contacts in a starter box or switch is not obvious because the arc is hidden. Sparks from an electric motor, particularly of the commutator type, the momentary glow of a broken light bulb filament, and arcing from a broken or damaged power cable are more apparent as sources of ignition. Conventional equipment and cable are suitable for areas that are considered safe. If electrical equipment could be made with perfectly gastight casings, then the possibility of gas seeping in and being ignited by a spark would not be a problem. Unfortunately, seals on the rotating shafts of motors or on the operating rods of solenoids are not gastight. Hermetic sealing of covers is not guaranteed. Gaskets or jointing can deteriorate and face-to-face joints depend on quality of the machined surfaces and tightness of fastenings.