Designating and Equipping
The two steps required for effectively implementing a hazardous-area classification strategy are designating and equipping.
NFPA 497 (which addresses flammable liquids, gases and vapors) and NFPA 499 (which addresses combustible dusts) provide guidance to facilities that are contemplating whether or not an area should be designated as hazardous-classified and, if so, how to define its boundaries.
NFPA 70, Article 500 (which addresses electrical equipment installed in hazardous classified areas) enumerates accepted ignition mitigation techniques and gives design and performance requirements for listed electrical equipment that is installed in hazardous-classified areas.
One question that is misguided, but often asked, is whether the interior of a combustion device should be designated as a hazardous-classified area. Actually, the correct question is not “should” but “can” the interior of a combustion device be designated hazardous-classified, and the answer is emphatically NO.
Since the interior of a combustion device has the potential for several major ignition sources to be routinely present (flames, reacting gases, ignition sparks, hot surfaces, etc.), there is no way it could ever be designated hazardous-classified because area designation is a technique used to eliminate ignition sources. In order to prevent furnace explosions, precautions that prevent unintended releases of fuel (e.g., redundant safety shutoff valves, purging, ventilation, trial-for-ignition, etc.) must be taken, as required by NFPA 86.
Next time, the applicability of classifying areas exterior to furnaces will be analyzed in the context of Class A, B, C and D furnaces/ovens.