When an electric motor must be used in what the National Electrical Code (NEC) calls (in Article 500) a “classified” area, the motor enclosure must be so constructed that the motor will not represent an explosion or fire hazard. (The “classification” of areas — determining when the risk is present — is beyond our scope here.)
Two types of risk exist. The first is the presence around the motor of an atmosphere containing potentially explosive vapors — gasoline, methane, alcohol, benzene, etc. Ignition of that atmosphere could occur in one of two ways. The surface of the motor might get so hot during normal operation that the ignition temperature of the vapor is reached. Hence, a motor suitable for this atmosphere must be designed and tested for a safely lower surface temperature. The NEC divides explosive vapors into groups, each corresponding to a stated temperature range, and the motor must be labeled as safe for the appropriate groups. Nameplate markings indicate the specific temperature range for which the motor may be used.
A second ignition source is an arc or spark inside the motor enclosure, typically caused by an insulation failure. That can ignite the atmosphere inside the motor. Since no motor can be vapor tight, inside and outside atmospheres are sooner or later the same. An internal explosion can directly spread flame to the exterior, through unavoidable assembly openings in the structure, or could shatter the frame to give the same result. Hence, again depending upon the vapor involved and the explosive force to be expected, labyrinthine “flame seal” paths are built into the motor assembly, as well as sufficient added structural strength to withstand an internal explosion.
This type of motor is the true “explosion-proof” version. Note that it is not proof against occurrence of an internal explosion, but is proof only against any extension of that event to the surrounding atmosphere.
A second type of hazard is flammable dust — coal, grain, starch, sugar, magnesium, etc. If sufficiently fine, any such material mixed with the proper amount of air can form an explosive mixture. Motors for service in flammable dust atmospheres are not properly terms “explosion-proof,” but rather “dust-ignition-proof. ” These also are designed for suitably low surface temperature, so as not to ignite dust clinging to that surface. Recognizing that thick coatings of any dust can contribute to motor overheating by blocking the cooling air, designers must make sure that the motor can safely withstand the thermal handicap of such a “dust blanket.”
Flame seals are again provided to confine any internal explosion. In addition, special bearing seals are added to prevent the lubricant becoming loaded with dust so that it becomes ineffective, leading to bearing overheating and failure that could itself touch off a dust explosion.
All these motors are tested and labeled, for the particular environment, by Underwriters Laboratories (UL). They can legitimately be repaired only according to UL procedures. Stepping outside these guidelines exposes the user to the risk of possibly fatal accident, as well as penalties from insurance carriers or local fire marshals. One thing “explosion-proof” does not mean: the ability to withstand an external explosion without damage.
Richard L. Nailen, P. E