By Edward Cowern, P.E. – Baldor/ABB
We have found that one of the most confusing things about explosion proof requirements involves the application of motors in Division 2 areas.
To put things in perspective, Division 1 involves areas where hazardous liquids, vapors, gases or hazardous dusts are present a good deal of the time or even all the time in the normal course of events. Division 2 areas are where the hazardous materials are only apt to be in the area if there is a spill, accident, loss of ventilation or some other unusual condition. the treatment of both of these divisions is covered in Article 500 of the National Electric Code (NEC). A hazardous (classified) location is an area where the possibility of fire or explosion can be created by the presence of flammable or combustible gases or vapors, combustible dusts, or easily ignitable fibers/flyings. Electric arcs, sparks, and/or heated surfaces can serve as a source of ignition in such environments.
Once an area has been identified as being either Division 1 or Division 2, the National Electric Code requires certain types of motors be used in those environments. Division 1 areas always require hazardous location (explosion proof) motors having the class and group approvals that match the particular hazardous substance in the area. Thus, for Division 1 requirements, explosion proof equipment must be used. On the other hand, if an area has been classified as Division 2, the National Electric Code will frequently allow the use of totally enclosed (or even open drip proof) motors provided certain conditions are met. Basically, those conditions relate to there not being any hot surfaces or sparking parts in the motor. For example, sparking parts could be brushes (as found in DC motors), switching devices (such as centrifugal switches used in many single phase motors), thermostats or thermal overloads normally found in thermally protected motors, or space heaters that might have high surface temperatures.
In essence, what the code is saying is that three phase induction motors that do not have high temperature surfaces or sparking parts will not, in normal operation, be likely to ignite the surrounding environment. They can be used because the likelihood of a (spark producing) failure of the motor occurring at the same time that a spill or accident occurs is so remote it is a very unlikely event.
One way to avoid conflicts on interpretations of what is needed is to “play safe” and use hazardous location motors for both Division 1 and Division 2 requirements. This is a safe but expensive option and becomes more expensive as motors get larger.
A second choice is to use three phase TEFC (totally enclosed fan cooled) or even Open Drip Proof (ODP) motors that meet the nonsparking and no hot surfaces requirements for Division 2.
For machinery builders or contractors who want to use the less expensive motors for Division 2 requirements, it is always wise to make your intentions known to the customer in advance.
Perhaps the best way to do this would be to notify them by letter, with a statement such as follows:
“Since your stated requirement is Class * Group * , Division 2, it is our intention
to supply totally enclosed, fan cooled, three phase induction motors in accordance with Paragraph (1) of the National Electric Code. If you object to this, please notify us as soon as possible.”
By using this type of letter to make your intentions clear, it is much less likely that a dispute over interpretation will develop at a later time.
If you should have any questions regarding this requirement, please refer to the National Electric Code for the appropriate Section based on the class, group and division of the requirement.
*Fill in appropriate references
(1) Paragraph references
For Class I ——————— 501-8(b)
For Class II ——————– 502-8(b)