National Electrical Code Top Ten Tips: Article 500 -- Hazardous Locations
- Article 500 provides the basis for interpreting and correctly
applying Articles 501 - 516. For one thing, you will find the
definitions for those Articles in Article 500.
- How a location is classified depends on the properties of
materials in that location or that are likely to be in that location
[500.5]. Please note that the NEC is telling electrical workers and people in related jobs how to recognize a given classification and what that means. It is not telling electrical workers and people in related jobs how to designate a given classification. The designating must be done by a qualified person who is approved by the Authority Having Jurisdiction and in regard to these areas that almost always includes the insurer. If you don't know the classification of an area, ask. And get the reference document to verify that unless the person who tells you obviously would know (e.g., the plant manager, the plant safety director, the plant engineer).
- Class I locations are those in which flammable gases or
vapors are (or may be) present in sufficient quantities to
produce explosive or ignitible mixtures [500.5(B)]. These locations have the strictest requirements because they have the smallest particle size. The particles are molecular in size.
- Class II locations are those in which combustible dust is
(or may be) present in sufficient quantities to produce a hazard of
explosion or ignition 500.5(C)]. The requirements for these locations are not as strict as for Class I, but they are more strict than those for Class III. The particles are much larger than those of gases or vapors, but still pretty small.
- Class III locations are those in which combustible fibers or
flyings are (or may be) present in sufficient quantities to
produce a hazard of explosion or ignition 500.5(D)]. These locations have the least strict requirements. The particle sizes make containment or exclusion much easier than for the othe two types of locations. But don't forget these are also dangerous. Any skirting on the rules unnecessarily increases the risk to people and property..
- Class locations are further broken down into Division 1 (normal
operations) and Division 2 (abnormal operations). Does this location
meet its Class I, II, or III designation during normal operations or
only during abnormal operations?
- 500.7 provides 12 of what it calls "protection techniques."
These are not actually "techniques," just as the Chapter Three "wiring methods" aren't actual "methods." But if you read this,
you'll understand what they mean.
- A given hazardous location can contain only equipment that is
approved for use in that location [500.8(B)]. Note that if it's approved for Class I then it is not automatically approved for Class II and Class III. To use Class I equipment in a Class II or Class III location, it would also need to be listed and identfied for those locations. However, equipment approved for Division 1 can be used in Division 2. This does not mean you should simplify and go entirely with Division 1 equipment. Use what is appropriate to reduce installation cost and reduce maintenance issues.
- You must address explosion properties and ignition temperature
separately [500.8, Informational Note No. ].
- You must account for temperatures other than just ignition
temperatures [500.8(C) and (D)].