National Electrical Code Top Ten Tips: Article 506 -- Zone 20, 21, and 22 Locations
- Article 506 covers the requirements for the zone classification
system as an alternative to the division classification system
covered in Articles 500, 502, and 503 for Zone 20, 21, and 22.
- Dust-ignitionproof refers to keeping equipment enclosed in a
manner that keeps sparks and flames contained within the
enclosure and keeps dust out  .
- If you use the zone system, you must generate and maintain
specific documentation [506.4]. The documentation requirements are
far from casual.
- Exactly how you may classify a location depends on the
properties of the ignitible fibers or flyings that may be present and on
their concentration [506.5].
- Summary of Zone 20 definition: Ignitible fibers or flyings are
consistently present and are present for long periods [506.5(B)(1)].
- Summary of Zone 21 definition: Ignitible fibers or flyings are likely to exist under normal operations,
during repair, or during breakdown
- Summary of Zone 22 definition: Ignitible fibers or flyings are not likely to exist under normal
- Note that we have oversimplified the definitions of Zone 20, 21, and
so that you can easily see how they differ. Read the references
thoroughly to understand the implications for a given location.
- As with Article 505, you must consider the Material Group. The Article 506 requirements are much simpler than those of Article 505 [506.6].
- Article 506 lists 9 protection techniques (in the 2008 Revision, it listed 11 protection techniques, five of which were
new with the 2008 Revision). Two of the techniques that were new with the 2008 Revision are gone now; they were
encapsulation techniques (maD and mbD). Only
a qualified person can determine the area classification and only a
qualified person can determine which mix of protection techniques to
apply to a given installation.