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. This does not mean that if you are working in an area classified with the division classification system you are free to use these requirements instead. It means if the area is classified with the zone classification system as Zone 20, 21, or 22,, then you use these requirements.
- Dust-ignitionproof refers to keeping equipment enclosed in a
manner that keeps sparks and flames contained within the
enclosure and keeps dust out . It does not mean the equipment can't ignite dust.
- If you use the zone system, you must generate and maintain
specific documentation [506.4]. The documentation requirements are
far from casual. Some contractors have bid on work in a Zone-classified area, only to discover that meeting the paperwork requirements costs them all of their profit and then some. If you do this kind of work, scope out the documentation requirements and allow extra resources for those. A maintenance operation should make extensive use of electronic templates.
- Exactly how a facility owner may classify a location depends on the
properties of the ignitible fibers or flyings that may be present and on
their concentration [506.5]. If you disagree with the classification, submit a written statement with observations that cause you to call the classification into question and ask for a review. If it's a work stoppage issue, make a list of these observations and pursue that verbally up the chain of command. If you disagree with the decision, you can either abide by it anyhow or withdraw from the job.
- 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. For example, an area is Zone 21 if it is adjacent to a Zone 20 location from which ignitible concentrations of dust or fibers/flyings could be communicated [506.5(B)(2)(4)]
- As with Article 505, you must consider the Material Group. The Article 506 requirements [506.6] are much simpler than those of Article 505 [505.6]. And that stands to reason, since dust particles and fibers/flyings are larger than gas molecules,
- Article 506 lists 14 protection techniques. This list seems to be a source of angst for the CMP responsible for it. In the 2017 Revision, the list consisted of 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 were gone with the 201 Revision; they were
encapsulation techniques (maD and mbD). Now encapsulation is back, but it's Encapsulation "m".
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. That person may need to hold onto each NEC Revision and grandfather things in rather than change hardware every three years. You may need those old NEC books, also. Save a copy going back 4 cycles, if you do any Zone 20, 21, or 22 work.