Group Classifications in the Zone System
- Thread starter finhead
- Start date
- Status
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
You are now entering the most arcane region of Hazardous Location lore ? not the classification of a location itself, but the classification of the volatile materials that make the location Hazardous (Classified). (I?m actually rather surprised it hasn?t come up before in the 5 years I?ve been associated with Mike Holt?s NEC Forums)
I don?t have the NEC 2008 Handbook readily available today, but I happen to know Table 500.1 it?s extracted from NFPA 497, Table 4.4.2; and, to the best of my knowledge, it?s complete including the various notes. If I?m in error, I?m sure one of our colleagues will correct me.
Unfortunately, the NEC is not where to look for detailed information on how to classify either locations or volatile materials. Exception: Articles 510-516, do a good job with classifying locations within their Scope. The basic NFPA Standard on both subjects is NFPA 497.
The first thing to look at is the two top lines in the Table. Note that Acetaldehyde is ?Division Group C? and Acetic Acid is ?Division Group D?. Also note that these classifications were determined by test as indicated by the superscript ?d.? Further note they are both ?Zone Group IIA.?
So right from the start, we see that the ?common knowledge? that ?Division Group D? equals ?Zone Group IIA? and ?Division Group C? equals ?Zone Group IIB? is in error. It is usually true but there are plenty of ?mavericks? in the Table. Some, like 1, 3-Butadiene, deviate so far from the norms they actually have their own unique classifications for all practical purposes.
Before 1999, both Division and Zone Groups were defined in terms of representative materials. In 1999 they became a bit more analytical and the Groups were defined in terms of maximum experimental safe gap (MESG) and minimum igniting current ratio (MIC ratio). This is the crux of the issue today: the two systems don?t use the same test standards!!! They each use their own set of MESG and MIC ratios and with different test apparatus. It should not come as a surprise then that they occasionally come up with different results.
From NFPA 497:
NFPA 497 Table 4.4.2 (NEC 2008 HB Table 500.1), Note ?c? indicates the listed Zone materials were already ??based on 1996 IEC TR3 60079-20, Electrical apparatus for explosive gas atmospheres ?Part 20: Data for flammable gases and vapors, relating to the use of electrical apparatus?? This is one of the reasons we can?t ?retool? the US Zone system because it would cost manufacturers, both foreign and domestic, too much to change them for the US market alone. IEC TR3 60079-20 didn?t include all the materials that the historic NFPA 497 and its predecessors did either. Thus there are some ?gaps.? There were several historic sources for NFPA 497 Table 4.4.2, such as the National Academy of Sciences and UL.
One of the more heart-warming stories of Division Groups is found in NFPA 497, Annex A, Section A.4.4.2, the ?Explanatory Material,? for Table 4.4.2. Several materials were specifically tested as indicated by the superscript ?d? in the ?Class I, Division Group? column. However, ?All other materials were assigned group classifications based on analogy with tested materials and on chemical structure, or on reputable published data reflecting MESG or MIC ratio values?, Although the classifications of these latter materials represent the best judgment of three groups of experts, it is conceivable that the group classification of any particular untested material may be incorrect. Users of these data should be aware that the data are the result of experimental determination, and as such are influenced by variation in experimental apparatus and procedures and in the accuracy of the instrumentation.?
That?s where we are today.
Side note: US ?Zones? aren?t the same as IEC ?Zones? either. The common notion that Div 1 = Zone 0+1 and Div 2=Zone 2 just ain?t so. Well, they are in the NEC system because they were force fit; internationally though, Zone 1 is more of an upgraded Div 2.
- Status