Re: Not Grounding Cranes in Class III Locations
I?ve been trying to figure out how to respond to this without writing an essay. It seems like I can?t.
Part of this is a ?legacy? issue, i.e., there are times when part of the Code is changed and some of the other seemingly effected parts are overlooked. In this case, originally Section 250.7 (250.22 since 1999) referred to a specific requirement in Section 503.13 that mandated that power circuits be ungrounded. Sometime between the 1978 and 1984 Code ( I don?t have all my copies with me) that specific requirement was dropped in 503.13 but no corresponding change was made in 250.7. Since no one seems to have found a serious need to correlate them there has been no ?update? in 250.7[250.22].
From 2002 NEC:
500.5 Classifications of Locations.
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(B) Class I Locations. Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitible mixtures. Class I locations shall include those specified in 500.5(B)(1) and (B)(2).
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(C) Class II Locations. Class II locations are those that are hazardous because of the presence of combustible dust. Class II locations shall include those specified in 500.5(C)(1) and (C)(2).
(1) Class II, Division 1. A Class II, Division 1 location is a location
(1) In which combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitible mixtures, ?
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(D) Class III Locations. Class III locations are those that are hazardous because of the presence of easily ignitible fibers or flyings, but in which such fibers or flyings are not likely to be in suspension in the air in quantities sufficient to produce ignitible mixtures. Class III locations shall include those specified in 500.5(D)(1) and (D)(2).\
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The next part is rather involved, so I?ve added a few sub sections from Article 500. Again, since the definitions have developed over the years, the ?numbering? system has changed a bit and the form is not identical as it describes each Class. However, one thing is clear: Class III is evaluated significantly different from Classes I and II. Class III is the only one where the materials under consideration ??
are not likely to be in suspension in the air in quantities sufficient to produce ignitible mixtures.? In the other two cases the mixtures are either ?explosive or ignitible.?
In application, this can be seen by simply reviewing Article 503 and seeing that there is virtually no difference in installation methods for Class III, Division 1 and Division 2 locations.
Without going to exhaustive detail, the cranes described in 503.13 are very specific purpose for Class III locations and they just aren?t used with Class I or II installations. Specifically, "running contactors" are not used with Class I or II.
