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Standard Number: 1910.304
June 22, 1981
|MEMORANDUM FOR:|| CURTIS A. FOSTER|
|THRU:|| JOHN B. MILES|
|FROM:|| BRUCE HILLENBRAND|
FEDERAL COMPLIANCE AND STATE PROGRAMS
|SUBJECT:||Subpart S -- Electrical Standards|
This is in response to your February 18, 1981, request for clarifications of a number of the new electrical standards. The answers to your questions are given in the same order as presented.
1. Grounding requirements involving permanent wiring are addressed in the newly revised Subpart S of 29 CFR 1910 from the standpoint of systems that require to be grounded as well as those equipments that need not be grounded. No general requirement is made for receptacles to be of the grounding type. Examples of this approach can be seen in Subpart S in Sections 1910.304(f)(1)(iv) and (v), and 1910.304(f)(5)(iv) and (v), and 1910.304(f)(6)(i) and (ii). The method is analogous to that in use by the National Electrical Code. As a consequence, since some equipment is not required to be grounded, there is no need for the standard to stipulate that all receptacles be of the grounding type.
2. The use of grounding terminals and devices are covered in the standard in Section 1910.304(a)(3). This requirement does not permit the use of grounding terminals for purposes other than grounding. If the terminal is ungrounded, then a violation exists. This requirement applies to both permanent and temporary wiring.
3. Section 1910.305(j)(1)(i) does not allow exposed live wiring; it does, however, permit rosettes, cleat-type lampholders and receptacles to have live parts exposed provided they are located 8' or more above the floor. These parts are very small, and the likelihood of an accidental contact with them is minimal.
4. Paragraph 1910.304(c)(3) applies to clearances from building openings, not from other structures. However, if a building such as the drilling rig "doghouse" contains window, door, porch or other similar openings, then 1910.304(c)(3) will apply to these openings. If, on the other hand, the drilling rig "doghouse" is separate from the drilling rig structure, 1910.304(c)(3) will still apply to the "doghouse" but not the separate drilling rig structure.
Where outside branch circuits or feeders are located so close to drilling rig structures as to be unsafe, e.g., 3 feet for insulated conductors and 5 feet for uninsulated conductors, regardless of where the "doghouse" is located, then a "General Duty Clause" (OSH Act Section 5(a)(1)) violation can be issued. The basis for substantiating such a citation is made by referencing Section 225-19(b) of the 1981 National Electrical Code. This section contains a new requirement for conductor clearance from Nonbuilding or Nonbridge Structures.
5. The National Electrical Code does not contain clearance requirements relative to outside conductors over 600 volts. In the case of outside branch circuits and feeders, NEC Section 225-18 "Clearance from Ground" contains a fine-print Note stating "For clearances of conductors of over 600 volts, see National Electrical Safety Code (ANSI C2-1977)." (Actually, the Note should reference the 1981 version of ANSI C2). Since the new OSHA Subpart S is based on the NEC, it [(1910.304(c))] likewise does not cover these outside conductors of over 600 volts. Furthermore, since OSHA never did adopt the National Electrical Safety Code (ANSI C2), no specific clearance values exist in Subpart S.
6. The classification of areas in grain elevators where concentrations of dust in the air are usually well below quantities sufficient to produce explosive or ignitible mixtures, is considered to be Class II, Division 2. This classification is in accordance with definitions given by the 1981 NEC in Section 500-5(b) and in the revised Subpart S in Section 1910.399(a)(25)(ii).
In the 1971 NEC, as previously adopted by OSHA, a Class II, Division 2 location was said to exist if, in addition to the basic requirement that normally no combustible dust be in suspension in explosive or ignitable mixtures, either of two conditions were present as follows: (1) combustible dust accumulation is sufficient to interfere with the safe heat dissipation from electrical equipment or apparatus or, (2) combustible dust accumulation on or in the vicinity of electrical equipment might be ignited by arcs, sparks or burning material from such equipment. If this definition is interpreted literally, it might be possible to identify a dust location which is neither Division 1 nor Division 2 by observing a dust layer which is neither deep enough to be ignited nor insulating enough to adversely affect the heat dissipation characteristics of electrical equipment.
In the recently revised Subpart S, the new definition for a Class II, Division 2 location was taken from the 1981 NEC. In this definition, a Class II, Division 2 location is identified when either of two basic conditions exist as follows: (1) normally combustible dust is not in suspension in explosive or ignitible mixtures and dust accumulation is normally insufficient to interfere with normal electrical equipment operation, or (2) infrequently combustible dust is in suspension due to abnormal processing or handling and the resulting dust accumulation is ignitible due to abnormal equipment operation. If this definition is interpreted literally a dust location will always be identified as Division 2 as long as a combustible dust layer is observed and the dust accumulation is normally insufficient to interfere with the normal operation of electrical equipment or other apparatus.
The substantiation for revising this definition for Class II locations in the 1981 NEC was given by NFPA Code-Making Panel 14 in the 1980 Annual Meeting Report (proposal No. 16-pg. 231). It was claimed by Panel 14 that the wording of Article 500-5(b), as changed, "better describes the condition that exists in present Class II, Division 2 locations."
In order to obtain some idea of grain elevator locations in which the location might be considered to be Division 1 instead of Division 2, reference is made to a study(1) in which actual measurements of airborne dust concentrations were made. In this investigation, various locations throughout a number of grain elevators were measured to determine where minimum explosive concentration (in excess of 40 grams per cubic meter) might occur.
The study showed that work areas, generally, had concentrations below 0.1 grams per cubic meter, with the highest measurements of 0.4 and 1.4 g/m(3). On the other hand, concentrations inside bucket elevator enclosures were found to be extremely high (10-1200 g/m(3)) at both the head and the boot. Horizontal conveyors had concentrations of up to 40 g/m(3) at loading points and 20-85 g/m(3) inside the discharge chutes. Inside scales measured 9-40 g/m(3) in the upper garner. The study concluded that the minimum explosive concentration of grain dust is typically exceeded inside the feeding and discharge enclosures of bucket elevators and (to a lesser degree) of horizontal conveyors at grain elevators.
It appears therefore, that based on measured data, most grain elevator locations will be Division 2 with the limited exception noted above which will be Division 1. Some locations in and around grain elevators may be neither Division 1 nor Division 2. These are especially designed to be dust free areas and usually involve engineered systems employing some type of positive pressure venting as sometimes found in control and computer rooms in feed mill operations. Outside locations, in most cases, may likewise be neither Division 1 nor Division 2 since the enclosed volume criteria which is a necessary part of the explosion dynamics, is not satisfied.
FOOTNOTE (1) Technical paper, "A survey of grain Dust Properties at Large Grain Terminals," by F.J Wade, A.L. Hawk and C.A. Watson, 10/2/79, Symposium on Grain Dust, Kansas State University, Manhattan, Kansas.
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