2.3 Class II dust and Class III fibers and flyings

1 Class II, and the clause that surprises people

"Class II locations are those that are hazardous because of the presence of combustible dust" (29 CFR 1910.399). A Class II, Division 1 location is a location meeting any one of three conditions:

1. 1
   In which combustible dust is or may be in suspension in the air under normal operating conditions, in quantities sufficient to produce explosive or ignitable mixtures;
2. 2
   Where mechanical failure or abnormal operation of machinery or equipment might cause such mixtures to be produced, and might also provide a source of ignition through simultaneous failure of electric equipment, through operation of protection devices, or from other causes;
3. 3
   In which combustible dusts of an electrically conductive nature may be present.

The third condition has no Class I equivalent, and it is absolute: a conductive combustible dust puts the location in Division 1 by its nature. Not by how much is in the air, not by how often — by what it is. Metal dusts and carbon black are the obvious cases. A shop that machines aluminum does not get to argue its way into Division 2 on the grounds that the dust rarely gets airborne.

The reason is mechanical as much as chemical. Conductive dust that works its way into an enclosure bridges terminals, tracks across insulators and creates the fault that becomes the ignition source. It attacks the equipment and supplies the fuel.

2 Which dusts, in OSHA's words

The note to the Class II, Division 1 definition names the industries: "areas of grain handling and processing plants, starch plants, sugar-pulverizing plants, malting plants, hay-grinding plants, coal pulverizing plants, areas where metal dusts and powders are produced or processed, and other similar locations that contain dust producing machinery and equipment (except where the equipment is dust-tight or vented to the outside)."

Read the parenthesis: dust-tight equipment, or equipment vented to the outside, can take an area out of that description. Containment is a design decision that changes the classification, which is the same lesson as ventilation in Class I.

The same note then lists the electrically nonconductive combustible dusts — those "produced in the handling and processing of grain and grain products, pulverized sugar and cocoa, dried egg and milk powders, pulverized spices, starch and pastes, potato and wood flour, oil meal from beans and seed, dried hay, and other organic materials which may produce combustible dusts when processed or handled" — and closes with a warning it gives nowhere else: "Dusts containing magnesium or aluminum are particularly hazardous, and the use of extreme caution is necessary to avoid ignition and explosion."

3 Class II, Division 2: accumulation, not suspension

The Division 2 definition for dust is different in kind from the Class I version, and this is the detail that makes dust its own discipline. A Class II, Division 2 location is one where both of the following are true:

1. 1
   Combustible dust will not normally be in suspension in quantities sufficient to produce explosive or ignitable mixtures, and accumulations will normally be insufficient to interfere with the normal operation of electric equipment, but dust may be suspended as a result of infrequent malfunctioning of handling or processing equipment; and
2. 2
   Resulting dust accumulations on, in, or near the electric equipment may be sufficient to interfere with the safe dissipation of heat from that equipment, or may be ignitable by abnormal operation or failure of electric equipment.

Notice what the second condition is actually about: a blanket of dust is thermal insulation. A motor or a light fitting that runs at a perfectly safe surface temperature when clean will run far hotter under a layer of flour, and that hotter surface is the ignition source. The dust does not have to be in the air to be the problem. It has to be on the equipment.

The note adds that this classification "includes locations where dangerous concentrations of suspended dust would not be likely, but where dust accumulations might form on or in the vicinity of electric equipment" — including areas adjacent to a Class II, Division 1 location into which dust may be put into suspension under abnormal conditions.

4 Class III: fibers and flyings

"Class III locations are those that are hazardous because of the presence of easily ignitable 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 ignitable mixtures" (29 CFR 1910.399).

The definition contains its own explanation of why it is a separate class. Fibers and flyings are large and light: they pile up, they wrap around shafts, they hang off ledges and they burn readily — but they do not usually form the explosive suspension that dust does. The hazard is fire and rapid flame spread rather than a deflagration, so the equipment requirements are less severe than Class II while still being real.

The two divisions split on manufacture versus storage, and that is the whole distinction:

And the materials themselves, named in the note: "rayon, cotton (including cotton linters and cotton waste), sisal or henequen, istle, jute, hemp, tow, cocoa fiber, oakum, baled waste kapok, Spanish moss, excelsior, and other materials of similar nature."

5 Woodworking sits in an awkward place

A wood shop is a good test of whether you have understood the difference. Sawdust and shavings coming off a table saw are flyings; wood flour coming off a sander and collected in a bag house is combustible dust — and OSHA's Class II note names "potato and wood flour" explicitly among the nonconductive combustible dusts, while its Class III note names "woodworking plants and establishments" among Division 1 fiber locations.

Both can be true in the same building, in different rooms, which is precisely why 1910.307(a)(1) insists that "each room, section or area shall be considered individually." The machine room and the dust collection room are not the same place, and they may not carry the same classification.

6 The three classes side by side

Purged and pressurized is permitted in any hazardous (classified) location for which it is approved — 1910.307(f)(4) — and intrinsic safety, under (c)(1), is permitted in any location for which it is approved. The rest of the last row comes straight from 1910.307(f), and it is a useful sanity check in the field: if the protection technique on the nameplate is not on the list for your class, the equipment is in the wrong room. Part 3, Chapter 4 goes through what each of those techniques physically does.