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Subchapter 7. General Industry Safety Orders
Group 16. Control of Hazardous Substances
Article 109. Hazardous Substances and Processes
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§5174. Combustible Dusts--General.

(a) Any action which liberates combustible dust that exceeds or may reasonably be expected to exceed a concentration of 25 percent of its lower explosive limit in air shall not be permitted unless all sources of ignition are eliminated or are identified and specifically controlled.

NOTE: Most combustible materials and some materials not usually considered combustible when reduced to dust or powder form, present an explosion hazard when mixed with air.

(b) No source of ignition shall be permitted in locations where a hazard of dust explosion exists until all dust-producing operations have been stopped, airborne dust allowed to settle, and accumulated dust removed to an extent which will remove the hazard of the dust explosion.

(c) All machines, conveyors, housings, and conductive surfaces in locations where combustible dusts are generated or are present shall be electrically bonded to ground to prevent the accumulation of electrostatic charges which are sufficient to potentially cause dust ignition.

(d) Hoses and nozzles used in the collection or blowing of dusts shall have electrical continuity maintained along the entire length from coupling to nozzle and shall be bonded to ground.

(e) Static electricity shall be removed from belts by grounded metal combs or other effective static elimination devices.

NOTE: Ground/grounded see definition of Grounded, Effectively in Section 3207.

(See Electrical Safety Orders for regulations governing electrical installations in locations where combustible dusts are generated, or are present.)

(f) All enclosed areas wherein combustible dusts are generated or are present, except in closed or covered containers shall be cleaned as often as necessary to prevent accumulation of dust on floors, ledges, beams, equipment, machines or elsewhere which may present a fire or explosion hazard. Safe means of access for cleaning of all surfaces shall be provided.

NOTE: (1) The cleaning of surfaces by a permanently installed grounded vacuum cleaning system is the preferable method. Soft push brooms or brushes may be used without serious hazard except where brushed dust may form explosive clouds in air. (Warning: Brush bristles of synthetic fiber have a tendency to accumulate strong electrostatic charges.) Blowing dust with compressed air may create dangerous dust clouds and such cleaning should be done only when other methods cannot be used and after all possible sources of ignition have been eliminated, including bonding hoses and nozzles to ground.

NOTE: (2) See Section 5144 for respiratory protective equipment requirements.

(g) Pneumatic or magnetic separators shall be provided to remove metal or foreign matter capable of igniting combustible material being processed in grinding, shredding, or pulverizing equipment.

(h) Machines and equipment in which the hazard of dust explosion exists shall be so located, constructed, enclosed or vented, that the force of an explosion in the machine or equipment may be dissipated without endangering employees in the regular performance of their duties.

(i) Dust collectors for combustible dusts which present an explosion hazard shall be located outdoors or in detached rooms of fire-resistant construction and provided with adequate explosion vents, except that liquid-spray type collectors may be located within buildings.

NOTE: It is recommended that all plants of this type be of fire-resistant construction, and that sections of the walls or roof be made of glass or other material which will act as a vent in case of explosion. For heavy construction, one square foot of vent area should be provided for each 80 cubic feet of room capacity; for medium construction, one square foot vent area should be provided for each 65 cubic feet of room capacity and for light construction, one square foot of vent area should be provided for each 50 cubic feet of room capacity. For sulfur plants the vent area should be not less than 15 percent of the total wall, floor and ceiling area; for starch plants, where dry starch grinding and grading, pearl and powdered starch bulk packing, or lump starch cooking, pressing, grading, and packing are carried on, daylight construction, with window surface equal to at least half of the total wall surface is recommended.

(j) Smoking shall not be allowed in any location where hazard of a dust explosion exists.

NOTE: Authority cited: Section 142.3, Labor Code. Reference: Section 142.3, Labor Code.


1. Repealer and new section filed 4-27-79; effective thirtieth day thereafter (Register 79, No. 17). For history of former Section 5174, see Register 72, No. 23.

2. Amendment filed 12-10-87, operative 1-9-88 (Register 87, No. 51).

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