Hazardous (Classified) Locations
Some places contain flammable gases, vapors, dusts or fibers that can explode or burn if an electrical spark or a hot surface touches them. These are hazardous (classified) locations. Ordinary electrical equipment can start the explosion, so special equipment and work rules are required.
- Electrical
- Industrial
- Fuel & Chemicals
Where you find them
- Gas stations, bulk fuel storage and loading areas
- Paint spray booths, finishing rooms and solvent mixing rooms
- Chemical plants, refineries, oil and gas sites and wastewater plants
- Grain elevators, flour mills, sawmills, woodworking and plastic and metal powder plants
- Textile and fiber operations, and some battery rooms and laboratories
How areas are classified
In the U.S., the National Electrical Code (NEC) classifies areas by what could be in the air and how often. See National Electrical Code.
| Class | Hazard | Examples |
|---|---|---|
| Class I | Flammable gases or vapors | Gasoline, solvents, natural gas, propane, hydrogen |
| Class II | Combustible dust | Grain, flour, coal, sugar, wood, metal powders |
| Class III | Ignitable fibers and flyings | Cotton, textile fibers, sawdust, wood chips |
| Division | Meaning |
|---|---|
| Division 1 | The hazard is normally present, or present often during repair, or from equipment failure |
| Division 2 | The hazard is present only in an abnormal condition, such as a leak or spill, or is contained |
Many sites also use the zone system (Zones 0, 1 and 2 for gases, and 20, 21 and 22 for dusts), which the NEC allows. Each class and zone is also broken into groups that describe the specific gas or dust, because some are easier to ignite than others. The classification is set by a qualified person, usually shown on drawings and posted at the area. Ask before you start work.
OSHA rules
OSHA requires electrical equipment in classified areas to be approved for the location (29 CFR 1910.307 for general industry and 1926.407 for construction). The equipment must be marked, installed and maintained as approved. See Listed & Labeled Equipment.
Protection methods
- Explosion-proof (flameproof) enclosures are built to contain an internal explosion and cool the gases so they don’t ignite the surrounding atmosphere. They don’t keep gas out. They must be kept sealed and bolted.
- Intrinsically safe equipment limits electrical and thermal energy so it can’t ignite a hazardous atmosphere, even in a fault.
- Purged and pressurized enclosures keep clean air or inert gas inside at slightly higher pressure.
- Dust-ignition-proof enclosures keep dust out and keep surfaces cool.
- Conduit seals keep gases and flames from traveling through conduit.
- Bonding and grounding prevents static spark. See Static, Bonding & Grounding.
Equipment markings
Approved equipment is marked with the class, division or zone, group and a temperature code (T-code) showing the maximum surface temperature. The equipment must be rated for the gas or dust in the area, and its temperature code must stay below that material’s ignition temperature. Never assume equipment is rated just because it looks heavy-duty.
Working in classified areas
- Get a hazard assessment and permit where required, including hot work permits. See Hot Work Permits.
- Use only approved tools, lights, radios, meters and phones for the class and division. A normal flashlight or phone can be an ignition source. See Test Meters and Gas Detectors.
- Test the air for flammable gas and keep monitoring while you work.
- Keep covers and seals closed and bolted. Don’t open an enclosure that’s energized in a hazardous atmosphere.
- Lock out equipment before servicing. See Lockout/Tagout.
- Control dust build-up, since a layer of dust on hot equipment can ignite, and an airborne cloud can explode.
- Don’t replace a part with a non-approved one, drill or modify an explosion-proof enclosure or leave a bolt out.
- Don’t use nylon, polyester or static-generating clothing where static is a concern.
- Don’t use temporary wiring or extension cords not approved for the area. See Extension Cords.
- Don’t carry lighters, matches or unapproved devices into the area.
Know the materials
Check the Safety Data Sheet for flash point, ignition temperature and explosive limits. See Safety Data Sheets and NFPA 30. Storage and handling of flammable liquids has its own rules. See Flammable Liquids.
Before you work in a classified area
- I know the area’s class, division or zone, and group
- My tools, lights, meters and radios are approved for it
- Permits are in place and the air has been tested
- Enclosures, seals and covers are intact and closed
- I have no lighters, phones or unapproved devices on me
Standards & references
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