Hazardous classified locations

This rule covers rooms, sections, or areas where flammable vapors, liquids, gases, combustible dusts, or fibers may be present and where the likelihood of a flammable or combustible concentration exists. See Scope in 1910.307(a) for examples such as aircraft hangars, gasoline dispensing stations, paint-finishing plants, marinas, and areas with excessive combustible dusts.

Hazardous locations are assigned Class/Division and Zone designations such as Class I, Division 1; Class I, Zone 0; Class II, Division 1; and Class III, Division 2, and so on. See the list of designations in 1910.307(a)(2)(i) and look up the detailed definitions in 1910.399 as referenced by 1910.307(a)(2)(ii).

You must choose one system for Class I installations: either the division system (follow paragraphs (c)–(f)) or the zone system (follow paragraph (g)). 1910.307(a)(4) requires that Class I locations use only one of these classification systems for the installation.

All areas designated as hazardous (classified) locations established after August 13, 2007 must be documented, and that documentation must be available to people authorized to design, install, inspect, maintain, or operate electrical equipment there. See 1910.307(b) for the documentation requirement and 1910.307(c) for equipment-related provisions.

Intrinsically safe equipment and its associated wiring may be used in any hazardous (classified) location for which that equipment is approved. 1910.307(c)(1) permits intrinsically safe apparatus in approved hazardous locations.

Equipment must be approved not only for the class of location (for example Class I) but also for the specific ignitable or combustible properties (group) of the gas, vapor, dust, or fiber present. See 1910.307(c)(2)(i) which notes NFPA 70 groupings for these properties.

Approved equipment must be marked to show the class, group, and operating temperature or temperature range for which it is approved (based on 40°C ambient), subject to limited exceptions. See 1910.307(c)(2)(ii) for the marking requirement and the exceptions in subparagraphs (A)–(E).

Certain nonheat-producing items and specified fixed equipment are excepted from full marking requirements—for example junction boxes and conduit need not show temperature markings if nonheat-producing, some fixed lighting for Class I, Division 2 need not show group, and fixed general-purpose equipment acceptable for Class I, Division 2 may omit class/group/temperature markings. See the specific exceptions in 1910.307(c)(2)(ii)(A)–(E).

Equipment shown to be "safe for the hazardous (classified) location" must be of a type and design the employer demonstrates will protect against hazards from the combustibility or flammability involved. 1910.307(c)(3) cites NFPA 70 (the National Electrical Code) as guidance for selecting equipment and wiring methods to meet this requirement.

All conduits must be threaded and made wrench-tight; if making a threaded joint tight is impractical, a bonding jumper must be used. See 1910.307(d) for the conduit and bonding-jumper requirement.

Yes—equipment approved for a Division 1 location may be installed in a Division 2 location of the same class and group. 1910.307(f) states that Division 1-approved equipment is permitted in Division 2 of the same class/group.

General-purpose equipment or equipment in general-purpose enclosures may be used in Division 2 locations if the employer demonstrates the equipment will not be a source of ignition under normal operating conditions. See 1910.307(f) for this allowance.

Acceptable protection techniques include explosionproof (for Class I Divisions), dust-ignitionproof, dust-tight, purged and pressurized, nonincendive circuits/components, oil immersion, hermetically sealed equipment, and any other technique that meets paragraph (c). See the list of techniques in 1910.307(f)(1)–(10).

The zone system may be used as an alternative to the division system for Class I, Zone 0, 1, and 2 locations for flammable gases, vapors, or liquids. Protection techniques permitted include flameproof "d", purged/pressurized, intrinsic safety, and type "n" protection (with further subtypes), among others listed in 1910.307(g)(1)–(3).

All threaded conduit must use an NPT standard conduit cutting die (¾-in. taper per foot) and be made wrench-tight; equipment with threaded entries must use listed NPT conduit/fittings or identified metric entries with listed adaptors. See 1910.307(g)(2)(iii)–(iv) and 1910.307(g)(2)(iv)(A)–(B).

Classification of areas and selection of equipment and wiring methods for zone installations must be under the supervision of a qualified registered professional engineer. See 1910.307(g)(4)(i).

Class I, Zone 2 locations may abut but not overlap Class I, Division 2 locations; Zone 0 or Zone 1 locations may not abut Division 1 or Division 2 locations. A Class I Division location may be reclassified to a Zone class only if all of the space classified because of a single flammable source is reclassified. See 1910.307(g)(4)(ii)–(iii).

Equipment listed for Zone 0 may be installed in Zone 1 or Zone 2 of the same gas/vapor; Zone 1-listed equipment may be installed in Zone 2. Equipment must be marked to show class/zone (except intrinsically safe apparatus), the symbol "AEx", protection techniques, applicable gas groups, and temperature classification as required in 1910.307(g)(5)(i)–(ii).

An example marking is "Class I, Zone 0, AEx ia IIC T6," which indicates the class/zone, symbol "AEx," the type of protection (ia), gas group (IIC), and temperature class (T6). See 1910.307(g)(5)(ii)(B) and the example in Figure S-1 in the same paragraph.

Yes—equipment that the employer demonstrates will protect from the flammability hazards of the gas/vapor and the zone involved and that is accepted as providing such protection by employees need not be marked. See 1910.307(g)(5)(ii)(C), with NFPA 70 guidance referenced for determining acceptable protection.

Equipment relying on protection techniques listed in 1910.307(g)(3)(i) (e.g., flameproof 'd') may not be suitable below -20°C (-4°F) unless approved for lower temperatures, and low ambient conditions can affect whether flammable concentrations exist. See the low-ambient-temperature note in 1910.307(g)(4).

If equipment is suitable for ambient temperatures above 40°C, it must be marked with both the maximum ambient temperature and the operating temperature or temperature range at that ambient temperature. See 1910.307(c)(2)(ii)(E).

Nonincendive circuits and nonincendive components are permitted in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations, as described in 1910.307(f)(5)–(7).

Intrinsic safety is permitted for equipment in Class I, Zone 0 or Zone 1 locations for which it is approved, and powder-filling (protection 'q') is permitted for equipment in Class I, Zone 1 locations for which it is approved. See 1910.307(g)(3)(iii) and 1910.307(g)(3)(viii).

Reclassification is allowed only if all of the space that was classified because of a single flammable gas or vapor source is reclassified; you cannot reclassify only part of that space. See 1910.307(g)(4)(iii).

Storage of pre-charged equipment that results in an on-site aggregate quantity of a Category 1 flammable gas at or above the 10,000-pound threshold can be a covered PSM process because storage and on-site movement meet the PSM definition of a "process." See the PSM interpretation on aggregation and storage in OSHA's letter PSM flammable gas aggregation (June 6, 2024) and 1910.119 for PSM applicability.