How are explosive atmospheres and Ex equipment regulated in the United States?

The United States has no equivalent of the EU ATEX directives. The workplace rule is OSHA's standard for hazardous (classified) locations, 29 CFR 1910.307: the employer must classify locations where flammable gases, vapours, combustible dusts or fibres may be present, document the classification, and install equipment that is approved for the class, division or zone, group and temperature (or, as a narrower alternative, equipment the employer itself demonstrates to be safe for the location). "Approved" in practice means certified (listed or labelled) by a Nationally Recognized Testing Laboratory (NRTL) recognised by OSHA. Installation follows the National Electrical Code (NEC, NFPA 70).

Short answer

• Classification: Class I (gases and vapours), Class II (dusts), Class III (fibres and flyings), each in Division 1 or 2. Class I may instead use Zones 0, 1 and 2. Under 1910.307, Class II and III must use divisions.
• Documentation: zone classifications, and division classifications made after 13 August 2007, must be documented and available to those who design, install, inspect, maintain or operate the equipment. Under the zone system, the classification and the selection of equipment and wiring methods must be supervised by a qualified registered professional engineer.
• Equipment: approved for the class, the gas or dust group and the operating temperature (based on a 40 °C ambient); the temperature marking may not exceed the ignition temperature of the substance.
• Approval: acceptable to OSHA, normally through NRTL certification; ATEX or IECEx certificates alone do not satisfy OSHA.
• Dust: no single combustible dust standard; OSHA applies several standards and the General Duty Clause.

Divisions, zones and groups

Division 1 covers places where ignitable concentrations can exist in normal operation; Division 2 covers places where they occur only in abnormal conditions. The zone system for Class I mirrors IEC practice. US equipment certified for zones carries the "AEx" marking.

The gas cards on excodex give the group and temperature class: hydrogen and acetylene are IIC, ethylene IIB, propane IIA. The NEC also has a zone system for dusts (Zones 20, 21 and 22, Article 506), but the 1910.307 text recognises zones only for Class I, so confirm with the enforcing authority before relying on dust zones.

General-purpose equipment may be used in Division 2 only if the employer can show it is not a source of ignition in normal operation. Equipment "safe for the location" without approval is allowed only where the employer can demonstrate the protection (1910.307(c)(3)); the employer then carries the burden of proof.

Equipment approval: NRTLs, not ATEX

OSHA does not certify products. It recognises private testing laboratories as NRTLs (29 CFR 1910.7), each for a defined scope of test standards and with its own registered mark. Under 1910.399, equipment is "acceptable" when it is accepted, certified, listed or labelled by an NRTL, and "approved" means acceptable to OSHA. Electrical equipment, and industrial trucks used in hazardous atmospheres, are among the products that need NRTL approval.

Equipment certified only under ATEX or IECEx must therefore also be certified by an NRTL for the US class, division or zone before installation. Underground mines are different again: MSHA approves equipment for mines under its own rules.

Workplace duties beyond equipment

OSHA has no direct equivalent of the EU workplace directive 1999/92/EC and its explosion protection document. The duties sit in several places: classification and documentation (1910.307), process safety management for flammables above 10,000 pounds in one location (1910.119, including hot work permits and management of change), hazard communication, and the General Duty Clause for recognised hazards such as combustible dust.

Common mistakes

• Shipping ATEX-only equipment to a US site. NRTL certification for the US marking is needed.
• Mixing division and zone markings. Equipment must match the system used to classify the area.
• Leaving the classification undocumented. Zone classifications and post-2007 division classifications must be on paper.

Related on excodex

• Occupational safety and health in United States: law, authorities and employer duties
• North American hazardous location approvals: NEC 500/505/506
• Zones vs Divisions: IEC and ATEX zones compared with NEC
• Which rules apply to major-hazard chemical sites in the United States?
• Ex equipment explorer

Standards and further reading

• 29 CFR 1910.307 Hazardous (classified) locations: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.307
• 29 CFR 1910.399 Definitions (acceptable, approved): https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.399
• 29 CFR 1910.7 and the OSHA NRTL Program: https://www.osha.gov/nationally-recognized-testing-laboratory-program
• NFPA 70, National Electrical Code, Articles 500 to 506 (hazardous locations)
• OSHA combustible dust standards page: https://www.osha.gov/combustible-dust/standards

This article is general occupational safety and health information prepared by the excodex editorial team. It does not replace the applicable legislation, standards, manufacturers' instructions or the assessment of a competent OSH professional or occupational physician.