Understanding NEC Article 506: Zone 20, 21, and 22 Locations for Combustible Dusts or Ignitable Fibers/Flyings
NEC Article 506 provides a framework for the zone classification system for hazardous locations where combustible dusts, fibers, or flyings might pose a risk of fire or explosion. This system parallels the division classification outlined in Articles 502 and 503 but uses zones to better categorize the presence and risks associated with hazardous materials.
What Are Zone Classifications for Combustible Dusts and Fibers/Flyings?
Zones 20, 21, and 22 represent different levels of hazard based on the likelihood and duration of combustible dust, fibers, or flyings being present:
- Zone 20: Continuous presence or for long periods.
Examples: Inside dust collection equipment or storage silos. - Zone 21: Likely presence during normal operations.
Examples: Areas near grain handling or conveyor systems. - Zone 22: Occasional presence, usually under abnormal conditions.
Examples: Areas adjacent to Zone 21 where leaks or maintenance might release dust.
Unlike the division system, the zone classification incorporates material groupings (IIIA, IIIB, and IIIC) to identify the specific characteristics of the hazardous material.
Key Requirements for Zone 20, 21, and 22 Locations
- Documentation
- Area classification drawings are mandatory, detailing all hazardous and unclassified areas.
- These documents must be accessible for design, installation, and maintenance purposes.
- Wiring Methods
Acceptable wiring methods vary by zone:- Zone 20: Rigid metal conduit (RMC), Type MC-HL cable, or intrinsically safe wiring systems.
- Zone 21: Dusttight enclosures, IMC, or liquidtight flexible conduits as needed.
- Zone 22: Standard wiring with additional protections like dusttight wireways and cables.
- Sealing and Bonding
- Seals must prevent the ingress of hazardous materials but need not be explosionproof.
- Bonding and grounding are required to eliminate static discharge hazards.
- Protection Techniques
Equipment must use specified protection techniques appropriate to the zone, including:- Dust Ignitionproof Enclosures: For Zones 20, 21, and 22.
- Intrinsic Safety: Limited energy to avoid ignition.
- Pressurized Enclosures: Prevent dust from entering equipment.
- Equipment Requirements
- All equipment must be listed or labeled for use in the specific zone.
- Markings include zone classification, material group, temperature class, and protection level.
- Temperature Ratings
- Equipment surface temperatures must not exceed the ignition temperature of the hazardous material present.
Applications and Industries
Common industries using Zone 20, 21, and 22 classifications include:
- Agriculture: Grain processing and storage.
- Woodworking: Dust-intensive processes.
- Pharmaceuticals: Handling powders or dry chemicals.
- Metals: Metalworking with combustible metal dusts.
Conclusion
NEC Article 506 ensures comprehensive safety in environments with combustible dusts, fibers, or flyings by introducing a zone classification system that specifies hazard levels and tailored safety measures. This approach enhances both safety and compliance in hazardous locations.
