Understanding NEC Article 505: Zone 0, 1, and 2 Locations
NEC Article 505 introduces the zone classification system for hazardous locations as an alternative to the division system described in Article 500. Zones focus on the presence and likelihood of flammable gases, vapors, or liquids in specific areas, offering a more granular approach to safety in hazardous locations.
What Are Zone Classifications?
The zone classification system identifies three main zones based on the frequency and duration of exposure to flammable substances:
- Zone 0: Flammable gases or vapors are present continuously or for long periods.
Examples: Inside vented tanks or inadequately ventilated enclosures with flammable vapors. - Zone 1: Flammable gases or vapors are likely during normal operations or due to frequent maintenance or leaks.
Examples: Areas near spraying operations or pump rooms handling volatile liquids. - Zone 2: Flammable gases or vapors are unlikely during normal operations but may occur briefly in abnormal conditions.
Examples: Areas with sealed systems that might leak only if ruptured or malfunctioning.
Key Requirements for Zone Classifications
- Documentation
- Area classification drawings must clearly identify zones and their boundaries.
- These documents should be accessible to personnel responsible for design, installation, inspection, or maintenance.
- Wiring Methods
Each zone has specific wiring requirements to match the hazard level:- Zone 0: Intrinsically safe circuits (Technique “ia”) are mandatory. Wiring must comply with Article 504 standards.
- Zone 1: Rigid metal conduits (RMC) or intermediate metal conduits (IMC) are common, alongside explosionproof enclosures and intrinsically safe wiring.
- Zone 2: More relaxed methods, such as Type TC-ER or ITC cables, are acceptable, provided fittings are listed for the application.
- Sealing and Drainage
- Seals prevent the passage of gases and vapors between hazardous and nonhazardous areas.
- In Zone 0 and Zone 1 locations, seals must be installed within specified distances from boundaries and enclosures.
- Equipment Requirements
- Equipment must be marked for the appropriate zone, protection technique, and temperature class.
- Common techniques include:
- Flameproof (“d”): Contain explosions within the enclosure.
- Increased Safety (“e”): Reduce ignition risk through robust design.
- Intrinsic Safety (“ia”, “ib”, “ic”): Limit energy to prevent ignition.
- Temperature Ratings
Equipment surface temperatures must not exceed the ignition temperature of gases or vapors present. Temperature classifications (T1-T6) specify maximum surface temperatures.
Comparison to Division System
Zones provide a more detailed breakdown compared to the division system:
- Division 1 corresponds roughly to Zone 0 and Zone 1.
- Division 2 aligns with Zone 2. This enhanced granularity helps tailor safety measures more precisely to specific environments.
Conclusion
NEC Article 505 provides a structured approach to managing hazardous locations using the zone classification system. By defining areas based on the likelihood and duration of flammable substance exposure, it allows for the implementation of tailored safety measures. Engineers and designers can use these classifications to enhance protection and compliance in high-risk industries.
