Zone Classification Systems: Understanding Class/Division vs Zone Methods

Introduction to Hazardous Area Classification

When working with equipment in potentially explosive atmospheres, proper area classification is the foundation of safety. Two primary systems exist worldwide for classifying hazardous locations: the North American Class/Division system and the international Zone system used by IECEx and ATEX standards.

Understanding these classification methods is crucial for engineers, safety professionals, and facility managers working in industries such as oil & gas, chemical processing, pharmaceuticals, and food processing.

North American Class/Division System

The Class/Division system, defined in NFPA 70 (National Electrical Code) and used throughout North America, categorizes hazardous locations based on three factors:

Classes (Type of Hazardous Material)

• Class I: Flammable gases and vapors
• Class II: Combustible dusts
• Class III: Ignitable fibers and flyings

Divisions (Probability of Presence)

• Division 1: Hazardous material present under normal operating conditions
• Division 2: Hazardous material present only under abnormal conditions

Groups (Material Properties)

For Class I locations, materials are further subdivided into groups A, B, C, and D based on their ignition characteristics and flame propagation properties.

International Zone System

The Zone system, used by IECEx (IEC 60079 series) and ATEX standards, provides a more granular approach to hazardous area classification:

Gas/Vapor Zones

• Zone 0: Explosive atmosphere present continuously or for long periods
• Zone 1: Explosive atmosphere likely to occur in normal operation
• Zone 2: Explosive atmosphere not likely to occur, or only briefly if it does

Dust Zones

• Zone 20: Combustible dust cloud present continuously or frequently
• Zone 21: Combustible dust cloud likely during normal operation
• Zone 22: Combustible dust cloud not likely, or only briefly

Comparison: Class/Division vs Zone System

Aspect	Class/Division (North America)	Zone System (International)
Risk Levels	2 levels (Division 1 & 2)	3 levels (Zone 0, 1, 2)
Dust Classification	Class II (all dusts together)	Separate zones (20, 21, 22)
Standards	NFPA 70 (NEC), CEC	IEC 60079, EN 60079
Global Usage	North America	Europe, Asia, most of world
Equipment Selection	Based on Class/Division/Group	Based on Zone/Gas Group/T-Class

Practical Classification Examples

Key Considerations for Engineers

When working with hazardous area classification, consider these important factors:

1. Documentation Requirements

Both systems require detailed documentation of the classification rationale, including release scenarios, ventilation analysis, and material properties assessment.

2. Equipment Selection Impact

The classification directly determines the required protection level for electrical equipment. Zone 0 requires the highest protection (Ex ia only), while Division 2/Zone 2 allows various protection methods.

3. Maintenance and Inspection

Different zones/divisions have varying maintenance requirements and inspection frequencies for installed equipment.

4. Global Projects

For international projects, the Zone system is typically preferred due to its worldwide acceptance and more detailed risk assessment approach.

Best Practices

• Involve qualified professionals: Area classification should be performed by experienced engineers familiar with the specific industry and processes
• Regular reviews: Classifications should be reviewed when processes change or during facility modifications
• Consider both systems: For North American facilities, consider both systems to optimize equipment selection and costs
• Document thoroughly: Maintain detailed records of classification decisions and supporting analysis

Conclusion

Understanding the differences between Class/Division and Zone classification systems is essential for anyone working with hazardous locations. While the North American Class/Division system provides a straightforward two-tier approach, the international Zone system offers more granular risk assessment capabilities.

The choice between systems often depends on regional requirements, but the Zone system's growing acceptance and more detailed approach make it increasingly popular for new installations worldwide.