1. Classification Overview
Electrical area classification is a method of analyzing and classifying the environment where flammable gases or vapors may be present, enabling the proper selection of electrical equipment to prevent ignition sources. In natural gas facilities, this classification is critical for personnel safety and regulatory compliance.
Classification Purpose
The primary purpose of area classification is to identify locations where flammable gas-air mixtures may exist and specify the type of electrical equipment and wiring methods that can be safely used in each location.
Governing Standards
Electrical area classification in the United States is governed by multiple standards that work together:
| Standard | Scope | Application |
|---|---|---|
| NEC Article 500 | Division System | Traditional US classification method |
| NEC Article 505 | Zone System | IEC-based classification (optional in US) |
| API RP 500 | Petroleum Facilities | Refineries and chemical plants |
| API RP 505 | Petroleum Facilities (Zone) | Zone system for petroleum industry |
| NFPA 497 | Classification Guidelines | Recommended practice for classification |
Class I Gases and Vapors
Natural gas facilities are classified as Class I locations because they handle flammable gases and vapors. Class I is further divided into groups based on the specific gases present:
| Group | Representative Gas | Application |
|---|---|---|
| Group A | Acetylene | Welding operations |
| Group B | Hydrogen | Refineries, hydrogen plants |
| Group C | Ethylene | Petrochemical facilities |
| Group D | Methane (Natural Gas) | Gas processing, compressor stations |
Natural Gas Classification
Natural gas facilities are typically classified as Class I, Group D locations. Methane, the primary component of natural gas, has a Lower Explosive Limit (LEL) of approximately 5% and Upper Explosive Limit (UEL) of approximately 15% by volume in air.
2. Division Classification System
The Division system (NEC Article 500) is the traditional classification method used in the United States. It divides hazardous areas into two divisions based on the likelihood of flammable atmospheres being present.
| Classification | Definition | Probability |
|---|---|---|
| Division 1 | Flammable concentrations exist under normal operating conditions | Continuous or frequent |
| Division 2 | Flammable concentrations exist only under abnormal conditions | Abnormal/failure only |
| Non-Hazardous | Flammable concentrations not expected | Not anticipated |
Key Distinction
The fundamental difference between Division 1 and Division 2 is the frequency and likelihood of a flammable atmosphere being present:
- Division 1: Hazardous atmosphere expected during normal operations (e.g., inside a compressor building where seals may leak)
- Division 2: Hazardous atmosphere only during equipment failure, maintenance, or abnormal conditions (e.g., outdoor areas near flanged connections)
Equipment Requirements
Division 1 areas require explosion-proof or intrinsically safe equipment. Division 2 areas may use less restrictive equipment types but still require protection against sparks and hot surfaces. Using Division 2-rated equipment in Division 1 areas is a code violation and serious safety hazard.
3. Division 1 Areas
Division 1 locations are areas where flammable gas-air mixtures may exist under normal operating conditions or where equipment failure could simultaneously release flammable materials and create an ignition source.
Typical Division 1 Locations
The following areas in natural gas facilities are typically classified as Division 1:
Compressor Buildings
The interior of compressor buildings housing gas compressors is classified Division 1 because compressor seals, packing, and connections may release gas during normal operation. This includes the entire enclosed space unless adequately ventilated and separated.
| Location | Reason for Division 1 Classification |
|---|---|
| Compressor buildings (interior) | Seal leakage during normal operation |
| Fuel gas scrubber buildings | Potential leakage from piping and vessels |
| Inadequately ventilated meter buildings | Accumulation of leaked gas |
| Areas within 5 ft of relief valves/vents | Designed release points |
| Pits, trenches, and sumps | Gas accumulation in low points |
| Unventilated basements below grade | Heavier-than-air vapors collect |
| Areas around open process vessels | Continuous vapor release |
Division 1 Extent
Standard Division 1 extents from various sources in natural gas facilities:
| Source | Horizontal Extent | Vertical Extent |
|---|---|---|
| Relief valve discharge | 5 ft radius | To grade or 5 ft above |
| Vent openings | 5 ft radius | 5 ft above opening |
| Compressor seal area | 3 ft radius | Floor to 3 ft above |
| Sample points | 3 ft radius | 3 ft above point |
Equipment Requirements for Division 1
- Explosion-proof enclosures - Contain any internal explosion and prevent ignition of external atmosphere
- Intrinsically safe circuits - Energy limited to prevent ignition
- Purged and pressurized enclosures - Maintain positive pressure with clean air
- Oil-immersed equipment - Arcing contacts submerged in oil
4. Division 2 Areas
Division 2 locations are areas where flammable gas-air mixtures are handled, processed, or used but are normally confined within closed containers or systems. Hazardous concentrations occur only during accidental rupture, breakdown, or abnormal operation.
Typical Division 2 Locations
| Location | Reason for Division 2 Classification |
|---|---|
| 5-15 ft from Division 1 sources | Transition zone from release points |
| Within 25 ft of piping with flanges/valves | Potential leak points (gasket failure) |
| Within 100 ft horizontal of Division 1 | Extended dispersion zone |
| Adequately ventilated compressor areas | Ventilation prevents accumulation |
| Ventilated basements below grade | Mechanical ventilation removes vapors |
| Adjacent to Division 1 with vapor barriers | Physical separation but proximity risk |
Division 2 Extent Guidelines
| Configuration | Division 2 Extent |
|---|---|
| From Division 1 boundary | Additional 10-20 ft depending on source |
| Outdoor flanged piping (>2" NPS) | 15-25 ft radius from flanges |
| Pump seals (outdoor) | 10 ft radius, 3 ft above grade |
| Relief valve discharge (beyond Div 1) | 15 ft radius from 5 ft boundary |
Ventilation Impact
Adequate ventilation can reduce a Division 1 area to Division 2, or reduce Division 2 to non-hazardous. "Adequate ventilation" is typically defined as a minimum of 1 CFM per square foot of floor area, or 4 air changes per hour, with intake at low level and exhaust at high level.
Equipment Requirements for Division 2
Division 2 areas allow more flexibility in equipment selection but still require protection:
- General-purpose enclosures - Permitted if no arcing components
- Non-incendive circuits - Normal operation cannot ignite
- Hermetically sealed devices - Contacts sealed against atmosphere
- Non-sparking equipment - No arcing during normal operation
- Restricted breathing enclosures - Limited air exchange with atmosphere
Common Misunderstanding
Division 2 does not mean "safe" or "non-hazardous." It means hazardous conditions are possible but not expected during normal operations. All equipment must still be suitable for the Group D gas classification and rated for Division 2 service.
5. Non-Hazardous (Unclassified) Areas
Non-hazardous or unclassified areas are locations where flammable gas-air mixtures are not expected under normal or abnormal conditions. Standard industrial electrical equipment may be used in these areas.
Typical Non-Hazardous Locations
| Location | Requirements for Non-Hazardous Classification |
|---|---|
| Control rooms | Vapor-tight barrier, positive pressure from clean air |
| Motor control centers (separate building) | Physical separation, no gas piping inside |
| Administration buildings | Outside classified area boundaries |
| General yard areas | Beyond Division 2 boundaries, outdoor |
| Buildings with low-pressure fuel gas | <100 psig fuel gas, vented to outside |
Control Room Requirements
Control rooms in natural gas facilities can be classified as non-hazardous if they meet specific requirements:
Control Room Design Criteria
- Vapor-tight construction - All penetrations sealed
- Positive pressurization - Minimum 0.1 inches water gauge
- Clean air source - Air intake from non-hazardous area
- Self-closing doors - Maintain pressure when opened
- Gas detection - Alarm at 20% LEL in HVAC intake
- Emergency ventilation - Manual or automatic purge capability
Adequately Ventilated Areas
Some areas that would otherwise be classified can be declassified with adequate ventilation:
| Ventilation Requirement | Result |
|---|---|
| Continuous ventilation, 1 CFM/ft² | May reduce Division 1 to Division 2 |
| 4+ air changes/hour with low intake | May reduce Division 2 to non-hazardous |
| Open construction (50%+ wall area open) | May reduce extent of classified areas |
6. Zone Classification System (IEC)
The Zone system (NEC Article 505, IEC 60079) provides an alternative classification method that is more granular than the Division system. It divides hazardous areas into three zones based on the duration of the hazardous atmosphere.
| Zone | Duration | Approximate Division Equivalent |
|---|---|---|
| Zone 0 | Continuous or >1000 hours/year | Most restrictive Division 1 |
| Zone 1 | Likely during normal operation (10-1000 hr/yr) | Division 1 |
| Zone 2 | Only abnormal conditions (<10 hr/yr) | Division 2 |
Zone vs. Division Comparison
| Feature | Division System | Zone System |
|---|---|---|
| Categories | 2 (Division 1, 2) | 3 (Zone 0, 1, 2) |
| Origin | North American (NEC) | International (IEC) |
| Equipment marking | Class/Division/Group | Zone/Group/EPL |
| Typical US use | Traditional, widely used | Growing adoption, especially multinational |
US Practice
Most US natural gas facilities continue to use the Division system due to familiarity and existing equipment inventory. The Zone system is permitted by NEC but is more commonly used by companies with international operations or when using European-manufactured equipment.
7. Equipment Selection
Selecting the correct electrical equipment for hazardous areas requires matching the equipment rating to the area classification. Equipment must be listed and labeled for the specific Class, Division (or Zone), and Group.
Protection Techniques
| Technique | Division 1 | Division 2 | Description |
|---|---|---|---|
| Explosion-proof (XP) | Yes | Yes | Contains explosion, prevents propagation |
| Intrinsically safe (IS) | Yes | Yes | Energy limited below ignition threshold |
| Purged/Pressurized (Type X, Y, Z) | Yes (X, Y) | Yes (Z) | Positive pressure prevents gas entry |
| Non-incendive | No | Yes | Normal operation cannot ignite |
| Hermetically sealed | No | Yes | Fused/welded closure |
| Oil immersion | Yes | Yes | Contacts immersed in oil |
Intrinsically Safe Systems
Intrinsically safe (IS) equipment is designed so that electrical energy is limited to levels that cannot cause ignition of the hazardous atmosphere. This is achieved through energy-limiting barriers and careful circuit design.
IS System Requirements
- Associated apparatus - Barriers, isolators in safe area
- Field devices - Simple apparatus (thermocouples, RTDs, switches)
- Wiring - Dedicated conduit, blue jacket cables
- Documentation - Entity parameters must be verified
- Installation - Separation from non-IS circuits
Equipment Marking
Equipment suitable for hazardous locations will be marked with its ratings:
Example Equipment Marking:
Class I, Division 1, Groups C & D, T3
This equipment is suitable for Division 1 areas with Group C or D gases, with a maximum surface temperature of 200°C (T3 rating).
Temperature Class
Equipment must have a maximum surface temperature below the auto-ignition temperature of the gas. Natural gas (methane) has an auto-ignition temperature of approximately 580°C (1076°F).
| T-Code | Max Surface Temp (°C) | Max Surface Temp (°F) |
|---|---|---|
| T1 | 450 | 842 |
| T2 | 300 | 572 |
| T2A | 280 | 536 |
| T2B | 260 | 500 |
| T2C | 230 | 446 |
| T2D | 215 | 419 |
| T3 | 200 | 392 |
| T3A | 180 | 356 |
| T3B | 165 | 329 |
| T3C | 160 | 320 |
| T4 | 135 | 275 |
| T4A | 120 | 248 |
| T5 | 100 | 212 |
| T6 | 85 | 185 |
Natural Gas T-Code
For natural gas (methane) with an AIT of 580°C, equipment rated T1 through T3 is typically acceptable. T1 rated equipment (450°C max) provides adequate margin below the 580°C auto-ignition temperature. However, if heavier hydrocarbons are present, lower T-code ratings may be required.
8. Documentation Requirements
Proper documentation of electrical area classification is essential for initial design, ongoing operations, and regulatory compliance. Documentation should be maintained throughout the facility lifecycle.
Required Documentation
| Document | Purpose | Update Frequency |
|---|---|---|
| Area Classification Drawings | Show extent of Division 1, 2, and non-hazardous areas | With any process change |
| Classification Basis Document | Engineering basis for each classification decision | With any process change |
| Equipment Schedule | List all electrical equipment with ratings and locations | Annual verification |
| IS Loop Drawings | Entity parameter verification for IS circuits | With any circuit change |
| Inspection Records | Document ongoing compliance inspections | Per inspection schedule |
Area Classification Drawing Content
Area classification drawings should include:
- Plan views showing horizontal extent of all classified areas
- Elevation views showing vertical extent
- Clear identification of Division 1, Division 2, and non-hazardous boundaries
- Location of all release sources (vents, reliefs, seals, flanges)
- Building outlines and ventilation openings
- Reference to applicable codes and standards
- Revision history and approval signatures
Management of Change
Any modification to process equipment, piping, or ventilation systems requires review of area classification. Even seemingly minor changes (adding a sample point, relocating equipment) can affect classification boundaries. All changes should be documented through the facility's Management of Change (MOC) process.
Inspection and Maintenance
Regular inspection of electrical installations in hazardous areas is required to ensure ongoing compliance:
| Inspection Item | Frequency | Standard Reference |
|---|---|---|
| Visual inspection of enclosures | Monthly | NFPA 70B |
| Verify equipment ratings match area | Annual | NEC 500 |
| Check conduit seals | Annual | NEC 501.15 |
| IS barrier function test | Per manufacturer | - |
| Purge system verification | Monthly | NFPA 496 |