1. Classification Overview
Electrical area classification is the process of identifying locations where flammable gases or vapors may be present in sufficient concentrations to create an ignition hazard. The classification determines what type of electrical equipment and wiring methods can be safely installed in each area. In oil and gas facilities, proper area classification is fundamental to process safety and regulatory compliance.
Purpose of Area Classification
The primary goal is to prevent electrical equipment from becoming an ignition source in areas where flammable atmospheres may exist. Area classification does not eliminate the presence of flammable gases; rather, it ensures that electrical installations in each zone or division are designed to contain or prevent ignition. The extent of each classified area is determined by the type and rate of potential release, fluid properties, and ventilation conditions.
Governing Standards
| Standard | System | Application |
|---|---|---|
| API RP 500 | Class I, Division 1 & 2 | Traditional US system for petroleum refineries and production facilities |
| API RP 505 | Zone 0, 1, & 2 | IEC-based system increasingly adopted in US oil and gas |
| NEC Article 500 | Division system | Installation requirements for Division-classified areas |
| NEC Article 505 | Zone system | Installation requirements for Zone-classified areas |
| NFPA 497 | Division system | Recommended practice for classification using Division system |
| IEC 60079-10-1 | Zone system | International standard for hazardous area classification |
Classification Hierarchy
Area classification follows a systematic hierarchy based on the type of hazardous material, its physical state, and the likelihood of a flammable atmosphere being present.
| Level | Classification | Description |
|---|---|---|
| Class | Class I | Flammable gases or vapors (applies to virtually all oil and gas) |
| Class | Class II | Combustible dusts (grain elevators, coal handling) |
| Class | Class III | Ignitable fibers or flyings (textile mills) |
| Division/Zone | Div 1 or Zone 0/1 | Higher probability of flammable atmosphere |
| Division/Zone | Div 2 or Zone 2 | Lower probability (abnormal conditions only) |
| Group | Groups A, B, C, D | Specific gas or vapor properties (explosion pressure, MESG) |
2. Class/Division System (API RP 500)
The Division system is the traditional North American approach to area classification. It divides hazardous areas into two divisions based on the probability that a flammable gas or vapor concentration will be present.
Division Definitions
| Division | Condition | Examples in Oil & Gas |
|---|---|---|
| Division 1 | Flammable concentrations exist under normal operating conditions, or during frequent repair/maintenance, or where equipment failure could simultaneously cause release and ignition | Inside process vessels, open pits/trenches below grade near process equipment, inadequately ventilated areas around continuous release sources |
| Division 2 | Flammable concentrations exist only under abnormal conditions (equipment failure, rupture, accidental release), or adjacent to Division 1 areas | Areas surrounding flanges, valves, and fittings; well-ventilated process areas; areas adjacent to Division 1 boundaries |
| Unclassified | Flammable concentrations are not expected under normal or abnormal conditions | Control buildings with positive pressurization, areas beyond classification boundaries |
Normal vs. Abnormal Conditions
The distinction between Division 1 and Division 2 hinges on whether a flammable atmosphere is expected during normal operations. Normal conditions include startup, shutdown, and routine maintenance. Abnormal conditions include equipment failures, seal ruptures, or accidental spills. An area around a compressor seal vent, for instance, is Division 1 because seal gas leakage during normal operation is expected. An area around a bolted flange joint may be Division 2 because leakage occurs only when gaskets deteriorate or bolts loosen.
Equipment Protection Methods — Division System
| Protection Method | Division 1 | Division 2 |
|---|---|---|
| Explosion-proof (XP) | Yes | Yes |
| Intrinsically safe (IS) | Yes | Yes |
| Purged/pressurized (Type X/Y) | Yes (Type X) | Yes (Type Y/Z) |
| Non-incendive | No | Yes |
| General purpose (enclosed) | No | Limited (non-arcing) |
3. Zone System (API RP 505 / IEC)
The Zone system, based on IEC 60079-10-1 and adopted in API RP 505, provides a three-tier classification that offers more granularity than the Division system. This system is increasingly used in new US facilities, particularly those with international engineering or operating companies.
Zone Definitions
| Zone | Duration of Flammable Atmosphere | Probability | Typical Locations |
|---|---|---|---|
| Zone 0 | Continuously or for long periods (>1000 hrs/yr) | High | Inside vessels, tanks, pipes containing flammable liquids or gases |
| Zone 1 | Likely under normal operations (10–1000 hrs/yr) | Medium | Near relief valves, sample points, drain connections, seal vents |
| Zone 2 | Only under abnormal conditions (<10 hrs/yr) | Low | Areas around flanged connections, valve stems, pump seals under normal conditions |
Zone 0 Equipment
Zone 0 requires intrinsically safe (Ex ia) equipment or equipment specifically certified for Zone 0 use. Very few electrical devices are installed in Zone 0 areas. Most instrumentation in Zone 0 applications (such as level transmitters inside vessels) uses intrinsically safe circuits with barriers located in the safe area. Explosion-proof equipment alone is not sufficient for Zone 0.
Equipment Protection Methods — Zone System
| Protection Type | Designation | Zone 0 | Zone 1 | Zone 2 |
|---|---|---|---|---|
| Intrinsic safety | Ex ia | Yes | Yes | Yes |
| Intrinsic safety | Ex ib | No | Yes | Yes |
| Flameproof | Ex d | No | Yes | Yes |
| Increased safety | Ex e | No | Yes | Yes |
| Purged/pressurized | Ex p | No | Yes | Yes |
| Non-sparking | Ex nA | No | No | Yes |
4. Gas Groups & Properties
Flammable gases and vapors are classified into groups based on their explosion characteristics, primarily the maximum experimental safe gap (MESG) and the minimum igniting current (MIC) ratio. Equipment must be rated for the specific gas group present in the classified area.
NEC Gas Groups (Class I)
| Group | Representative Gas | MESG (mm) | Common in Midstream |
|---|---|---|---|
| Group A | Acetylene | < 0.45 | Rare (welding operations only) |
| Group B | Hydrogen, butadiene | 0.45 – 0.75 | Hydrogen-rich gas streams, some reformer operations |
| Group C | Ethylene, ethyl ether | 0.75 – 0.90 | Ethylene plants, some NGL processing |
| Group D | Methane, propane, natural gas | > 0.90 | Most common — covers natural gas and NGL products |
Group D Dominates Midstream
In midstream oil and gas operations, the vast majority of classified areas are Group D because the primary fluids handled are natural gas (methane), propane, butane, and natural gasoline. Group D equipment is the most widely available and least expensive. However, facilities that handle hydrogen-rich streams (such as tail gas from amine treating or PSA off-gas) may require Group B classification in specific areas.
IEC Gas Groups (Zone System)
| IEC Group | NEC Equivalent | Representative Gases |
|---|---|---|
| IIC | Groups A & B | Hydrogen, acetylene |
| IIB | Group C | Ethylene |
| IIA | Group D | Methane, propane, natural gas |
Key Flammability Properties
| Property | Methane | Propane | Hydrogen | Significance |
|---|---|---|---|---|
| LEL (% vol) | 5.0 | 2.1 | 4.0 | Lower limit for ignition risk |
| UEL (% vol) | 15.0 | 9.5 | 75.0 | Upper limit (hydrogen has very wide range) |
| Vapor density (air=1) | 0.55 | 1.52 | 0.07 | Heavier-than-air vapors accumulate at grade |
| Autoignition (°F) | 999 | 842 | 932 | Equipment temperature class rating |
5. Extent of Classified Areas
The extent (distance and shape) of classified areas around potential release sources is defined by API RP 500 and API RP 505. These dimensions represent engineering judgment based on dispersion modeling, historical incident data, and industry experience. The extent depends on the type of equipment, the fluid handled, and ventilation conditions.
Common Release Sources — Outdoor, Adequately Ventilated
| Source | Division 1 Extent | Division 2 Extent |
|---|---|---|
| Relief valve discharge (gas) | 5 ft radius from point of discharge | 15 ft radius from point of discharge |
| Compressor seal vent | 3 ft radius from vent outlet | 10 ft radius from vent outlet |
| Flanged connections (≤ 2 in.) | None (not a continuous source) | 3 ft radius from connection |
| Flanged connections (> 2 in.) | None | 5 ft radius from connection |
| Open drain or sump | 3 ft radius, grade to 18 in. above | 5 ft radius, grade to 18 in. above |
| Separator or scrubber | None (closed vessel) | 15 ft from vessel in all directions |
| Pipeline pig launcher/receiver | 5 ft radius from closure | 15 ft radius from closure |
| Tank (fixed roof, volatile liquid) | Inside tank above liquid surface | Shell to 10 ft beyond, grade to roof |
Below-Grade Considerations
Below-grade areas such as pits, trenches, and sumps within or adjacent to classified areas require special attention. Heavier-than-air vapors (propane, butane, natural gasoline) accumulate in low points. Any pit or trench within a Division 2 area is typically classified as Division 1 because vapors cannot disperse naturally. This is one of the most frequently overlooked aspects of area classification in midstream facilities.
Indoor and Enclosed Areas
When process equipment containing flammable fluids is located indoors or in enclosed buildings, the entire room or enclosure is typically classified unless adequate ventilation can be demonstrated.
| Ventilation Level | Air Changes | Classification Effect |
|---|---|---|
| Inadequate | < 4 ACH | Entire enclosed area is Division 1 |
| Adequate | 6 – 12 ACH | Extent per API RP 500 outdoor tables |
| Positive pressure | N/A (maintained above atmospheric) | Interior may be unclassified if criteria met |
Vertical Extent
Classified areas extend both horizontally and vertically from the release source. For lighter-than-air gases (methane, hydrogen), the classified area extends upward. For heavier-than-air vapors (propane, butane), the area extends downward to grade level and into below-grade spaces. API RP 500 figures show both horizontal and vertical extents for each source type.
6. Ventilation Effects
Ventilation is the single most significant factor in determining the extent of classified areas. Adequate ventilation dilutes and disperses released gases or vapors, reducing both the probability and extent of a flammable atmosphere.
Ventilation Categories
| Category | Definition | Effect on Classification |
|---|---|---|
| Adequate (outdoor) | Natural air movement in open areas without obstructions to airflow | Standard extent dimensions apply; this is the baseline assumption |
| Inadequate | Enclosed or congested areas where natural ventilation cannot disperse vapors | Classification extent increases; enclosed spaces may become entirely Div 1 |
| Mechanical (forced) | Engineered ventilation system providing controlled air changes | Can reduce classification extent or reclassify areas from Div 1 to Div 2 |
Mechanical Ventilation as Safeguard
Mechanical ventilation systems used to reduce area classification must be designed with high reliability. API RP 500 requires that the ventilation system operate continuously during plant operation, include failure alarms, and be designed so that loss of ventilation automatically de-energizes electrical equipment in the affected area (unless the equipment is rated for the unventilated classification). The ventilation system itself becomes a safety-critical system requiring regular maintenance and functional testing.
NFPA 496 Purging and Pressurization
NFPA 496 provides requirements for purging and pressurizing enclosures to reduce the classification of their interior. This is commonly used for control buildings, analyzer houses, and motor control centers located in or near classified areas.
| Type | Exterior Classification | Interior Result | Application |
|---|---|---|---|
| Type X | Division 1 | Unclassified | Control rooms, MCC buildings in Div 1 |
| Type Y | Division 1 | Division 2 | Equipment enclosures in Div 1 |
| Type Z | Division 2 | Unclassified | Buildings at edge of classified areas |
7. Classification Drawings
Area classification documentation is a critical deliverable in facility design. Classification drawings serve as the permanent record of hazardous area boundaries and form the basis for all electrical equipment selection and installation throughout the facility lifecycle.
Drawing Requirements
| Element | Requirement |
|---|---|
| Plan views | Show horizontal extent of all classified areas at grade level |
| Elevation views | Show vertical extent, especially for heavier-than-air vapors |
| Classification boundary | Clear delineation between Division 1, Division 2, and unclassified |
| Gas group | Identified for each classified area |
| Temperature class | Equipment temperature class based on autoignition temperature |
| Source identification | Each release source labeled with equipment tag and fluid |
| Ventilation notes | Assumptions about ventilation (adequate, inadequate, mechanical) |
| Reference standard | API RP 500 or 505 revision used |
Living Document
Area classification drawings must be updated whenever process changes, equipment modifications, or facility expansions alter the location or nature of potential release sources. Adding a new compressor, changing the fluid composition, or modifying ventilation systems all require review and potential revision of the classification drawings. OSHA PSM and EPA RMP programs both require management of change procedures that include electrical area classification review.
8. Division vs Zone Comparison
Both the Division and Zone systems achieve the same safety objective but differ in granularity and equipment selection options. Many US facilities are transitioning to the Zone system for new construction.
Side-by-Side Comparison
| Feature | Division System | Zone System |
|---|---|---|
| Number of classifications | 2 (Division 1 & 2) | 3 (Zone 0, 1, & 2) |
| Continuous release category | Included in Division 1 | Separate Zone 0 |
| Equipment protection types | Fewer options (XP, IS, purged) | More options (Ex d, e, ia, ib, n, p, o, q) |
| Potential cost savings | Higher cost (more XP equipment in Div 1) | Zone 2 allows less expensive equipment |
| International compatibility | Primarily US/Canada | Aligned with IEC (global acceptance) |
| Mixing systems | NEC 505.7 — Division and Zone equipment shall not be mixed in the same area | |
Cannot Mix Systems
Once a facility selects either the Division or Zone system for a given area, all electrical equipment and wiring in that area must conform to the chosen system. The NEC does not permit mixing Division-rated and Zone-rated equipment in the same classified area. However, different areas within the same facility can use different systems, provided the boundaries are clearly documented.
Temperature Classification
Regardless of whether the Division or Zone system is used, all electrical equipment in classified areas must have a maximum surface temperature below the autoignition temperature of the gas or vapor present. Temperature classes are designated as follows:
| T-Code | Max Surface Temp (°C) | Max Surface Temp (°F) | Typical Gases Covered |
|---|---|---|---|
| T1 | 450 | 842 | Methane, hydrogen (most gases) |
| T2 | 300 | 572 | Ethanol, butane |
| T3 | 200 | 392 | Gasoline, hexane, hydrogen sulfide |
| T4 | 135 | 275 | Acetaldehyde, diethyl ether |
| T5 | 100 | 212 | Carbon disulfide |
| T6 | 85 | 185 | (No common midstream gases) |