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API Publication 317-1993: Recommended Practices for Classification of Locations for Electrical Installations at Petroleum Facilities
A Technical Guide to Hazardous Area Classification and Electrical Safety in the Petroleum Industry
API Publication 317-1993, titled Recommended Practices for Classification of Locations for Electrical Installations at Petroleum Facilities, provides a systematic framework for identifying and classifying areas where flammable gases, vapours, or liquids may be present in sufficient concentration to create an explosive or ignitable atmosphere. First issued in 1993, this publication served as a foundational document for electrical design and installation in the petroleum industry, covering upstream, midstream, and downstream facilities including production platforms, refineries, pipeline terminals, and storage depots. The scope extends to: The standard applies primarily to Class I locations (flammable gases and vapours) and provides guidance for delineating Divisions 1, 2, and (in some references) non‑classified areas. It utilizes the Division classification system consistent with the National Electrical Code (NFPA 70) and the Canadian Electrical Code.Scope and Applicability
Tip: Although API 317-1993 has been superseded by later editions (e.g., API 500 and API 505), it remains an authoritative reference for legacy installations and for understanding the evolution of area classification practices.
Technical Requirements and Classification Methodology
Classification Parameters
API 317-1993 establishes classification based on three primary factors:
- Chemical properties of the flammable material (flash point, lower and upper explosive limits)
- Release characteristics (continuous, intermittent, or rare under abnormal conditions)
- Environmental conditions (ventilation, temperature, topography)
The following table summarises the classification levels defined in the publication:
| Division | Definition | Typical Examples |
|---|---|---|
| Division 1 | A location where ignitable concentrations of flammable gases or vapours exist under normal operating conditions, or where such concentrations are frequently present due to repair or maintenance. | Dome of a storage tank, sump pits, pump seals handling volatile liquids, open vent lines. |
| Division 2 | A location where ignitable concentrations exist only under abnormal conditions (e.g., accidental rupture, ventilation failure), or where volatile liquids are confined within closed systems. | Immediately outside a Division 1 boundary, areas around flanges and valves, enclosed electrical rooms adjacent to process areas (with positive pressure ventilation). |
| Non‑classified | Locations where the probability of an ignitable concentration is negligible under both normal and abnormal conditions. | Offices, control rooms with adequate overpressure, areas far from any source of release. |
Important: API 317-1993 does not use the Zone classification system (Zones 0, 1, 2) introduced later in IEC 60079 and adapted in API 505 for petroleum facilities. Users working globally must reconcile the legacy Division system with Zone‑based regulations.
Boundary Determination and Extent of Classification
The publication provides detailed tables and diagrams for establishing the radial and vertical distances from release sources where classification applies. Factors such as natural ventilation (outdoor vs. indoor) and mechanical ventilation rates modify these distances. For example:
- For a vent opening in an outdoor environment with good natural ventilation, the Division 1 boundary may be limited to 1–3 m, while indoor environments with poor ventilation may require significantly larger separation.
- Underground pits and trenches are generally considered Division 1 due to the accumulation of heavier‑than‑air vapours.
The recommended practise emphasises a risk‑based approach, using published examples and allowing engineering judgement for unique situations.
Implementation Highlights
Applying the Classification to Electrical Equipment
Once areas are classified, API 317-1993 guides the specifier to appropriate electrical enclosure types, temperature ratings (T‑codes), and group classifications (Groups A, B, C, D for gases and vapours). The publication cross‑references with NFPA 70 Article 500 to ensure a clear compliance path in North America.
Documentation and Marking
Implementers are required to produce a formal area classification drawing that identifies all classified zones with divisions, groups, and the explosive atmosphere temperature class. This drawing must be updated when changes occur in the process or equipment.
Best Practice: A multidisciplinary team (process, electrical, safety, and operations) should jointly conduct the area classification exercise to ensure all release scenarios are identified.
Ventilation and Mitigation
The standard discusses the use of reliable mechanical ventilation to reduce the extent of classified areas. It defines criteria for “good” vs. “poor” ventilation based on air change rates and availability of makeup air. Systems must include redundancy or performance monitoring if they serve as the sole means of keeping a location non‑classified.
Critical: Never rely solely on ventilation to reclassify a Division 1 location to Division 2 unless the system meets strict reliability requirements (e.g., backup power, automatic alarms). Failure to do so has been a root cause of multiple catastrophic explosions.
Compliance Notes and Legacy Considerations
Relationship with Current Standards
API 317-1993 was officially withdrawn and replaced by API 500 (2014) and API 505 (2018) for petroleum facilities. However, many existing installations still operate under original classification studies based on the 1993 edition. Regulators (e.g., OSHA in the US, local fire authorities) typically accept API 317 classifications if they can be demonstrated to be equivalent to the current risk‑level.
Common Compliance Gaps
- Failure to update classification drawings after process modifications
- Use of “non‑classified” designation without proper ventilation verification
- Misapplication of Division 2 to locations that actually experience occasional ignitable concentrations during normal operations
- Ignoring the effects of prevailing wind on outdoor gas dispersion (a point stressed in the 1993 edition)
To demonstrate compliance, operators should retain original classification documentation, a record of changes, and justification for any departure from the recommended boundary distances.
Note: For new projects, it is strongly recommended to use the current editions (API 500 / 505) rather than the 1993 publication. However, API 317-1993 remains a valuable educational resource for understanding the evolution of area classification in petroleum facilities.
Frequently Asked Questions
Q: Why was API Publication 317-1993 developed, and how does it differ from API 500?
A: API 317-1993 was one of the first comprehensive recommended practices specifically for petroleum facilities, bridging earlier general standards (NFPA 70) and industry‑specific needs. It was later superseded by API 500, which incorporated more detailed risk‑based methods and aligned better with international practices (e.g., IEC). The core classification logic (Division 1 vs. Division 2) remains largely similar.
A: API 317-1993 was one of the first comprehensive recommended practices specifically for petroleum facilities, bridging earlier general standards (NFPA 70) and industry‑specific needs. It was later superseded by API 500, which incorporated more detailed risk‑based methods and aligned better with international practices (e.g., IEC). The core classification logic (Division 1 vs. Division 2) remains largely similar.
Q: Can I still use the 1993 edition for a new facility design?
A: While technically feasible, it is not recommended. Regulatory bodies and insurance carriers generally expect compliance with current editions. Using an outdated standard may complicate your safety case and could be deemed inadequate if an incident occurs. Always verify with the local authority having jurisdiction.
A: While technically feasible, it is not recommended. Regulatory bodies and insurance carriers generally expect compliance with current editions. Using an outdated standard may complicate your safety case and could be deemed inadequate if an incident occurs. Always verify with the local authority having jurisdiction.
Q: Does API 317-1993 cover Zone classification (Zones 0, 1, 2)?
A: No. The 1993 edition exclusively uses the Division system (Class I, Divisions 1 and 2). Zone classification for petroleum facilities was introduced later in API 505, based on IEC 60079 series.
A: No. The 1993 edition exclusively uses the Division system (Class I, Divisions 1 and 2). Zone classification for petroleum facilities was introduced later in API 505, based on IEC 60079 series.