API 500: Electrical area classification in petroleum installations

API 500 is a recommended practice used for the electrical classification of hazardous areas in petroleum facilities, particularly in environments where flammable or explosive atmospheres may form. Its application allows for accurately defining the zones where it is safe to install electrical equipment, preventing ignition sources.

This article describes the scope of the standard, the differences with API RP 505, the meaning of Class I Division 1, and the relevance of these criteria in the oil and gas sector.

API 500: Definition and purpose in petroleum installations

API 500, developed by the American Petroleum Institute, establishes procedures for classifying hazardous locations where flammable vapors, gases, or liquids may be present in quantities capable of forming explosive mixtures.

The document adopts the structure of the NEC, NEC means National Electrical Code, and in this context, Article 500 establishes the classification of hazardous areas or locations where there is a risk of fire or explosion due to flammable gases, vapors, dusts, or fibers. The text indicates that API 500 (American Petroleum Institute) is based on the definitions and criteria of NEC Article 500 to classify these zones. The main objective is to prevent ignition of flammable atmospheres and ensure the integrity of workers, infrastructure, and operational processes.

Electrical classification of hazardous areas according to API 500

API 500 classifies areas based on the probability of the presence of explosive atmospheres:

• Class I: Corresponds to areas where flammable gases or vapors may exist.
  • Division 1: The explosive atmosphere is present under normal operating conditions, continuously, frequently, or intermittently.
  • Division 2: The explosive atmosphere appears only under abnormal conditions, such as accidental leaks or process equipment failures.
• Classes II and III: Related to combustible dusts and flammable fibers.
  • Their application in petroleum installations is limited; the dominant focus is Class I due to the presence of hydrocarbon vapors.

Understanding the difference between Division 1 and Division 2 is critical for specifying electrical equipment and explosion-protection systems.

Technical differences between API 500 and API RP 505

Both standards address the classification of hazardous areas; however, their approaches differ. Table 1 provides a summary of the main differences:

Table 1. Technical Comparison: API 500 vs. API RP 505

Aspect	API RP 500	API RP 505
Classification System	Classes and Divisions (NEC)	IEC Zones (International Electrotechnical Commission): 0, 1, 2 (IEC 60079)
Philosophy	Based on probability of presence	Based on duration, frequency, and persistence of flammable gases/vapors
Level of Detail	Practical approach	Analytical approach, greater granularity
Equipment Type	Class/Division equipment	IECEx/ATEX-certified equipment
Geographic Use	Mainly U.S.	International
Interchangeability	Does not replace IEC/ATEX	May replace API 500 in global plants

In summary, API 500 is a traditional system oriented to local installations, while API RP 505 aligns better with international projects based on IEC/ATEX requirements.

Equipment and protections required according to API 500

API 500 establishes that explosion-proof equipment installed in hazardous areas must comply with the protection suitable for the corresponding Class and Division:

• Division 1: Fully sealed equipment or devices with specific certifications for continuous operation in the presence of flammable gases are required.
• Division 2: Equipment with a lower degree of protection may be used, provided ventilation and detection controls minimize the exposure to gases.

Common equipment includes:

• Certified electric motors and transformers.
• Explosion-proof industrial lighting.
• Hermetically sealed switchboards and control panels.
• Sensors and monitoring equipment in petroleum installations.

Technical importance of API 500 in the oil industry

Applying API 500 in electrical installations within the petroleum sector is essential for several reasons:

• Personnel safety: Ensures that workers are not exposed to ignition risks in areas with explosive atmospheres.
• Asset protection: Minimizes the probability of fires and explosions that may affect oil and gas installations.
• Regulatory compliance: Ensures conformity with local and federal regulations based on NEC Article 500.
• Proper design of electrical equipment: Facilitates the selection of explosion-proof equipment according to Class I Division 1 and 2.
• Process standardization: Helps companies implement uniform, safe electrical systems throughout the facility.

In a context where hydrocarbon leaks are a frequent risk, applying API 500 is an essential preventive measure.

How to identify and classify hazardous areas?

The technical procedure for classifying these areas using API 500 must follow the steps below:

• Identification of flammable vapor sources:
  • Storage tanks.
  • Pumps, compressors, and seals.
  • Hydrocarbon piping and manifolds.
• Determination of the frequency of explosive atmosphere presence:
  • Division 1 → frequent or normal exposure.
  • Division 2 → occasional or accidental exposure.
• Establishment of the limits of the classified area:
  • Delimitation of zones where equipment must be explosion-proof.
• Selection of certified electrical equipment:
  • Must be based on Class and Division.
• Technical documentation:
  • Warning labels.
  • Electrical drawings with Class I Division 1 and 2 zones.
  • Operational and maintenance procedures.

This procedure ensures consistent electrical classification in compliance with safety standards.

Technical importance of applying API 500

The implementation of API 500 is essential for ensuring operational safety in installations where flammable atmospheres can be generated. Its application significantly reduces ignition risk by defining precise criteria for hazardous area classification, enabling the selection of electrical equipment capable of operating under critical conditions.

By preventing catastrophic failures, the standard helps minimize economic losses related to explosions or plant shutdowns while ensuring regulatory compliance with audits, inspections, and certifications.

Additionally, it enables the safe integration of new technologies and electrical systems, supporting modernization without compromising system integrity. This approach also strengthens environmental protection and operational continuity by reducing incidents that may affect processes, infrastructure, or the surrounding environment.

Practical application examples

The application of API 500 criteria is evident in cases such as hydrocarbon storage plants, where areas near vents, relief valves, or loading/unloading points are typically classified as Class I Division 1. This is due to the possible presence of flammable vapors under normal operating conditions. For example, the selection of electrical equipment: components such as luminaires, motors, or sensors installed directly inside tanks or in areas with continuous gas presence must meet Division 1 requirements.

In contrast, outdoor areas with adequate ventilation and no frequent releases are usually defined as Class I Division 2, indicating lower but still controlled risk. Under this criterion, peripheral control panels and systems are generally installed under Division 2 specifications.

This classification must be rigorously reflected in technical documentation, including hazardous area signage and updated electrical diagrams with the corresponding divisions, ensuring consistency and technical support for audits.

API 500 vs. API RP 505 in global installations

In the international context, most refineries, terminals, and industrial complexes prefer adopting API RP 505 because of its compatibility with ATEX and IECEx standards widely used in Europe and other regions. This standard, based on the concept of zones, provides more precise and flexible classification for installations with complex processes or high automation levels. However, in the United States, API 500 remains fully valid and mandatory in any installation operating under the National Electrical Code (NEC).

The choice between both methodologies depends on the regulatory requirements of the country, the level of detail required in the engineering design, and the complexity of the industrial process. Therefore, the decision must be based on a technical analysis considering the operational environment, applicable regulations, and compatibility with available equipment and certifications.

Conclusion

API 500 is essential for electrical classification in the oil industry, defining hazardous areas and ensuring explosion protection through certified equipment. Understanding its differences with API RP 505 and the significance of Class I Division 1 enables the design of safe electrical installations that comply with regulations and reduce risks in petroleum facilities.

References

1. American Petroleum Institute (API), RP 500: Electrical Classification of Locations for Petroleum Facilities.
2. API RP 505, Classification of Locations for Electrical Installations at Petroleum Facilities.
3. National Electrical Code (NEC), Class I, II, III Electrical Systems.
4. IEC 60079, Explosive Atmospheres – Equipment Requirements and Protection Concepts.
5. NFPA 70, National Electrical Code – Hazardous Locations.

Frequently Asked Questions (FAQs)