About two years ago, I was asked to develop and provide a workshop for the U.S. Space Force on hazardous classification locations and highly oxygenated locations. The basic areas for combustible materials are generally covered in Article 500 of the National Electrical Code. Ignitable and combustible gases, vapors and liquids are covered in Article 501; ignitable and combustible dust is covered in Article 502; and ignitable large fibers or flyings are covered in Article 503. Section 500.4 requires areas designated as hazardous (classified) locations and areas determined to be “unclassified” to be documented on an area classification drawing.

Classified areas

Just showing the classified area on a drawing is not acceptable without also including unclassified surrounding areas. This documentation must be available to the authority having jurisdiction and anyone permitted to design, install, inspect, maintain or operate equipment at the hazardous location. This ensures locations have clear designations.

The NEC provides general and specific information on hazardous locations but does not establish the actual area “classification.” Other NFPA committees, whose members are subject matter experts, provide specific classification information. It is interesting to note that early in the NEC’s development, electricity was called “hazardous” and ignitable materials were called “extra-­hazardous.” In later editions, the designation of “extra-hazardous locations” was changed to “hazardous (classified) locations.” All uses of that phrase in any NFPA document will automatically refer the user back to the NEC. 

The NEC uses that specific information on the classification in the various hazardous location articles from Article 500 to Article 517. For example, NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, provides the area classification, the likelihood that a flammable or combustible concentration or quantity is present, and area documentation. This information is then incorporated into the hazardous (classified) drawings within NEC Chapter 5 based on the specific application. 

Each room, area or section of an indoor or outdoor location must be individually evaluated in determining its area classification. For example, 500.5(A)(2) in the NEC states that refrigerant machinery rooms containing ammonia refrigeration systems and equipped with continuously operating mechanical ventilation or where the ventilation system is initiated by a detection system at a concentration not exceeding 150 parts per million are permitted to be an “unclassified” location. 

This information is in 500.5(A) Informational Note, which recognizes the ANSI/IIAR 2-2021, American National Standard for Design of Safe Closed-Circuit Ammonia Refrigeration Systems, for classification and ventilation of areas involving closed-circuit ammonia-refrigeration systems. 

The specific ventilation requirements for ammonia-refrigeration systems are provided in ANSI/IIAR 2-2021 as follows: “the emergency ventilation rate must be 30 air changes per hour (ACH), the temperature control ventilation rate must limit the room to 104ºF, and all exhaust fans must discharge upwards at a velocity of 2,500 ft./min.”

Oxygen-rich environments

Air is made up of about 78% nitrogen and 21% oxygen, with trace amounts of other gases such as carbon dioxide. The hazardous location information in articles 500–517 is based on 21% oxygen levels. An oxygen level higher than that is considered to be “highly oxygenated” and can drastically change materials’ burning and explosive characteristics. Highly oxygenated levels lower the minimum ignition energy (the lowest amount of energy needed to ignite a flammable mixture). They can also increase explosion pressures caused by material ignition, which are the highest pressures reached during an explosion within a closed vessel or a piece of electrical equipment. 

Highly oxygenated areas can also reduce the maximum experimental safe gap, which is the largest space between two parts of a test apparatus that can prevent a gas mixture from igniting. It is a standardized measurement used to classify flammable gases and vapors within explosion-proof equipment. Highly oxygenated areas can cause higher pressure than the explosion-proof equipment is designed to handle.

An oxygen-rich environment is not, in itself, a hazardous location, according to Article 500. However, an oxygen-rich environment does have specific design and installation challenges. Electrical equipment, safe in ordinary oxygen levels, is not necessarily safe in higher oxygen concentrations or pressures. NFPA 53, Recommended Practice on Materials, Equipment, and Systems Used in Oxygen-Enriched Atmospheres, recommends that no electrical equipment be used in an oxygen-enriched atmosphere unless approved for use at the maximum proposed pressure and oxygen concentration.

I will continue the discussion on this phenomenon next month. 

Porawit / stock.adobe.com