Why a tank battery is a classified location
OSHA 1910.307 covers the requirements for electric equipment and wiring in hazardous classified locations, and it names petroleum and chemical processing plants in its list of typical occupancies. A tank battery, a wellhead, and a production facility are Class I locations because crude oil and condensate give off flammable vapors, and the classification depends on the properties of those vapors and the likelihood that a flammable concentration is present. Each room, section, or area is considered individually, which is why the classification of a heater treater area can differ from the classification of the truck parking area on the same lease.
The rating has to match the vapor
Under 1910.307(c), equipment, wiring methods, and installations in hazardous classified locations must be intrinsically safe, approved for the hazardous location, or demonstrated safe for the location. Equipment must be approved not only for the class of location but also for the ignitable or combustible properties of the specific gas or vapor that will be present, and it must be marked to show the class, group, and operating temperature for which it is approved. The temperature marking may not exceed the ignition temperature of the gas or vapor. NFPA 70, the National Electrical Code, defines the gas groups. A general-purpose generator wheeled next to a tank battery is a mismatch between the equipment and the vapor it sits beside.
The documentation is part of the rule
Section 1910.307(b) requires that all areas designated as hazardous classified locations be properly documented, and that the documentation be available to those authorized to design, install, inspect, maintain, or operate electric equipment at the location. The classification drawing is not an engineering extra; it is the map that tells the electrician, the pumper, and the inspector which rules apply where. On a lease that has grown one skid at a time, the classification document is often the first thing missing and the first thing an inspector asks for.
Electrical safety shares the same discipline as lockout/tagout, hot work, and chemical hazard communication. If the classification drawings and equipment ratings live in the electrician's head, an operations audit will show which locations are documented and which are running on assumption.