Refineries and similar facilities often have electrical control equipment and instrumentation located in rooms that are within or near Class I hazardous locations. If you install a purging and pressurizing system in these rooms, the NEC allows you to install general-purpose equipment (intended for use in an unclassified location) in such locations.
FPN 1 to Sec. 500-4(d) in the NEC suggests you can reduce, limit, or even eliminate hazards by adequate positive-pressure ventilation from a source of clear air, coupled with effective safeguards against ventilation failure. FPN 2 to Sec. 500-4(d) in the NEC refers to NFPA 496-1998 (Purged and Pressurized Enclosures for Electrical Equipment) for requirements pertaining to the design of a purged and pressurized room.
Requirements included in Chapter 5, NFPA 496, provide guidelines for preventing the entry of flammable vapors or gases into the room housing electrical-related equipment.
Factors to consider (NFPA 496, Sec. 5-3)
As a designer or installer, you must consider many factors in the design and layout of the control room. First, the number of people in the room is important when calculating the volume of air required as well as the access requirements. An appendix in NFPA 496 states a control room located in a hazardous (classified) location should have as few doors as possible so you can maintain positive pressure within the room - while maintaining the need for egress of personnel per Appendix A-5-4.1.
A control room typically contains data processing, communications, HVAC, lighting, power, and electrical equipment, as well as process-control instruments and panels. It's the designer and installer's job to understand the varied needs for protection from flammable atmospheres. The volume of air introduced must satisfy the need for cooling the electrical equipment and preventing heating problems.
Location, location, location (NFPA 496, Secs. 5-3.1(c) and 5-3.2)
You must also consider the location of the control room in relation to the source of flammable gases or vapors. Pay particular attention to the direction of the prevailing wind. One side of the room may face a location generally free from trace amounts of flammable vapors or gases, or the height of the fan intake may be sufficient to provide a clean source of air. If you need ducting to reach an uncontaminated source, it must be noncombustible material, free of leaks, and protected against damage or corrosion.
Positive pressure air systems (NFPA 496, Sec. 5-4.1)
You must maintain positive pressure of at least 0.1 in. of water column (25 Pascals) in the control room with all openings closed. Sensitive pressure switches and other devices are available to measure these low values. The Code permits this minimum air pressure to drop to a lower level when doors and other apertures remain open - providing you maintain a minimum air velocity of 60 ft/min. (0.3 m/sec) through the openings.
Type X equipment (NFPA 496, Sec. 5-4.4)
If you locate a control room in a Class I, Division 1 (or Zone 1) location, containing equipment that can only function safely in an unclassified location, you must use a Type X purging system. Type X purging systems reduce the hazards from Division I (or Zone 1) to unclassified. You must cut power off immediately when the positive-pressure air system fails. You also must detect failure of the system at the discharge end of the fan.
The Code does not consider an electrical interlock that indicates when the pressurizing fan motor is running to be reliable for this purpose because of the possibility of a broken belt or other equipment failure. The sensing device must start an audible or visual alarm located in a constantly attended position. (See exception to Sec. 5-4.4 for a variance pertaining to this rule.)
You must take the electrical power circuit for the positive-pressure air system equipment off ahead of any service disconnects feeding the control room. The airflow-monitoring switch, electrical disconnect, and motor for the air system fan must be suitable for the area in question (as it would be classified if there was no positive ventilation system). This provision allows for the re-pressurization of the room after the air system fails.
One method to determine the degree of safety for such a situation involves the use of combustible gas detectors. You can use these detectors to be sure the atmosphere around the electrical equipment is nonflammable. As an alternative, you could also employ a purge timer to prevent reapplying power too soon after the pressurizing air system restarts. The time period should be sufficient to allow at least four air changes within the room.
Type Y and Z equipment (NFPA 496, Sec. 5-4.5)
Type Y purging systems reduce the classification within a room from Division 1 (or Zone 1) to Division 2 (or Zone 2). Type Z purging systems reduce the classification from Division 2 (Zone 2) to unclassified. If the control room location and/or equipment is suitable for these type of purges, then it's not necessary to de-energize the power supply circuit to the control room equipment immediately upon a positive pressure air system failure. However, for safety's sake, you should de-energize that equipment as soon as possible after you detect air failure, or that some means of monitoring the atmosphere within the room be started.
This article covers some of the general rules you must be familiar with when working in hazardous locations. For more detailed information, refer to specific requirements outlined in NFPA 496.