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April 2004: Electrical Design in Hazardous Areas

If you store, process or generate combustible liquids or dusts, the fire and explosion potential is reason enough to be concerned about the electrical design for the areas involved. The extent of the concern, and what to do about it should be developed by a careful review of Article 500 of the National Electrical Code (NEC), which is published by the National Fire Protection Association as NFPA 70.

The first step is to understand the classification of the area(s) being considered. There are two classes for these types of materials - Class I for gases and vapors in ignitable concentrations and Class II for combustible dusts. There are additional classes in the National Electrical Code, but these two are the subjects dealt with here. Within each class there are two divisions. Division 1 usually contains an ignitable concentration of gas or frequently does. For dusts, there is a bit of a difference in that it also includes cases where dust could be the result of abnormal conditions that allow dust and simultaneously cause an ignition source through the failure of electrical equipment or other elements. Division 2 is a lesser hazard in that gases are normally confined, and released only due to ruptures, breakdowns, or abnormal equipment operation; or breakdown of ventilation that normally prevents ignitable concentrations from forming; or areas adjacent to Division 1 locations in most cases. Dust is similar but must also involve accumulation on or in electrical equipment sufficient to interfere with safe heat dissipation or cause malfunction of electrical equipment.

There is also a Zone classification system applicable to gases and vapors. The definitions are similar but slightly different than the Division definitions so that in effect Division 1 is replaced by Zone 0 and Zone 1. The benefit results in the potential ability to utilize Zone 1 equipment which can result in savings over the traditional Division 1 requirements. This system has not been widely adopted in the United States since its inclusion in the NEC in 1996 although it is popular in most other areas around the world. One reason for this is the resistance in the United States to use non-armored, flexible cable wiring instead of the traditional wiring in conduit. Should you choose to use this classification system, the same methods to reduce the extent of Division 1 or 2 areas apply equally as well to reducing the extent of a Zone.

The above is an attempt to simplify several rather detailed definitions which require careful reading to fully comprehend. For the actual wording, refer to Article 500-5 and 500-6 of the NEC.

The type of electrical equipment and installation wiring requirements are directly linked to the type of gas or dust under consideration. There are seven "Groups" of gas or dust, but there are also a number of special or non typical materials. The groupings are primarily a function of flashpoint, auto ignition temperature and explosion pressures. These determinations have already been made for numerous materials and are tabularized in NFPA 497M "Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Locations" as well as NFPA 325M "Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids". Should you be dealing with a material that is not listed, there are standard test protocols for identifying the parameters used to determine the "Group" classification.

Now that you have gotten this far in your considerations you are ready for equipment selection and wiring specifications for the classified areas. This can be fairly straight forward, especially where not much electrical equipment is involved. However, electrical equipment listed for hazardous areas is fairly expensive compared to ordinary electrical equipment in non hazardous areas. On occasion, the process equipment is in the hazardous area and may not be commercially available for use in hazardous locations. You can imagine the cost of having custom made electric machinery for a process area.

At this point some major cost/benefit studies may be needed. Since electrical design for hazardous areas can be expensive, you may want to limit the hazardous area or change the planned location of some equipment to avoid the need for equipment designated for hazardous area use. If it is within reason from a cost and operational point of view, relocate any equipment you can outside the hazardous area. For instance - a pump in a piping system can be located outside of a hazardous area room by routing the pipe out of the room and back in, with the pump being installed in the pipe outside the room. Where costs are not excessive, equipment designated "Intrinsically Safe" can be used in any Class/Division location and thus eliminated from further analysis.

Another method involves reviewing the area thoroughly to develop a three dimensional profile of specific spaces within an area or room that are actually the classified spaces. For instance, if you have a flammable liquid in a small open container, there is a space radiating outward for a defined distance that is Class I, Division 1 space. In general this distance is typically 5 ft. Beyond that boundary is a defined space that would be Class I, Division 2. This is typically 3 ft. vertically and 20 ft. horizontally, and extending down to the floor. Beyond that there might not be a hazardous area. If this situation were in a small room, the Division 2 space might be the entire room volume beyond the Division 1 space. In any event, there may be significant cost savings by undertaking this type of evaluation and positioning equipment to be in lesser or non hazardous areas of the room. This is easier said than done. The actual evaluation would have to consider what the defined hazardous spaces are and their extent. There is much useful information about the boundaries where flammable liquids are involved in NFPA 30 "Flammable and Combustible Liquids Code". There is no corresponding hazard specific standard for dust because the behavior of dust in an environment is treated differently. However, there are numerous occupancy specific codes, standards and practices published by the National Fire Protection Association that outline information on hazardous locations and the extent of hazardous areas for both liquid and dust handling occupancies - everything from dry cleaning plants and solvent extraction plants to milling of agricultural commodities to pulverized fuel systems. You have to do the research to find the standard or practice that matches your occupancy.

Another means of reducing the extent of areas classified as hazardous employs the use of environment modification - most often ventilation. For this approach, you need to become familiar with NFPA 496 "Purged and Pressurized Enclosures for Electrical Equipment". Purging can be used for a Class I location and pressurization can be used for a Class II location, depending on your circumstance. The object is to create an enclosure for the hazardous area and reduce the hazard classification within, thus reducing the sophistication of the electrical equipment within.

Purging uses a sufficient flow and pressure of clean air or an inert gas to reduce the concentration of ignitable gas. There are three types of purging: Type X reduces Division 1 to non hazardous, Type Y reduces Division 1 to Division 2, and Type Z reduces Division 2 to non hazardous. Pressurizing an enclosure is a means to keep dust from entering a confined space so equipment with does not have to be designated for Class II locations The requirements for this type of classification reduction can be quite sophisticated. Techniques are available for Class I spaces that contain or do not contain an internal source of flammable gas but are limited to spaces that do not contain an internal source of combustible dust. The designs can require fail safe features ranging from alarms to automatically shutting off equipment.

The cost of providing hazardous area electrical equipment can be considered inversely proportional to the cost involved in reducing the extent or amount of electrical equipment that is designated for hazardous area use. There is a point where the cost/benefit ratio is optimized without sacrificing efficient operation or production requirements. The techniques presented here provide several approaches that can be undertaken in this optimization process.

Risk Logic, Inc. can help in designing the layout for electric equipment in a hazardous area that will meet all applicable codes and insurance company requirements. Please contact our office and we can work with you or directly with your insurance carriers, contactors, or an in-house design department.