Chapter 5 of the 2011 National Electrical Code (NEC), which focuses on special occupancies, is made up of 28 Articles, all of which focus on electrical environments that are considerably more demanding than ordinary residential and commercial spaces. Many of these areas are hazardous in varying degrees, requiring exceptional wiring methods and materials.
Take a flammable liquid or gas facility as an example. Volatile fuel may mix with air in a critical proportion so that an overheated surface or a small spark caused by an arcing electrical device may instantaneously trigger a massive fireball, resulting in great loss of life and/or property. Although many may think of feed grain and flour as innocuous substances, fine dust suspended in air in critical proportions may become explosive. Some operative parameters are ambient temperature, type of fuel, proportion to an oxidizer, and overall volume of the mix.
NEC Guidance
The Code and associated documents deal with these hazards in a highly competent manner. People enter these hazardous areas every day with much less risk than they encounter driving on a public road or engaging in other everyday activities. However, it is possible for electrical professionals to accomplish this task while minimizing the cost without cutting corners or compromising safety. The preferred strategy is to mitigate risk as opposed to reducing protection. Before examining the methods for doing so, however, let’s review some of the ways the Code delineates and organizes hazardous locations. The NEC recognizes three classes of hazardous areas:
- Class I locations are characterized by the presence of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors. If any of these are or may be present in the air (in quantities sufficient to produce explosive or ignitable mixtures), the location is considered Class I. As such, it requires specialized wiring techniques (see Art. 501) within its boundaries.
- Class II locations are characterized by the presence of combustible dust. These locations also require specialized wiring techniques (see Art. 502); however, they are generally less extreme than those needed in Class I locations. Nevertheless, huge explosions are possible due to combustible dust suspended in air in critical proportions; therefore, electricians must take great care when wiring these locations. Additional hazards accompany the presence of electrically conductive dusts, notably aluminum and magnesium. Zirconium and thorium dusts suspended in air are subject to spontaneous ignition. Conductive dusts may settle on circuit boards, bridging adjacent traces and resulting in short circuits. Another hazard associated with Group II locations occurs when dusts settle on motor housings and the like, resulting in heat buildup.
- Class III locations are characterized by the presence of easily ignitable fibers or flyings. These may be the same materials as found in Class II, but they're less finely divided. Although explosion is not a possibility, ignition may result in a rapidly spreading fire. The hazard is less than in Class II locations, but nonetheless is substantially greater than within unclassified locations. Specialized wiring techniques are also needed (see Art. 503) in these locations.
Each of the three classes contains two divisions, based on the immediacy of the hazard. Within Class I, Div. 1, for example, ignitable concentrations of flammable gases (or flammable/combustible liquid-produced vapors) can exist under normal operating conditions or because of repair/maintenance operations or leakage. Another possibility is breakdown or faulty operation of equipment or processes, resulting in the release of these substances into the air and allowing electrical equipment to become a source of ignition.
Within Class I, Div. 2 locations, the hazard, while still substantial, is somewhat less immediate. Volatile flammable gases or flammable/combustible liquid-produced vapors may be handled, produced, or used, but they will normally be confined within closed containers or closed systems. If they escape, it’s typically because of accidental rupture or breakdown of the containers or systems or abnormal operation of equipment. Division 2 conditions also exist when ignitable concentrations of flammable gases or flammable/combustible liquid-produced vapors are normally prevented by positive mechanical ventilation. These could become hazardous through failure or abnormal operation of the ventilating equipment.
Another possibility is that the area in question is adjacent to a Class I, Div. 1 location - and that ignitable concentrations of these substances above their flashpoints might occasionally be communicated, unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.
Class II and Class III locations may also be Div. 1 or Div. 2, depending upon the immediacy of the hazard. Typically, in Div. 2 areas, the substances are present in ignitable concentrations only under abnormal circumstances and presumably for much briefer periods of time.
The zone classification system is a viable alternative to conventional hazardous location delineation, and its use is appropriate under certain circumstances. Traditionally, division classifications were used in North America while zone classifications were used elsewhere. In recent years, the NEC added Art. 505, permitting optional use of Zones 0, 1, and 2 within Class I areas and Zones 20, 21, and 22 within Class II and III areas. Use of the zone method allows designers to specify offshore equipment. The resulting installation may or may not be less expensive. Decisions should be made on a case-by-case basis.
Material Groups
Article 500 also introduces the topic of material groups, stating explicitly that various air mixtures are to be grouped for testing, approval, and area classification. This means that equipment must be approved not only for the class of location, but also for the material involved.
For example, acetylene, which is widely used in chemical processes, is easily ignited and considered a Group A material. Hydrogen is an example of a Group B gas, ethylene is Group C, and propane (see Photo 1) is Group D - all of which fall within Class I. The 2011 NEC Handbook contains an extensive multi-page list of hazardous materials with group designation and other properties, as part of its commentary, but the requirements don’t end there.
Surface temperature
An additional condition is placed upon equipment contemplated for use within a hazardous area. The equipment temperature marking in Class I locations is not to exceed the ignition temperature of the specific gas or vapor to be encountered. In Class II locations, the temperature marking must not exceed the lower of either the ignition temperature of the dust encountered or 165°C (329°F). One of the problems with dust is that it may settle on potentially hot surfaces, such as motor housings. Repeated exposure to heat may cause partial carbonization, which lowers the ignition temperature.
To facilitate compliance with this Code mandate, Table 500.8(C) provides temperature class (T Code) categories for various maximum surface temperatures of heat-producing equipment, ranging from T1, with a maximum surface temperature of 450°C (842°F), to T6, with a maximum surface temperature of 85°C (185°F).
Protection Techniques
Whenever electrical equipment is installed within a hazardous area, one or more protection techniques must be employed to ensure that life and property are not endangered. The principal protection techniques include:
- Explosionproof equipment - permitted for equipment in Class I, Div. 1 or 2 locations.
- Dust ignitionproof equipment - permitted for equipment in Class II, Div. 1 or 2 locations.
- Dusttight equipment - permitted for equipment in Class II, Div. 2 or Class III, Div. 1 or 2 locations.
- Purged and pressurized equipment - permitted for equipment in any hazardous location for which it is identified.
- Intrinsic safety - permitted for equipment in any hazardous location.
- Nonincendive circuit, nonincendive equipment, nonincendive component, and hermetically sealed - permitted for equipment in Class I, Div. 2, Class II, Div. 2, or Class III, Div. 1 or 2 locations.
- Oil immersion - permitted for current-interrupting contacts in Class I, Div. 2 locations.
- Combustible gas detection system - permitted as a means of protection only in industrial establishments with restricted public access and where conditions of maintenance and supervision ensure that only qualified persons service the installation.
Any protection technique that is permitted for a Div. 1 location is also good for Div. 2 within the same class, because the hazards are the same - although not as immediate - as in a Div. 1 location. However, the same relation is not applicable among classes. A protection technique permitted for Class I, for example, is not necessarily relevant for Class II, and explosionproof enclosures permitted for Class I locations are not meaningful in Class II locations.
It is possible for a location to be simultaneously Class I and Class II or III, so care must be taken in complying with all requirements. Dual-rated equipment is useful, but it’s necessary to observe material and temperature parameters in all cases.
We have seen how hazardous areas are delineated and have listed applicable protection techniques. Additionally, many other Code mandates are in place. It is necessary to consider NEC requirements on a case-by-case basis to ensure that a safe environment is maintained. Particular attention must be focused on raceway sealing so that hazardous area gases or liquids do not infiltrate non-classified areas where protection techniques are not in place. Enhanced grounding and bonding techniques are essential as well. Redundant bonding, using grounding lugs designed for the purpose, ensures that there will be no interruption in the low-impedance ground path. This technique must be continued outside the hazardous location all the way back to the service, load center, or other grounding electrode conductor connection.
Obviously, materials and labor involved in any hazardous location installation will be much more costly than the same work within a non-hazardous industrial location. There are, however, ways of greatly reducing these costs without compromising safety.
Taking Action
The first and most effective strategy is to take a hard look at the overall installation to see if it is possible to relocate part or all of the electrical equipment and circuitry that powers it outside of the hazardous area - or perhaps moving it from a Div. 1 to a Div. 2 location. It may be possible to do pre-assembly outside a hazardous area so that less equipment is required within it.
A second hazard mitigation technique is ventilation, which may be used to good effect. In a commercial garage, for example, where floor areas include pits, below-grade work areas, or subfloor work areas in lubrication or service rooms, differing classification rules apply, depending on whether ventilation is provided. The entire floor area is unclassified where there is mechanical ventilation, providing a minimum of four air changes per hour. Ventilation must provide for air exchange across the entire floor area, and exhaust air is to be taken at a point within 12 inches of the floor.
In contrast, where ventilation is not provided, the floor area up to a level of 18 inches above any unventilated pit, below-grade work area, or subfloor work area, and extending a distance of 3 feet horizontally from its edge, is Class I, Div. 2.
Similarly, areas adjacent to classified areas within commercial garages in which flammable vapors aren’t likely to be released, such as stock rooms, switchboard rooms, and other similar locations, are unclassified where mechanically ventilated at a rate of four or more air changes per hour, designed with positive air pressure, or effectively cut off by walls or partitions.
A third hazard mitigation technique is to use intrinsically safe systems within a hazardous location wherever possible. To see how this works, let’s look at a more conventional but far more expensive protection technique.
Explosionproof enclosures, in conjunction with properly sealed rigid or intermediate metal raceway, are acceptable as a protection technique for use in Class I, Div. 1 locations (see Photo 2). These enclosures have wide bolted flanges, threaded covers, and entries that preclude the transmission of flame or heat. The idea is that, over a period of time, flammable gas or liquid-produced vapors will infiltrate the enclosure. An arcing switch or relay will cause ignition if the flammable material exists in critical mix with air. The explosionproof enclosure ensures the resulting explosion will be contained so that ignition will not occur outside the enclosure. A system of explosionproof enclosures is highly effective if installed and maintained correctly, with proper raceway sealing and protection from physical damage.
A much less expensive and every bit as effective protection technique is available for use in all classes and divisions. Rather than relying on the brute-force approach of the explosionproof enclosure, intrinsically safe systems depend upon an entirely different approach (click here to see Fig. 1).
For fire or explosion to occur, three elements must be present simultaneously. There must be a source of ignition (such as an electrical spark), a supply of fuel (such as gasoline), and an oxidizing agent (such as the oxygen) available in the earth’s atmosphere. Intrinsically safe systems protect against hazard by limiting the amount of power to a level where any spark or thermal effect is incapable of igniting the mixture of flammable or combustible material. Intrinsically safe apparatus is permitted to be installed in any hazardous location for which it is listed. General-purpose (non-explosionproof) enclosures are permitted.
Because of the low power levels necessary to ensure that ignition will not occur, it is not feasible to use intrinsically safe systems to power motors and other heavy equipment. Intrinsically safe systems are used for instrumentation, controls, and power-limited fire alarm systems. Because these items comprise a large proportion of the wiring within hazardous locations, significant cost savings are possible.
Power transfer is a function of the connected load and applied voltage. By choosing these values, it is possible for an intrinsically safe system to work as intended. However, the possibility remains that higher voltage and current levels would infiltrate the hazardous location if a power surge, misconnection, or arcing from adjacent wiring outside the hazardous location boundary should occur. One technique for preventing this eventuality is to observe required separations of intrinsically safe conductors, as outlined in Sec. 504.30.
These and other requirements are laid out within the control drawings supplied by the manufacturer, which documents, among other things, allowed interconnections between intrinsically safe apparatus and associated field wiring. These control drawings must be scrupulously followed.
An essential component is the Zener, or shunt diode barrier, which is installed on the nonhazardous location side adjacent to the boundary. This barrier provides secure isolation to ensure that unsafe current does not flow into the hazardous area, making possible ignition of the gas, vapor, dust, or other material within.
This barrier works by shunting excess voltage to ground. A Zener diode, with a specified reverse-bias breakdown voltage, is wired across the input terminals. Because this type of diode could become subject to an open-circuit fault, a second diode is provided for redundant protection. A fuse in series with one line is provided with a rating such that it will open if the power rating of the Zener diodes is exceeded. A resistor (plus a second one for redundancy) is provided so that in the event that the Zener diode enters the conduction mode, the source within the unclassified area will not be subject to a direct short circuit.
This type of barrier depends for its effectiveness upon a totally reliable ground connection. Two separate ground connections are desirable, each with a resistance of less than 1 ohm. Ground cables must be insulated and protected from physical damage. The ground electrode should be as near to the barrier as possible, preferably directly adjacent.
Because of the resistors, it is possible that the output of the barrier would not be sufficient for the application contemplated. The protection would be in place, but the barrier would not permit the system to function. Therefore, this aspect has to be considered.
Where it is not feasible to create a suitable ground, another type of barrier is available. This is the so-called “active barrier.” It does not work by diverting the fault current to ground, so the dedicated ground is not required, just (as always) the required equipment grounding means.
Now that we've discussed some of the NEC 2011 hazardous location requirements, it’s easy to see that there are ways to comply while reducing labor and materials costs without compromising safety. Careful planning, compliant design work, and flawless installation will ensure a safe electrical infrastructure for those whose lives depend upon it.
Herres is a licensed master electrician in Stewartstown, N.H. He can be reached at [email protected].
