What do the new zone classification system markings mean, and how do they apply to specific equipment applications?
Art.505 of the 1996 NEC introduces North American electrical personnel to the concept of "Zones" in classifying hazardous (classified) locations. The International Electrotechnical Commission (IEC) developed the Zone approach, which has been heretofore typically associated with European electrical installations.
In the NEC, hazardous (classified) locations are broken down first by Class, with Class I for gases and vapors in areas where they are or may be present in sufficient quantity to form ignitable or explosive mixtures with air. Class II locations include combustible dusts, and Class III locations cover fibers or flyings, such as at some textile mills, which could be present in a suspension in air so as to be readily ignitable.
The Zone classification system presently covered in the NEC is used only for Class I locations, and so this article will not cover Class II and Class III locations. Under the new system, locations are divided into zones based on the likelihood of an explosive gas/vapor mixture being present:
* Class I, Zone 0. This is an area in which an explosive atmosphere is always present or could be for long periods of time. Examples include the space above the fluid level of a vented tank holding combustible liquid and a similar area in a wet-pit. This zone, plus Zone 1 below, comprise the areas that under the traditional system are classified as Division 1. In effect, the zone system splits the traditional Division 1 into two zones. This allows equipment to be designed for the lesser hazards associated with Zone 1 instead of having to always be designed for Zone 0, the worst case. If possible, electrical equipment should not be installed in a Zone 0 area, and the NEC prohibits all power wiring in these areas.
* Class I, Zone 1. This is an area in which an explosive mixture with air is likely to occur periodically under normal operating conditions, but not for long periods of time. This zone also includes areas in which ignitable concentrations may exist frequently due to repair or maintenance, or leakage. In addition, this zone includes areas where a hazardous mixture could result from an equipment breakdown or faulty operation that would simultaneously cause an electrical failure of a type that could cause ignition. Finally, this zone generally exists adjacent to Zone 0 locations unless special safeguards are in place.
* Class I, Zone 2. This is an area in which an explosive or ignitable mixture with air is possible due to the use of hazardous materials, but which would be unlikely to occur because they are normally contained in closed containers or systems and would only be released because of a rupture or other accidental breakdown. This zone also includes areas under positive pressure that excludes hazardous agents but that would become hazardous if the ventilation failed and areas adjacent to Zone I locations unless special safeguards are in place. In sum, these areas are similar to Class I Division 2 areas, but with additional general language that allows this classification where hazardous conditions are unlikely. If they do occur, the exposure will be only for a short period.
Under the IEC classification system, equipment is rated based on the protective features it uses. In the U.S., the basic ratings will be for location (i.e., Class I, Zone 1, etc.), but you should be aware of the protective categories that will be used, particularly since some of those categories may look to be similar to existing NEC terminology and yet carry listings for zone locations that don't correlate with current NEC allowances. The equipment categories are:
* Flameproof. This is essentially the same as explosionproof. The parts of the electrical device that could ignite an explosive atmosphere are contained within an enclosing structure, typically with a flame path that won't allow ignition of the surrounding atmosphere. The enclosure can withstand the pressure created within it by exploding the worst-case explosive mixture of the appropriate gas group, and then allow for the release of hot gases slowly enough to cool them so ignition can't occur. IEC Symbol Exd.
* Increased safety. Here, constraints are put on insulation requirements and clearances and creepage distances between conducting parts. Therefore, the possibility of a potentially incendive fault occurring is reduced to a level that is thought to be acceptable. This is only applicable to apparatus that does not arc or spark or generate temperatures high enough to cause ignition in normal operation, including fluorescent light fixtures. IEC Symbol Exe.
Note that some categories may be combined. For example, there are switch contact blocks constructed with flame paths suitable for flameproof applications but installed in increased safety enclosures. The resulting mark, Exde, reflects IEC rules that require arcing components of increased safety apparatus themselves to be flameproof.
* Encapsulation. This method involves a gastight seal around the arcing contacts by embedding in epoxy or similar mastic, or using methods similar to hermetic sealing by fusion as recognized traditionally and now covered in Sec. 500-2(a)(8). This isn't suitable for Zone 0 locations and therefore is excluded from Div. 1 locations under the traditional methods, since some Div. 1 locations are similar to Zone 0 areas. Under the new system, however, this method can be allowed in both Zone 1 and Zone 2 environments. IEC Symbol Exm.
* Pressurized enclosure. The penetration of an explosive atmosphere into an enclosure is prevented by the constant presence of a safe gas inside the enclosure at a pressure that is greater than the surrounding atmosphere. This equipment can be used with the same applications as flame-proof enclosures. In addition, this protective technique can be used for very large equipment or even entire rooms. The technique is similar to the purged and pressurized systems recognized in Sec. 500-2 (a)(3) for conventional work. IEC Symbol Exp.
* Intrinsic safety. Intrinsically safe apparatus is apparatus in which all the circuits are intrinsically safe. A circuit is intrinsically safe if under test conditions - which incorporate normal operation and certain fault conditions - no sparks or thermal effects are produced that will ignite the specified atmosphere. Applications include some very low-energy measurement and control equipment, such as thermocouples.
There are two levels of intrinsic safety under IEC procedures: "ia," which requires two separate failures in order to allow a future hazard to develop; and "ib," which requires one failure before a future hazard could develop. Under the zone system, Type "ia" protection is required in Zone 0 locations and Type "ib" is allowable in Zone 1 locations. This will be a new distinction for our market. Class I Div. 1 locations under the traditional method include both Zone 0 and Zone 1 areas, and therefore traditional equipment classified as intrinsically safe always meets the "ia" protective level. IEC Symbol Exi with suffix "a" or "b."
* Oil immersed. Electrical equipment of this nature has all arcing parts submerged in oil to a sufficient depth that will prevent ignition of an explosive mixture that may be present above the surface of the oil. In addition, all live parts on which arcs do not occur in normal service are either immersed in oil or protected by some other recognized method. Under traditional classifications, this method is reserved for Div. 2, but under the new rules, it can be used in either Zone 1 or 2. Again, the reason for the difference is the fact that Zone 1 is more limited in the probability of hazard than Div. 1. IEC Symbol Exo.
* Powder (sand) filled. This is available for sealed enclosures with a sand or powder filling that will immediately quench an arc. This concept was added to the 1996 NEC for Class I Div. 2 locations in Sec. 501-6(b)(3) for hermetically sealed fuses that are of the "nonindicating, silver-sand, current-limiting type." IEC Symbol Exq.
* Nonincendive. These are circuits and components with which an arc or thermal effect normally isn't capable of igniting specified atmospheres under intended operating conditions, or upon opening, shorting, or grounding of the field wiring. These circuits aren't as restrictive as intrinsically safe circuits, and traditionally have been restricted to Div. 2 areas.
You will now see them in Class I Zone 2 locations as well, for obvious reasons. The principle is that since the occurrence of an ignition-capable fault, although possible, is very improbable. Since the presence of ignitable quantities of gas or vapors in Zone 2 areas is very infrequent, the probability of both occurring simultaneously is infinitesimal. Note that the NEC also allows nonincendive wiring in Zone 0 locations, provided it is essentially wired like Class I Div. 1 with heavy wall conduit or Type MI cable.
In addition to the traditional uses of the term "nonincendive," which relate to the ignition capabilities of the electric circuit (in terms of total energy storage, etc.), this category may be used for other protective systems that work under a similar principle of nonsimultaneous failures. For example (admittedly oversimplified), certain gasketed enclosures around nonarcing equipment may be acceptable in Zone 2; this type of procedure is sometimes referred to as "restricted breathing" and is being used in some parts of Europe. IEC Symbol Exn with possible suffixes, e.g., ExnR for restricted breathing.
Hierarchies and substitutions
Equipment tested for more extreme conditions can always be used in less extreme conditions. For example, any equipment that is allowable in Zone 0 can be used in either Zone 1 or Zone 2, and Zone 1 equipment can be used in Zone 2 locations.
In addition, equipment that is suitable for Class I Div. 1 can be used in Class I, Zone 1 or Zone 2 locations, and equipment suitable for Class I Div. 2 can be used in Class I, Zone 2 areas. The equipment must be evaluated in terms of an equivalent gas grouping, and any temperature limitations must be observed. This may be a common application given the widespread availability of traditional equipment.
Always remember that the majority of area classifications made under the NEC are specific to a given hazardous agent. An area that is entirely safe for gasoline vapor may violently explode when hydrogen is there instead. Never forget to track not just the class and zone, but also the specific flammable agent you will be working with. This must be supervised by a Registered Professional Engineer with proven qualifications, in accordance with Sec. 500-3, second paragraph.