How to Play Zone Defense in Hazardous Locations
May 1, 1999 12:00 PM, By John Paschal, Jr., P.E.
What are the differences between the Zone and Division systems in hazardous (classified) locations? Here is what you need to know.
Zone? Division? What are the similarities and differences? It seems so confusing, but it doesn't have to be. A good way to understand the Zone system is to compare and contrast it with the familiar Division system rules and methodologies. This makes sense because they both have the same goal: To promote safety by defense against the ignition of flammable gases by electrical arcing or electrically heated surfaces. Also, both systems work with the same natural laws of physics and chemistry, such as gas ignition temperatures and combustible percentages of volatile gases.
Looking at the rewritten Art. 505 in the '99 National Electrical Code (NEC), we see it provides for the use of the Zone Classification system, detailing the different gas groupings, protection philosophies, and types of equipment from the Division system. If you don't understand the new terms and concepts, you'll end up applying the installation rules at the end of Art. 505 strictly by rote, and that doesn't work well in the long run.
The NEC Committee rewrote Art. 505 because the Division Classification system and IEC system are too different to merge. For example, list the IEC and NEC Group classifications for the different gases in ascending order of "More Easily Ignitable." Then, note that the group denoted with an "A" in the IEC area is the least ignitable, while the group denoted with an "A" in the NEC group is the most ignitable. Also, the IEC system uses three groups for the representative gases while the NEC system uses four. To further confuse the two systems, the IEC and NEC do not use the same terminology, and neither have the same number of categories to identify the likelihood of the hazard being present.
The NEC is an installation standard. With the exception of a few specific occupancies (e.g. Art. 511 for service stations, or Art. 515 for bulk storage facilities), the NEC does not make area classifications. Even those special articles generally consist of material extracted from other NFPA documents, where the heavy lifting gets done as to the boundaries of various hazardous (classified) areas. This is because the NEC is an installation document, not an occupancy document, and NFPA operating procedures clearly distinguish the functions of both.
For example, what would you use to decide area boundaries in a refinery? The American Petroleum Institute (API) RP 500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I Division I and Division 2, would be the best place to start. This widely used API document defines the extent of classified areas in refineries operating under the Division system.
Now suppose you're proposing to do the same thing, but under the Zone system. IEC 79 provides rules and standards about how to define and develop the extent of a Zone-classified area. Appendix C of IEC 79 contains examples demonstrating the results of the applications of the rules found in the body of the IEC 79 document.
IEC79 compared to API 500 area classification. Where API 500 classifications interface with IEC installations, there are obvious differences. For example, IEC Zone 0 and Zone 1 are included within Division 1 of the NEC. The IEC has three Zones, and the NEC has two Divisions. This causes some of the major differences between the two systems. Also, reclassification of Division locations into Zone locations brings out these differences. See Fig. 1 (page 58) and Fig. 2 (right) for an example of applying NEC 505-6(b-c) rules in delineating between Division and Zone boundaries.
Direct comparisons between NEC and IEC rules. Here's a quick comparison and contrast between the two systems.
Comparison of overall methodologies. Both Division and Zone classification systems begin with defining what the hazard is and the probability the hazard will be present.
Comparison of how the hazards are defined. The NEC Division system uses Classes and Groups to identify the hazard. The classes are used to represent gases, types of dusts, or fibers.
- Class I represents flammable gases,
- Class II represents explosive dusts, and
- Class III represents hazardous fibers.
Groups further define the hazard in Class I and II locations. For Class I, Groups A (acetylene and similar gases), B (hydrogen and similar gases), C (ethylene and similar gases), and D (propane and similar gases) represent flammable gases. Note: Hazardous Dusts and fibers are not treated in the Art. 505 Zone system.
The IEC Zone system identifies the hazard by two main Groups:
- Group I for mining (underground locations) and Group II for surface (not underground) industries. (Note: Sec. 90-2(b)(2) states the NEC doesn't cover underground mining installations.)
- Group II is divided into three subgroups arranged in order of hazard due to threat of ignition: A (the most difficult to ignite, such as propane); B (gases such as ethylene); and C (the easiest to ignite, such as acetylene/hydrogen).
Comparison of hazard probability. The NEC Division system categorizes locations as Division 1 or Division 2 based on the rules found in Sec. 500-5(a) or (b), respectively.
A Class I Division 1 location is where:
- Concentrations [refer to NFPA 497 for the percentage volumes of gases that will burn, since too little (too lean) or too much (too rich) gas won't burn] of flammable gases or vapors that would be ignitable under normal operating conditions,
- Hazardous gas mixture(s) may exist frequently because of repair or maintenance operations or leakage, or
- The breakdown of equipment simultaneously releases hazardous gas and causes failure of electrical equipment.
A Class I Division 2 location is where:
- Volatile flammable liquids are handled or stored, but they are normally confined within closed containers from which they can escape through container rupture, or
- Positive ventilation normally prevents ignitable concentrations of vapors (that is, the quantity of clean air continually brought into the atmosphere prevents the hazardous gas concentration from reaching its lower explosive limit percentage), or
- The area is adjacent to a Class I Division 1 location from which hazardous concentrations of gas may occasionally be transported.
The IEC/CENELEC approach uses three Zones instead of two Divisions. The Zones are based on how often the hazard is present instead of whether the hazard is present "normally." A Fine Print Note (FPN) in Sec. 505-4 of the '99 NEC directs the reader to IEC 79-0 for specific information about classifying areas as hazardous. This is also CENELEC EN 60079-10, Electrical Apparatus for Explosive Gas Atmospheres, Classification of Hazardous Areas. It provides specific definitions of zones, as follows:
Zone 0 location is "...an area in which an explosive gas atmosphere is present continuously or for long periods."
Zone 1 location is "...an area in which an explosive gas atmosphere is likely to occur in normal operation."
Zone 2 location is "...an area in which an explosive gas atmosphere is not likely to occur in normal operation, and if it does occur, is likely to do so only infrequently and will exist for a short period only."
The Zone system makes extensive use of the Lower Explosive Limit (LEL) concept. That is, if the amount of flammable gas in the atmosphere mixture is below the LEL for the gas, then the mixture is too lean and won't burn. A large part of the IEC 79 document is about methods that are intended to keep the atmospheric mixture below the LEL for the gas. However, the "...likely frequency of release, the release rate, the concentration, the velocity, the ventilation, and other factors all affect the Zone classification."
Zone 0: the biggest difference. The concept of Zone 0 is a big difference. No amount of ventilation can change a Zone 0 area to some lower classification, and the IEC general method of treating Zone 0 locations is to eliminate the hazard from within these locations by eliminating all electrical devices, cables, and loads from Zone 0 locations. In the final analysis of the Zone rules, the only type of electrical items you can install within Zone 0 locations is the intrinsically safe type. You'll find Zone 1 locations roughly equivalent to Division 1 locations, and Zone 2 locations roughly equivalent to Division 2 locations.
Acceptable wiring methods. After Sec. 505-10, subsequent sections of Art. 505 define the acceptable wiring methods for Zone 0, 1, and 2 locations, required markings for equipment to be placed in each Zone, and specifics about how to treat unique loads, such as "increased safety" motors in these locations.
You can find explanations about the '99 NEC requirements for Division and Zone locations with figures, photographs, and examples in EC&M's new book: Understanding NE Code Rules on Hazardous Locations Based on the 1999 NE Code. For ordering information, call (800) 543-7771.
Sidebar: How We Got from Division to Division and Zone
In 1947, the NEC first recognized different levels of risk exist in hazardous locations. Consequently, it established Division 1 and Division 2, to provide a means to treat the issue. This permitted installation methods to be specified, based upon what was considered to be an acceptable level of risk.
The IEC rules advanced the NEC logic to a new level. The IEC recognized the NEC divisions were based more on whether the hazard was present under either normal or abnormal conditions, instead of on the duration of the hazard. Therefore, the IEC established three divisions, or Zones, that are based on how often the hazard is present rather than upon normal versus abnormal conditions.
The three zones break the NEC's Division 1 into two distinct zones, one of which (Zone 0) is for those locations that are the most hazardous because they remain hazardous for a long time. Separating Zone 0 from the remainder of what is Division 1 in the NEC system permits a more refined treatment of the hazards of the two zones. It restricts methods of protection in Zone 0 while permitting more relaxed method of protection in Zone 1 locations.
The Division system in Sec. 501-5(a), Sec. 505-7 of the Code describes the grouping for the Zone system by gas, according to permissible gap range and minimum ignition currents. This section provides specific "go-by" information for each gas group. Similarly, Sec. 505-8 and its FPN point to NFPA-497-1997 for gas temperatures for use with either Division or Zone classifications. On the other hand, the Zone system uses some types of protection not recognized by the Division system.
Sidebar: Other Comparisons Between Division and Zone Classifications
To help you in your new IEC-related work (if you're well-grounded in the use of the Division system), here are valuable comparisons you should know:
- The Division system uses flame path cooling in Division 1 and Division 2 locations; but, the Zone system does not allow this "flameproof" methodology in Zone 0.
- For all practical purposes, the T-ratings for both systems are identical
- The Zone system allows intrinsic safety with two fault conditions applied (ia), as the Division system permits, and a lesser form of intrinsic safety that is only safe with one fault applied (ib).
- In the Division system, purging and pressurizing is permitted in all locations; but, in the Zone system, this is not permitted in Zone 0 locations.
- The use of "increased safety" protection isn't recognized by the Division system; but the Zone system permits its use in all but Zone 0 locations.
- Encapsulation isn't a recognized protection method in the Division system; but the Zone system uses it frequently.
- "Special Protection" is not recognized within the Division system; but, the Zone system allows a mechanism whereby apparatus and equipment can be certified to be safe and workable in a given location, though it does not comply with the other recognized forms of protection. However, Sec. 500-4(i) opens the door to other techniques.
- The Division system maintains a listing of types of enclosures, such as NEMA 4, or 7, 12, while the Zone system maintains the Ingress Protection (IP) system. The IP system designates, by means of a number, the degree of protection provided by an enclosure against solid objects. It also has a second number to designate the degree of protection provided by the enclosure against water ingress. For example, an IP number of 54 would eliminate the possible ingress of dust and protect against splashing water from any direction.
- Equipment listed for a Zone 0 can also be installed in Zone 1 or 2.
- Equipment must be marked with the Class, Zone, Group, and Temperature classification (or the symbol AEx), except that intrinsically safe equipment is only required to be marked with the symbol AEx, the protection technique, and with the gas group. The FPN in NEC Sec. 505-10(b) provides an example of the equipment marking system for Zone applications.