1999, all references to the zone concept of

Photo 2. Intrinsic Safe Telephone
UNDERSTANDING THE ZONE AREA CLASSIFICATION METHOD IN THE
NEC 

www.iaei.org NOVEMBER/DECEMBER 2002
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Protection Techniques for ZoneLocations
Many of the protection techniques for zone locationsin the
 NEC 
 are modeled after the InternationalElectrotechnical Commission (IEC) standards and theInternational Standards Organization (ISO) standards.These international organizations have been primarily responsible for providing standards on electrical equip-ment for many countries.Underwriters Laboratories Inc. (UL) has further adapted these IEC documents with minor changes intoa multi-part American National Standards Institute(ANSI) document, ANSI/UL 2279-2001, the Standard for Electrical Equipment for Use in Class I, Zone 0, 1, and2 Hazardous (Classified) Locations. This UL standardis used for testing and listing of electrical equipmentfor use in zone classified areas and provides the manu-facturer and the user with a detailed description of theparticular protection techniques that can be used for these areas.Section 505.4(B), fine print notes provides standardreferences for important information involving hazard-ous locations. Each protection technique permitted ina zone installation is covered in 505.8 and in fine printnotes that immediately follow many of these protec-tion techniques.
1. Flameproof (Type “d”) Equipment
Flameproof (Type “d”) is a type of protection that issimilar to explosionproof equipment protection that hasbeen one of the most common protection techniquesused in the United States. In this protection technique,the equipment is in an enclosure that has been tested to withstand an internal explosion of a gas- or vapor-to-air mixture that can penetrate into the interior of the enclo-sure. The enclosure must contain the flame and the ex-plosion pressure without damaging the enclosure and without permitting the flame to leave the enclosurethrough any joints or other openings in the enclosureand without igniting the surrounding gases.
2. Purged and Pressurized (Type “p”) Equipment
Purged and pressurized (Type “p”) is a type of protec-tion that uses inert gas or instrument quality air as a pro-tective gas to purge the inside of the enclosure of any haz-ardous quantity of flammable gases or vapors. The enclo-sure is then kept pressurized at a pressure high enoughabove the outer atmosphere surrounding the enclosure toprevent the flammable gas or vapor from re-entering theenclosure. This guards against an ignitable concentrationof gas accumulating within the enclosure and then beingignited by an arcing or sparking part or hot electrical equip-ment. This protection technique also permits a general typeenclosure to be used rather than an explosionproof or flameproof enclosure, as would normally be the case.
3. Intrinsic Safety (Types “ia” or “ib”) Equipment
Intrinsic Safety (Types “ia” or “ib”) is a type of pro-tection technique that uses an apparatus that limits themaximum level of current and voltage measured asenergy (usually in millijoules) under normal or faultconditions that can be delivered into the hazardouslocation. The intrinsically safe associated apparatusproviding this energy limitation has a further identifi-cation marking on the equipment that indicates thetype of intrinsic safety. The marking will be “ia” or “ib”and should be located on the nameplate of the appara-tus. This equipment should provide a level of safety that, even in a double fault condition (“ia”) or a singlefault condition (“ib”), there will not be enough igni-tion energy to ignite the gas or vapor in that area.
4. Protection (Type “n”) Equipment
Protection (Type “n”) equipment is a protection tech-nique applied to electrical equipment which does nothave enough ignition energy to ignite the gas or vapor during normal operation and is not likely to have afault that could cause ignition. This type of protectionis similar to the nonincendive circuits and equipmentused in Class I, Division 2 locations. If there is a possi-bility of a fault occurring within the system, then someother method of protection should be chosen or extraprotection against faults should be incorporated.
5. Oil Immersion (Type “o”) Equipment
Oil immersion (Type “o”) equipment is a type of protection in which the electrical equipment is im-mersed in a protective fluid of nonconductive siliconeor mineral oil. The fluid level should be such that theelectrical parts and any arcing or sparking parts of theoil-encased equipment are immersed and the liquidalways covers the electrical parts. This ensures that thegas or vapor located above the arcing or sparking partsor the gas or vapor located exterior to the enclosure