1999, all references to the zone concept of

Photo 8. Spark Test Apparatus for Determination of MIC Ratio
UNDERSTANDING THE ZONE AREA CLASSIFICATION METHOD IN THE
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trical equipment to bedesigned, tested, andlisted based upon aspecific anticipated us-age. For example, elec-trical equipment thathas been tested as aGroup IIB would nor-mally be required for ethylene-based chemi-cal atmospheres butmay also have beentested for hydrogen-based chemicals. Thispermits the equipmentto be marked with a IIBmarking instead of hav-ing to be subjected tothe more rigid tests for acetylene, as would normally be required if a IIC equipment listing is used.
Listing, Marking, and Documentation
Listing of zone equipment is a requirement in 505.20.Equipment listed for use in a Zone 0 location can beused in a Zone 1 or Zone 2 location for the same gas or  vapor. Listed Zone 1 equipment can be used in a Zone2 location for the same gas or vapor. Electrical equip-ment may be marked with both Division 1 or Division2 marking and Zone 1 or Zone 2 for the same gas at-mosphere. Where marking division equipment for thezone designation, the marking should include the ClassI, Zone 1 or Zone 2 designation, the applicable gasclassification group, and the temperature class (T Code)of the equipment.Zone equipment must be marked to indicate theclass and the particular zone for which it has been listed.Zone equipment marked for use in the United States will have an “AEx” marking to indicate that the equip-ment has been listed in accordance with United Statesnationally recognized testing and listing standards.Many European and other countries permitmanufacturer’s self-certification of electrical equipmentfor suitability of the equipment for environmental con-ditions, wire bending spaces, electrical insulation, inother words, the ordinary electrical equipment require-ments. These countries do, however, require the equip-ment used in a zone location be third party certifiedfor the hazardous location into which the equipment will be installed. The “AEx” mark and the testing labmark should indicate to all persons involved in the in-stallation that the electrical equipment has been testedand listed by a third party certification organizationbased on U.S. standards for the particular protectiontechniques plus the aforementioned ordinary electri-cal equipment requirementsfor shock and fire hazards.
TemperatureClassification of ZoneEquipment
Electrical equipment used ina zone location must bemarked with the operatingtemperature of the equip-ment using a reference am-bient temperature of 40°C(104°F). This ambient tem-perature provides a startingpoint at which to test and asa reference to the operatingtemperature of the equip-ment. The temperature classmust be marked on the equipment in the form of a“T” code. “T1” is for equipment operating at equalto or less than 450°C; “T2,” for equal to or less than300°C; “T3,” for equal to or less than 200°C; “T4,”for equal to or less than 135°C; “T5,” for equal to or less than 100°C; and “T6,” for equal to or less than85°C. Electrical equipment designed for use in anambient temperature of between -20°C and +40°Cdo not require any additional temperature marking,other than the “T” code.Equipment designed be used in an ambient tem-perature below -20°C or above +40°C must have theambient temperature range marked on the equipment.This is necessary since the ambient temperature at which it has been designed may affect the operatingtemperature of the equipment. These extremely low or high ambient temperatures may also affect theamounts of gases or vapors present. Low temperaturesmay also produce higher explosion pressures in flame-proof or explosionproof equipment.Equipment suitable for use in an ambient tempera-ture exceeding 40°C (104°F) must be marked with boththe maximum ambient temperature in which it may beoperated and the operating range of the equipment atthat elevated temperature. Extremely high ambient tem-peratures may elevate the gas or vapor close to theautoignition temperature of the gas or vapor. It is criti-cal that the autoignition temperature of a gas or vapor is not exceeded by contact with extremely hot electri-cal equipment.Electrical equipment that is not of the heat produc-ing type or that operates at no greater than 100°C(212°F) is not required to have a marked temperaturerange or a marked operating temperature. Many haz-ardous chemicals have a much higher autoignition tem-perature than 100°C, but even if dealing with a chemi-