according to their maximum experi-
mental safe gaps and minimum igniting currents is
described in IEC 60079-12-1978, Classification of mixtures
of gases or vapours with air according to their maximum
experimental safe gaps and minimum igniting currents.
FPN No. 2: Verification of electrical equipment utilizing protection techniques "e," "m," "p," and "q," due to design technique, does not require tests involving MESG or MIC. Therefore, Group II is not required to be subdivided for these protection techniques.
FPN No. 3: It is necessary that the meanings of the differ- ent equipment markings and Group II classifications be carefully observed to avoid confusion with Class I, Divi- sions 1 and 2, Groups A, B, C, and D.
Class I, Zone 0, 1, and 2, groups shall be as follows:
(A) Group IIC. Atmospheres containing acetylene, hydrogen, or flammable gas, flammable liquid- produced vapor, or combustible liquid-produced vapor mixed with air that may burn or explode, having either a maximum experimental safe gap (MESG) value less than
or equal to 0.50 mm or minimum igniting current ratio
(MIC ratio) less than or equal to 0.45. [497:3.3.5.2.1]
FPN: Group IIC is equivalent to a combination of Class I, Group A, and Class I, Group B, as described in 500.6(A)(1) and (A)(2).
(B) Group IIB. Atmospheres containing acetaldehyde, ethylene, or flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor mixed with air that may burn or explode, having either maximum experimental safe gap (MESG) values greater than 0.50 mm and less than or equal to 0.90 mm or minimum igniting current ratio (MIC ratio) greater than
0.45 and less than or equal to 0.80. [497:3.3.5.2.2]
FPN: Group IIB is equivalent to Class I, Group C, as described in 500.6(A)(3).
(C) Group IIA. Atmospheres containing acetone, ammo- nia, ethyl alcohol, gasoline, methane, propane, or flam- mable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor mixed with air that may burn or explode, having either a maximum experimental safe gap (MESG) value greater than 0.90 mm or minimum igniting current ratio (MIC ratio) greater than 0.80. [497:3.3.5.2.3]
FPN: Group IIA is equivalent to Class I, Group D as described in 500.6(A)(4).
505.7 Special Precaution. Article 505 requires equipment construction and installation that ensures safe performance under conditions of proper use and maintenance.
FPN No. 1: It is important that inspection authorities and users exercise more than ordinary care with regard to the installation and maintenance of electrical equipment in hazardous (classified) locations.
FPN No. 2: Low ambient conditions require special consideration. Electrical equipment depending on the protection techniques described by 505.8(A) may not be suitable for use at temperatures lower than -20°C (-4°F) unless they are identified for use at lower temperatures. However, at low ambient temperatures, flammable concentrations of vapors may not exist in a location classified Class I, Zones 0, 1, or 2 at normal ambient temperature.
(A) Implementation of Zone Classification System. Classification of areas, engineering and design, selection of equipment and wiring methods, installation, and inspection shall be performed by qualified persons.
(B) Dual Classification. In instances of areas within the same facility classified