Intrinsically safe Foundation fieldbus H1 networks

Hazardous Area Classifications
A hazardous location is an area where an explosive atmosphere exists due to flammable gases, vapors, liquids, combustible dust or ignitable fibers or flyings. The area classification is determined in accordance with properties of the material creating the potential explosive atmosphere and the possibility that the materials may be present. The North American method of area classification is defined in the National Electrical Code (NEC) for the U.S. and the Canadian Electrical Code for Canada.  A typical area classification is Class 1, Division 1, Groups C/D T3C. But what does this classification mean?

The class term separates the fuels into families (Table 2). Class 1, for example, would signify an environment of flammable gases, vapor or liquids.

TABLE 2

The division term separates the hazardous area into two parts based on the probability that an explosive fuel and air mixture will be present (Table 3). A Division 1 classification signifies that ignitable concentrations of flammable gases, vapors or liquids can exist all of the time or some of the time under normal operating conditions.

TABLE 3

The group term separates the division into materials with similar explosive properties (Table 4). Group C/D signifies that the gases, vapors or liquids have similar explosive properties to ethylene and propane.

TABLE 4

Areas can also be classified based on maximum temperature (Table 5). If the maximum operating temperature of a device is greater than 85 ºC (T6), the device must be marked with the proper temperature category. A T3C-rated device has a surface temperature that does not exceed 160 ºC.

TABLE 5