When electrical and electronic equipment e.g. contactor, generator, switches, etc. is used in, around, or near an atmosphere that has flammable gases or vapors, flammable liquids, combustible dusts, ignitable fibers or flyings, there is always a possibility or risk that a fire or explosion might occur. To minimize the risk of fires or explosions that could result from this arcing, sparking and heat dissipation, it is critically important that electrical/ electronic equipment be designed, tested and labeled as being acceptable for use in the areas in which they are installed. This holds especially true for hazardous (classified) locations, i.e. locations where flammable, combustible or ignitable gases, vapors, liquids, dust, fibers or flyings may be present. There are 2 hazardous area classification systems that are used, depending on the country. In North America, the class/division system is used. In Europe and the rest of the world, they adapt the zone system.
Figure 1 – An example for Hazardous Area Classification for petrol station
2. Class/Division system
The Class/Division/Group system is based on Article 500 of the US National Electrical Code (NEC) where
- Classes – defines the general nature of the hazardous material in the surrounding atmosphere
- Divisions – defines the probability of hazardous material being present the surrounding atmosphere
- Groups – defines the type of the hazardous material in the surrounding atmosphere
MESG (Maximum Experimental Safe Gap) – The maximum clearance between two parallel metal surfaces that has been found under specified test conditions to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration.
MIC (Minimum Igniting Current) Ratio – The ratio of the minimum current required from an inductive spark discharge to ignite the most easily ignitable mixture of a gas or vapor, divided by the minimum current required from an inductive spark discharge to ignite methane under the same test conditions.
3. Zone system
Zone defines the general nature – if it is a gas or dust – and the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere. The Zone system has three levels of hazard for gas or dust where the Division system has two.
Gases, Vapors, Mists
Article 505 National Electrical Code (NEC)
Article 506 National Electrical Code (NEC)
Group define the type of hazardous material and (partly) the location of the surrounding atmosphere. Group is divided in three groups where Group I is reserved for mining locations. Group II is for explosive gases (Zone 0, 1 and 2) and Group III is for explosive dusts (Zone 20, 21 and 22).