Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 69)

The extensive CENELEC standards for electrical equipment for hazardous areas apply in all
West European states and practically cover the same subjects as the IEC standards.
2.4.7.2 Classification of hazardous areas
When handling combustible or oxidizing substances that are present in fine dispersion as
gases, vapors, mists or dusts, risks of explosion can arise. An effective source of ignition must
be present to initiate an explosion. Sources of ignition can arise in electrical plants as electrical
sparks and arcs, mechanical sparks and hot surfaces.
Hazardous areas are zones, in which, due to the local and operational conditions, a potentially
explosive atmosphere of a dangerous quantity can occur. Hazardous areas are classified into
zones (IEC/EN 60079-10, EN 50281-3 and IEC/EN 61241-10) according to the nature of the
combustible substances (gases, vapors and dusts) and according to the frequency of occur-
rence and duration of the explosive gas atmosphere in the Ex-zone (permanent, occasional or
very seldom and short-time) (
Tab. 2.4-2 ).
In accordance with Appendix I of the Directive 94/9 EC distinction is made between 2 groups of
devices:
- “Equipment-group I” (Methane or combustible dusts) in the mining industry and
- “Equipment-group II” (gases or dust/air mixtures) for the remaining areas with explosive
atmospheres.
“Equipment-group II” is subdivided among explosion groups IIA (for example propane), IIB (for
example ethylene) and IIC (for example hydrogen). The hazard of the gases increases from
explosion group IIA to IIC. The requirements on the electrical equipment increase accordingly.
Combustible gases and vapors are classified into temperature classes according to their ignition
temperatures, the electrical equipment according to its surface temperature (T1 ... T6,
Tab. 2.4-4 ).
As explosion-protected electrical equipment does not always have to satisfy the highest
requirements, it is classified according to zone, temperature classes and the explosion group of
the combustible substances. This enables the adaptation of explosion-protected electrical
equipment for various ignition protection types (
Tab. 2.4-3 ).
LVSAM-WP001A-EN-P - April 2009 2-41