BECKHOFF CX50x0 - Manual (Page 15)

Basic principles of explosion protection
CX50x0 units in potentially explosive areas 11
4.4 Classification of electrical equipment into protection groups
The usability of electrical equipment in potentially explosive atmospheres is classified into three groups:
This subdivision is graded in accordance with the properties of the explosive atmosphere in which the
equipment is used and is based on the minimum ignition current ratio of representative gases.
The lowest classification is group IIA (typical gas: propane) with a high minimum ignition current ratio, while
elements of the group IIB possess a moderate minimum ignition current ratio. The highest category IIC covers
highly explosive atmospheres with a low minimum ignition current ratio and offers the maximum protection
against ignition.