How is Ex ma, Ex mb and Ex mc differs in the protection?

Ex m denotes encapsulation type protection for using components in hazardous area. Equipment with ma is used in Zone 0, mb in Zone 1 and mc in Zone 2. But what is the difference between these three?

EXm as you are aware refers to the protection concept of encapsulation, The suffixes a / b / c refer to EPL (Equipment Protection Level) which apply to Ex d Ex g and Ex m

a = “Very High” Protection - safe after two consecutive malfunctions b = “High” Protection - safe after one malfunction c = “Normal” Protection - safe under normal operating conditions

Note Level c is for surface industries only and only Ex ma and Ex mb are permitted in mines