Hazardous area classification

Hazardous area classification has three basic components:
Class (1,2) : Type of combustible material (Gas or Dust)
Div (I, II) : Probability of combustible material being present
Gas Group (A,B,C,D): most combustible to least combustible (amount of energy required to ignite the gas)

Hazardous Area Protection Techniques: There are many, but most commonly used for Instrumentation are listed below:
1) Instrinsic Safety : Limits the amount of energy going to the field instrument (by use of Instrinsic Safety Barrier in the safe area). Live maintenance is possible. Limited for low energy instruments.
2) Explosion proof: Special enclosure of field instrument that contains the explosion (if it occurs). Used for relatively high energy instruments; Instrument should be powered off before opening the enclosure.
3) Pressurized or Purged: Isolates the instrument from combustible gas by pressurizing the enclosure with an inert gas.

Then there are encapsulation, increased safety, oil immersion, sand filling etc.

Weather protection: Every field instrument needs protection from dust and water.
IP-xy as per IEC 60529, where
x- protection against solids
y- protection against liquids
Usually IP-65 protection is specified for field instruments i onshore applications (which is equivalent of NEMA 4X); IP-66 for offshore application and IP-67 for submersible service.