Almost every industrial setting poses some potential safety risk to personnel working in such environments whether it is something that can be seen such as rotating machinery or unseen, such as possible lethal concentrations of flammable/combustible gas.
Accordingly, there are several methods of personal protection employed specific to the threat posed. In the case of Gas Detection instrumentation employing electrical circuits/devices, identification of the working environment and the hazards faced in that environment is the first step in determining the method of protection required to ensure Safety of personnel.
There are two systems used to define a hazardous environment, Division Classification and Zone Classification.
Image credit: "Jet over Post Industrial Gas Works Park, walkways and handrails, Fremont / Wallingford, Seattle, Washington, USA," Wonderlane © 2008, used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by/2.0/