What are “Zones” and how are they related to risk?
All of the electrical equipment being situated in that area would be required to be of special construction to ensure that ignition of the hazardous gas could not take place. The cost of this special equipment is very high so the total cost of equipment in the installation could be prohibitive.
It is most likely that there would be many areas in the plant where gas would never appear. Economics would suggest that the plant should be divided into areas with degrees of probability of the explosive gas appearing, and then to select electrical equipment suitably built to those areas or degrees of hazard.
A type of risk assessment has to be carried out to assess the degree of risk, then actions are taken to minimise the risk of using electrical equipment in such areas.
The policy of grading areas is widely used and the particular areas are referred to as Zones (IEC and NEC 505) or Divisions (NEC 500).