What are “Zones” and how are they related
If it is known that gases and vapours of a
hazardous nature could be present at various
locations in a particular plant it would be very
easy and convenient to say that the complete
area of the plant is a hazardous zone and
should be treated as such.
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).