Intrinsic Safety (IS) uses the concept of limiting the amount of energy at the Hazardous Area so that it is incapable of ignition.
Appropriately designed Intrinsically Safe devices depending on their certification can be used in all zones and are categorised as;
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Exia (Zones 0, 1 and 2)
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Exib (Zones 1 and 2)
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Exic (Zone 2)
ISA-RP12-6 defines intrinsically safe equipment as “equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration.”
Intrinsically Safe Instrumentation and the associated systems are usually allocated a Gas Group IIC classification, this means they are suitable for all Gas/Air mixtures.
Most IS Instruments achieve a T4 (135 deg C) which satisfies all industrial gases except Carbon Disulphide.
Because standard Instrument Cables can be used, no conduit and bulky enclosures are required the concept is the most cost effective.
One of the largest advantages of IS is that the systems can be worked on live without a hotwork permit, however test equipment must also be IS certified.
There are a proportion of engineers who select Exd (Flameproof) or Exn (Non Incendive) equipment because they see IS as being too hard to design, install and maintain, however this is a false premise as concept is the most effective method of ensuring electrical safety in the hazardous area.
Actually the correct installation and ongoing maintenance of the alternative concepts (Exd, Exn, Exe etc) require more care and maintenance over the life of a facility.
The Intrinsically Safe Equipment Certificate is a very important document in that must be utilised in all phases of the facility life, hence it should be kept in a readily available Ex Equipment Dossier.
It has details of design and installation requirements such as cable parameters and other specific provisions for the equipment. Each piece of IS equipment carries a certification label which designates the Ex category, Gas Group and Temperature Rating.
Exd , Exn or other certified equipment must not be used in a IS circuit! There are some devices that are designated “simple devices” for example a switch, these do not require certificates.
A very important issue in working with Ex is PERSONNEL COMPETENCY in the Design, Installation and Maintenance of Electrical Equipment in Hazardous Areas.
There are many competency based courses available around the world and it is ESSENTIAL that anyone involved in these areas are assessed and accredited to work on any Ex system. This competency should be reassessed every three years at a minimum.