The construction of electrical equipment specifically for use in areas where explosion risks might occur due to clouds of gas, vapour, mist or dust was a problem first addressed in the early part of the twentieth century and initially was aimed at the coal-mining industry. This initial addressing of the problem concentrated upon the problems associated with methane (firedamp) and the problems of coal dust. Although it was relatively rare for significant outbursts of firedamp to occur (i.e., those which would engulf the men and the equipment before the latter could be switched off) the difficulty of escape from a mine was significant and a relatively high level of protection was adopted. This level later became the norm for Zone 1 above ground. Therefore the oldest types of protection currently used for electrical equipment in surface industry are those appropriate to Zone 1. Surface industry did not, in the early days, consider the installation of electrical equipment in Zone 0, a situation contemporarily familiar to those dealing with dust hazards, and it was only with the rapid development of instrumentation that any such demand became significant. It must be remembered that Zone 0 generally occurs inside such things as process vessels and so, until the demand for automatic process monitoring increased with the development of sophisticated instruments at a much later date, there was no demand for protection suitable for Zone 0.
There are thus four basic approaches which can be adopted to prevent uncontrolled ignitions of gas, vapour or mist and air clouds.
1. The gas, vapour or mist/air cloud can be excluded by mechanical barriers or other equivalent means. (Ex-o, m and p)
2. The components of the electrical circuits can be prevented from sparking or becoming sufficiently hot to ignite the gas, vapour or mist/air cloud. (Ex-e)
3. The gas, vapour or mist/air cloud can be permitted to enter the equipment enclosure and ignited, but the ensuing propagation of the explosion outside the enclosure can be prevented by some form of quenching. (Ex-d and q)
4. The electrical components within the enclosure can be permitted to spark, but the energy fed to the enclosure will be limited to a level which is not capable of igniting the gas, vapour or mist/air cloud. (Ex-i)
5. Special protection (Ex-s)
The concepts are elaborated in detail
1. Exclusion of the explosive atmosphere (Ex-o, m and p)
Three methods of satisfying this requirement in the construction of explosion protected apparatus have been developed and are in general current use. These are immersion of the electrical circuits in oil, encapsulation of the electrical circuits, and pressurization of the enclosure containing the electrical circuits with non-flammable gas.
Oil immersion (symbol 'o’)
In this protection concept (type of protection) the electrical components are immersed in oil within their enclosure, thus preventing access of the explosive atmosphere. It is dealt with in detail in Chapter 9. There are limitations on the type of oil which may be used and requirements to ensure the security of the oil within the enclosure, so that the electrical circuits remain immersed. The technique is considered as suitable for Zone 2 only.
Encapsulation (symbol ‘m’)
The explosive atmosphere and electrical circuits are separated from one another, this time by encapsulating (potting) material. The types of material used are defined and the encapsulated block is arranged so that surfaces presented to the outside are hard enough to give protection. The technique is considered as suitable for both Zone 1 and 2.
Pressurization (symbol ‘p’)
In this technique the explosive atmosphere is kept out of the equipment enclosure by pressure of air or inert gas. Air is normally used as there are asphyxiation problems with inert gas. Because the protection is by a gas (air) it is almost always necessary to provide ancillary equipment to ensure that the air pressure is maintained in spite of small leakages, which are almost certain to occur. Because of this the technique almost always requires a pressure-control system, together with a purge system to ensure that on start-up any internal explosive atmosphere is removed, and this gives added complication which must be set against its added flexibility because of the ease of depressurizing for repair. The technique is considered as suitable for both Zone 1 and 2 or only Zone 2, depending upon the pressure-control systems used.
2. Prevention of Sparking (Ex-e)
There is only one formal protection concept which is suitable for both Zone 1 and 2 in this area.
Increased safety (symbol ‘e’)
The technique accepts that there is ignition-capable energy within the equipment but relies upon the quality of construction to ensure that the energy is not released in a way which could cause ignition. This is achieved by the quality of construction of the equipment. The technique is considered as suitable for both Zone 1 and 2.
3. Containment of Explosions (Ex-d and q)
There are two protection concepts which use this technique.
Flameproof equipment (symbol 'd’)
This is the oldest protection technique. The explosive atmosphere is permitted to enter the equipment enclosure and to be ignited by the components within. The equipment is, however, provided with a very special strong enclosure which will withstand the internal explosion without sustaining damage and, in addition, will prevent the flames associated with the internal explosion from exiting the enclosure in a way which would permit them to ignite any surrounding explosive atmosphere. The technique is considered as suitable for both Zone 1 and 2.
Powder filling (symbol ‘q’)
Here the equipment enclosure is filled with quartz, sand or some similar inert small-grained filling. The technique is an extension of an old technique involving the use of stone chambers to prevent the passage of flame up-ducting. These chambers served to quench the flame, or so it was thought. The filling does not prevent the explosive atmosphere from accessing the equipment and coming into the proximity of a source of ignition. The fine-grained filling, however, serves to quench any ignition, thus preventing an explosion. The technique is considered as suitable for Zone 2 only in the UK, but is used in more hazardous areas in other countries and is expected to be acceptable for use in Zone 1 in the UK in the not-too-distant future
4. Energy Limitation (Ex-i)
There is only one protection concept in this area but it is the protection concept offering the highest level of security.
Intrinsic safety (symbol ‘i’)
In this technique the electrical energy fed to the equipment is below that which will cause ignition if released in a spark or as a hot surface, and all energy storage in the equipment is closely controlled to ensure that stored energy is limited. As with pressurization, this equipment relies to a large extent upon other equipment, which feeds it with electricity and the technique is somewhat complex. It is also limited in its application to instrumentation because of the energy limitations imposed, but within this limitation it is very flexible. There are two grades of intrinsic safety, the higher grade ‘ia’ being suitable for Zone 0, Zone 1 and Zone 2, but the lower grade ’ib’ being limited to Zone 1 and Zone 2.
5. Special Protection (Ex-s)
Special protection is not a fixed type of protection, as in the above but is used to describe equipment which is suitable for use in explosive atmospheres on the basis that its individual type of protection is equivalent to one of the more classic types, although different. Its typical use is to permit advances in technology where such advances do not easily fit into standard types of protection. It has been typically used for encapsulated equipment ’m’ prior to the publication of the construction Standard for that type of protection, and for sintered flame arresters prior to those being included in type of protection ‘d‘ (flameproof enclosure). It has also been used where techniques contrary to existing standards such as bi-pin tubes in type ‘e‘ (increased safety) fluorescent fittings are used on the basis of equivalent safety. It can also be used where equipment which has two independent types of protection, each of which is complete in itself (e.g., an increased safety ’e’ terminal chamber in a flameproof enclosure), is used provided that the two protection concepts are each independent of the other. This latter approach must, however, be treated with caution. Type ‘sf equipment is usable as specified in each particular case, and is only usable in Zone 0 and 1 where this is specifically stated.