For equipment which is to be used in areas where flammable atmospheres may occur, a number of standard methods of protection against ignition have been established. These have been adopted into construction standards, or codes, which allow manufacturers to make apparatus of a uniform type and have this equipment or apparatus tested & certified for compliance with the standards. The principle methods of protection are summarised as follows :-
Designed to prevent the flammable mixture reaching a means of ignition. Ex ‘N’ or Ex ‘nR’ (Restricted Breathing)
Ex ‘m’ Encapsulation
Ex ‘p’ Pressurisation
Ex ‘o’ Oil Immersion
Designed to prevent any ignition from arising Ex ‘e’ Increased Safety
Ex ‘N’ or Ex ‘nA’ Non Sparking
Ex ‘m’ Encapsulation
Designed to prevent any ignition from spreading Ex ‘d’ Flameproof Enclosure
Ex ‘q’ Powder or Sand Filling
Designed to limit the ignition energy of the circuit. Ex ‘i’ Intrinsic Safety

A comprehensive range of definitions of these forms of protection is available on the Hazardous Area Forms Of Protection page:

Flameproof Type ‘d’ Equipment which may include arcing and sparking (or incendive) devices and can have explosive mixtures present internally. The enclosure construction is designed to contain an internal explosion and prevent transmission of sufficient energy to cause an explosion external to the equipment. Any joints, covers or openings are constructed with flameproof paths which must be periodically inspected and continuously maintained in order to retain the integrity of the form of explosion protection.

Whilst explosive mixtures may enter the equipment, the enclosure is not designed to withstand an internal explosion. Instead the likelihood of a fault condition, which could result in ignition of explosive mixtures, is significantly reduced by the following measures. The (non-incendive) components used in the apparatus shall not produce arcs or sparks or dangerous temperatures in normal working conditions. The apparatus or equipment usually has a maximum voltage rating of 11kV. Electrical connections and insulation are selected for high reliability and effectiveness. The level of protection against the ingress of dirt and moisture ensures that the prospect of component contamination is substantially reduced. Two fundamental requirements of Ex ‘e’ protection are that the equipment shall be protected to IP54 minimum for Gas / Vapour (IP6X for Dust Hazards), and have an Impact Strength of 7Nm minimum. As this form of protection can be used in both Zone 1 and Zone 2 Areas, it is often preferred to Ex ‘d’ due to the need for reduced levels of maintenance and inspection. Another major consideration is that Ex ‘e’ apparatus is generally constructed from lighter weight materials, which often leads to lower cost.

Primary Flameproof Type ‘d’ Enclosure with Secondary Increased Safety Type ‘e’ Protection, allowing reduced frequency of periodic maintenance and inspection cycle because of the ‘e’ philosophy strategically employed. In the case of Flameproof Type ‘d’ Enclosures having an Increased Safety Type ‘e’ Terminal Chamber, ‘line barriers’ would normally be utilised between the Type ‘d’ and Type ‘e’ compartments creating an indirect cable entry interface. Consequently this would permit the use of Cable Glands with Increased Safety Type ‘e’ form of protection.

NOTE : Other forms of primary and secondary protection methods exist. This example has been included to demonstrate a commonly used application of the concept.

Intrinsically Safe Apparatus (Sub grouped into Ex ia and Ex ib) of these types incorporate circuits which due to their low spark energy potential are not capable of igniting an explosive mixture. Ex ib equipment is safe under one fault condition and can be used in Zone 1 Areas. Ex ia equipment is safe under two fault conditions and can be used in Zone 0 Areas. The Intrinsically Safe components or circuitry may be housed in an enclosure having another form of protection e.g. Ex e or Ex d, although the enclosure need not always be an approved item in its own right. In all cases it is important that the integrity of the housing or enclosure is maintained in accordance with the form of protection pertaining to it.

Encapsulation of arcing and sparking components or apparatus in a fashion which ensures that there is no exposure to explosive mixtures which may be present, and the surface temperature is controlled under normal and fault conditions, thus preventing ignition from occurring.

Permitted only in areas where the likelihood of a flammable atmosphere is remote (Zone 2),Non incendive Type ‘n’ Apparatus is divided into four sub-forms as follows :-

Non-Sparking – Ex nA – Components selected for non arcing or sparking. Enclosed Break – Ex nC – Incendive components, e.g. lampholders, enclosed and sealed to exclude gas or vapour from reaching ignition capable components. Energy Limitation – Ex nL – Low energy circuitry eliminates potential for ignition. Restricted Breathing – Ex nR – Relies upon the sealing and gasketing of the apparatus to ensure that the flammable mixture is kept away from any hot surfaces and ignition capable components so that it does not enter the enclosure in sufficient quantity to reach the specific L.E.L. documented. Apparatus with the type ‘n’ protection method is considered to be non incendive in that it does not usually generate arcs or sparks or dangerous temperatures in normal service. Internal component temperatures must be controlled and wiring connections and terminals selected with ‘Non-sparking’ in mind. The concept shares similarities with the Ex e philosophy but is suitable only for use in Zone 2 (or 22 where the apparatus may also be approved for dust hazards). This apparatus type is not designed to withstand explosion, and would usually employ a light weight enclosure of metallic or non metallic construction, but one that ensures a high Ingress Protection level is maintained.

In cases where the internal surface temperatures of components cannot be controlled in line with prescribed T ratings, the Restricted Breathing method (Ex nR) is adopted whereby joints and covers have tightly fitted seals and gaskets to provide the additional protection. This effectively excludes any external explosive mixtures which may be present from entering the enclosure and reaching the hot components. The ‘Restricted Breathing Enclosure’ technique is typically used in the design and construction of certain types of Lighting Equipment for Zone 2 Areas where the operating temperature of the lamp would otherwise be dangerously high. With this form of protection the user is responsible for ensuring that the cable entry device (Cable Gland) provides an adequate gas tight seal and maintains the integrity of the equipment protection code as detailed under the applicable standard to which it was tested and certified.

Permitted only in areas where the likelihood of a flammable atmosphere is remote (Zone 2),Type ‘o’ Apparatus where the sparking components are immersed in Oil and controlled venting is also a feature, for example in early forms of switch gear.

Pressurised or purged Apparatus Type ‘p’ rely on a combination of a positive static pressure applied inside the enclosure and a continuous flow of air or inert gas to expel any explosive mixture which may have entered. The system relies essentially on purging schedules and monitoring systems to ensure the reliability and effectiveness of the overall protection.

Powder or Sand filled Enclosures Type ‘q’ which house arcing and sparking (or incendive) devices and include a vent. Often used to contain the energy released from the failure of electrical or electronic components such as the breaking of a fuse. This form of protection is often associated with components inside Ex e apparatus for example lighting control gear.

Apparatus or components with Special Protection Type ‘s’ which have been assessed to comply with an applicable standard but which do not precisely meet all of the necessary criteria. Nevertheless the equipment or component will have been proven to be safe in normal use. Whilst demonstrating a required level of safety under test conditions, the component or equipment cannot be incorporated into any one of the normally recognised forms of protection.

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