The early 1900’s saw the beginning of a long evolutionary process in the safety of workers in potentially explosive atmospheres following various fatal mining incidents. Nowadays there is a broader range of industries with potentially explosive atmospheres, including pharmaceuticals, automotive, aerospace, power, milling and oil and gas and many more.
Since 1st July 2003, it is a legal requirement in all European Union Member States for design, manufacture or sales of any equipment or protective system intended for use in a potentially explosive atmosphere to comply with the ATEX Directive. The new ATEX 2014/34/EU Directive sets a number of technical and quality objectives that must be complied with. Once these are met, a manufacturer is entitled to display the distinctive European Mark which is recognized throughout the European Union.
Presafe is an accredited laboratory according to the standard EN ISO/IEC 17025 and is a Notified Body according to the ATEX Directive with the authority to certify both electrical and non-electrical products for gas and dust environments.
|Category||Probability of an explosive atmosphere||Level of protection||Corresponding Zone|
|1||Continuously present or for long periods of time||Very high level of protection||Zone 0|
|2||Likely to occur||High level of protection||Zone 1|
|3||Not likely to occur, and only for short periods of time||Normal operation||Zone 2|
Manufacturers must also consider what the nature of the hazardous area is that they are dealing with, as there are two main gas groups I and II. Group II is further broken down into 3 sub-groups. Group IIC is intended for use in the most explosive atmosphere. Should a product be certified as group IIC, it may also be used in IIB and IIA. This does however not apply in the reverse order.
A mixture of air and flammable gases may ignite by coming in contact with a hot surface. An ignition depends on surface area, temperature and the concentration of the gas. Equipment is tested for maximum surface temperature and assigned a temperature class as follows:
Depending on the product a protection concept must be applied to demonstrate compliance with the standards, however, not all protection concepts are suitable for all zones.
Flameproof enclosures "Ex d”
Within an Ex d enclosure, arcs, sparks and hot temperatures may be permissible it is assumed that an internal explosion shall occur, the protection type prevents any explosion transmission happening from the inside of the enclosure to the outside. To do this the enclosure must be strong and robust to contain the explosion without deformation. It must function at a temperature below the ignition temperature of the surrounding atmosphere and it must provide a means for burning gases to escape. Looking at the flameproof enclosure diagram below (ref. image 1), there are burning gases inside which are expanding but can escape through the small gap between the enclosure lid and body, the path (the flame path) that the burning gas travels to escape is long enough to cool and quench the flame so that it will not ignite the surrounding atmosphere.
There are various types of flame paths: the standard specifies the maximum permissible gaps for flanges, spigots and other types of joints based on experimental testing.
Typical “Ex d” applications:
- Switchgear and relays in power electronics
- Incandescent and halogen lamps
- Analyzer heads, ovens of gas chromatograph
- Plug and socket connectors
Increased safety "Ex e"
It is assumed that the hazardous atmosphere penetrates to the enclosures interior and explosion is prevented by ensuring that either in normal operation or under fault conditions:
- There are no arcs or sparks
- No temperatures of components or parts exceeding the temperature limit of the corresponding temperature class
The minimum degree of ingress protection required is IP54 according to IEC 60529 unless the enclosure contains insulated conductive parts only; the degree of protection may be reduced to IP44.
Typical “Ex e” applications:
- Cable terminals, connection boxes, plugs and sockets and connectors
- Cage induction motors
- Transformers, ballasts, current and voltage transformers
- Heating elements for pipes, valves
- Light fittings with tubular fluorescent lamps
Intrinsic safety "Ex i"
The protection concept of intrinsic safety is based on limiting the energy in an electrical spark so that the combustible gas-air atmosphere present cannot be ignited. Covering intentionally generated sparks in switches and relays for instance or unintentional sparks generated by short circuits or wire breaks in electric circuits. If two countable faults are considered and the device is still intrinsically safe as per the standard the marking given is Ex ia and may be used in any zone including zone 0. Ex ib considers just one fault and is good for zones 1 and 2. Ex ic is a relatively new concept which considers normal operation only and may be utilized in zone 2 only. I.s. barriers are used to protect the field circuit from non i.s. circuits.
Encapsulation “Ex m”
The protection concept of encapsulation is a method of excluding electrical components which could become an ignition source either by arcing, sparking or by thermal effects from the hazardous atmosphere using a compound. A thermosetting, thermoplastic, epoxy resin or elastomeric material is poured into a mould or enclosure comprising the components. When the substance solidifies the components are enclosed within the compound thus preventing immediate contact with the surrounding atmosphere. This protection method also limits the surface area temperature of the equipment under normal operating conditions. The encapsulation of components can offer other benefits such as greater shock resistance, reduced environmental effects and better resistance to chemical attack. Considerations for an Ex m design:
- Resistors, capacitors, optoisolators, diodes etc. must be 2/3 rated, that is they must not operate at more than 2/3 of their rated voltage.
- The temperature increase of components and wiring must be limited.
- Voids and air gaps other than those for relays or other devices must be prevented.
- The effect of a component short-circuiting under fault conditions as with intrinsic safety, encapsulation may be considered for a one fault condition (marking will be Ex mb) or a two fault condition (marking will be Ex ma).
Typical applications are electronic circuit relays and timers. It is often used alongside other protection concepts in applications such as luminaires, products may utilize a combination of the increased safety and encapsulation protection methods, and should be marked with Ex mb.
Pressurized enclosures "Ex p"
The principle of the protection method Pressurization is to purge out any hazardous gas that may be present within an enclosure by passing large volumes of either air or an inert gas through the enclosure. When purging is complete an overpressure is maintained to prevent and hazardous gas from entering. This may be done in various ways:
Continuous flow of protective gas: generally air is supplied to the enclosure and exits via and outlet aperture Leakage compensation: An unspecified quantity of the protective gas is supplied to the enclosure to compensate for leakage via joints that are not gas tight such as windows and doors thus maintaining a specified pressure differential. This method is commonly employed to protect large motors.
Static pressurization: An enclosure considered gas tight is filled with protective gas and provides a safe operating time. The enclosure then must be re-filled for further use.
Powder filling "Ex q"
Like with Ex d, this method assumes that an ignition will take place. Potentially incendive components are placed within an enclosure filled with sand or a similar inert powder so that when an arc occurs, the arc will not be liable to ignite the external explosive atmosphere.
This protection method applies to electrical equipment, parts of electrical equipment and Ex components with:
- a rated supply current less than or equal to 16 A
- a rated supply voltage less than or equal to 1 000 V
- a rated power consumption less than or equal to 1 000 W
Oil immersion "Ex o"
A method of protection where again explosion is prevented by excluding the surrounding hazardous atmosphere, this time by immersing the ignition capable equipment in oil. The oil presents a barrier between the explosive atmosphere and the electrical apparatus.
Type of protection “n”
This type of protection is suitable for Zone 2 only. It does not consider any fault conditions, in other words it assumes only normal operation.