Understanding ATEX equipment protection concepts

Control panels built for use in hazardous areas have to be designed to meet the specific risks of the installation site. Panel builders need both a sound understanding of the safety principles and expertise based on broad experience. Stuart Harvey, managing director of Softstart UK, looks at the recognised concepts of explosion protection for electrical and non-electrical equipment intended for use in hazardous areas

The horror of industrial catastrophes means they tend to linger in the memory. Few will ever forget the Piper Alpha oil rig disaster in 1988, or the ICI Flixborough chemical plant catastrophe of 1974. These and other disasters took the lives of many people, and led to a groundswell of calls for greater understanding of the industrial hazards involving explosion risk, and formalisation of measures to prevent such accidents happening in the future.

The response was the drawing up and ratification first of the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) 2002, and then of the two ATEX directives; ATEX 99/92/EC (or ATEX 137, the workplace directive) and ATEX 94/9/EC (or ATEX 95, the equipment directive). ATEX 95, which we shall focus on here, applies to equipment and protective systems which may be used in potentially explosive atmospheres created by the presence of flammable gases, vapours, mists or dust. It also applies to any associated equipment having a protection function which may be located outside the hazardous area.

The directives recognise the danger of explosion where there is a possible ignition source (flame, spark, hot spot, etc) in combination with sufficient oxygen and fuel – whether that fuel is gas, a mist, vapour or dust. Such fuels include methane, hydrogen, coal dust, flour, cocoa powder, sugar dust and many others. That means that the directives cover a large range of equipment – almost anything in fact intended for use in offshore platforms, petrochemical plants, mines, flour mills, industrial woodworking factories and other areas where a potentially explosive atmosphere may be present.

The regulations apply to all equipment intended for use in explosive atmospheres, whether electrical or mechanical, including protective systems.

European classifications define different zones of risk; zone 0, zone 1 and zone 2 for gases, and zone 20, zone 21 and zone 22 for dust. Zone 0/20 is defined as an area in which an explosive mixture is continuously present or is present for long periods. Zone 1/21 is defined as an area in which an explosive mixture is likely to occur in normal operation. And zone 2/22 is defined as an area in which an explosive mixture is not likely to occur in normal operation and if it does occur it will exist only for a short time.

Gases are categorised into groups according to the required ignition energy. Group I covers mining only and relates specifically to methane. Group II covers all gases in surface industries, and is split into Group IIA (gases requiring higher energy input to ignite), Group IIB (gases requiring medium energy to ignite) and Group IIC (gases requiring low ignition energy).

Correspondingly, equipment and products that fall within the scope of the directive are divided into two groups: Group I comprises equipment intended for use in mines, above or below ground; and Group II comprises equipment intended for use in other hazardous environments. Group II equipment is further divided into Category 1, 2 or 3, corresponding with the zones of risk – ie, Category 1 equipment is intended for use in areas in which explosive atmospheres are present continuously, for long periods or frequently (zone 0/20), and so on.

Gases are further categorised into temperature classes (T-Class T1 to T6 – not to be confused with the temperature classes applied for equipment). A gas is allocated a temperature class to the temperature below its specific auto ignition temperature. Equipment, on the other hand, is given a temperature rating that relates to the maximum surface temperature of the surface that is likely to come into contact with the gas. Dusts are not grouped but given a specific temperature rating and, for practical application, can be treated in the same way as gas.

Equipment certified for use in explosive atmospheres is given Ex certification, but for the specifier of equipment intended for use in hazardous areas there is more to consider. In particular, the protection method needs to be considered, which is indicated by an extra letter, including:

Ex i – intrinsic safety
Ex d – flameproof
Ex e – increased safety
Ex n – non sparking
Ex p – pressurised
Ex m – encapsulation
Ex o – oil immersion
Ex q – powder fill

So what do these different protection method classifications actually mean?

It is important to note that these ratings mean slightly different things in relation to electrical equipment, non-electrical equipment and dust protection. There are sub-divisions, too, with intrinsic safety in electrical equipment for example being broken down into Ex ia (zones 0, 1 and 2), Ex ib (zones 1 and 2) and Ex ic (zone 2 only). So let’s consider some of the basic concepts of protection for different types of equipment.

Electrical equipment

The first concept of protection is to have no arcs, sparks or hot surfaces in normal operation, and this is covered under Ex e increased safety and to a lesser extent under Ex n non-sparking.

The next concept of protection is to contain the explosion and/or quench the flame. Here it is common to see explosion proof motors rated as Ex d or to see quartz/sand filled high power electronic equipment (solenoids, capacitors, etc) rated as Ex q. Then there is intrinsic safety certification, which limits the energy of sparks and surface temperatures.

The final approach with electrical equipment is to keep the flammable gas out. That might be through pressurisation of the equipment (Ex p) or encapsulation (Ex m – suitable for light current and instrumentation applications) or oil immersion (Ex 0 – suitable for high current switchgear and transformers).

Each method of protection and associated Ex rating define which zone or zones the equipment can be used in. Encapsulation and intrinsic safety Ex ia ratings are the only protection concepts suitable for equipment to be used in zone 0 hazardous areas.

The protection concepts for electrical equipment only consider use in areas where there may be explosive mists, vapours or gases. But what about electrical equipment such as motors or starters that might be used in areas where there is dust? Here we must consider the specific dust protection concepts.

Dust protection

The first concept of protection is to have a rugged tight enclosures that dust cannot enter, and this carries the certification Ex tD. For a motor, for example, this would mean that no potentially explosive dust can penetrate the motor (ie an IP rating of IP55 or IP65), that the maximum surface temperature outside the motor must not exceed the temperature class for which the motor is certified, and that no sparks may occur outside the motor enclosure.

Note that there is some electrical equipment – motors included – that cannot be used inside zone 20.

The next concept of protection is intrinsic safety (iaD for zones 20, 21 and 22 and ibD for zones 21 and 22) which is similar to tD but with some relaxations if the circuit inside is intrinsically safe. Further approaches include protection by pressurisation of the enclosure (pD) and protection by encapsulation of incendive parts (maD and mbD).

Again, the different concepts and methods each define which zones the equipment can typically be used in.

Non-electrical equipment

The various concepts of protection for non-electrical equipment define ratings for both gas hazard and dust hazard environments.

The first concept of protection for non-electrical equipment relies on tight seals, closely matched joints and touch enclosures to restrict the breathing of the enclosure. This flow restriction methodology (Ex fr) is fairly simple and is aimed at preventing the atmosphere inside the enclosure from becoming explosive. It is only suitable for use in zone 2/22, category 3 environments. It can also be covered by flameproof enclosures (Ex d) for equipment intended for use in zones 1, 2, 21 and 22. Here explosion containment will not allow the transmission of an internal explosion to the surrounding environment.

Where there is low potential energy and so no risk of arcs, sparks and hot surfaces in normal operation, non-electrical equipment is covered by the Ex g inherent safety rating. Ex c covers constructional safety, where ignition hazards are eliminated by good engineering methods, protecting against the possibility of ignition from hot surfaces, sparks and adiabatic compression. A further concept of protection is to fit products with equipment to detect malfunctions, and this is defined under Ex b control of ignition sources. Equipment complying with any of these ratings can be used in any zone or category.

The two final concepts of protection for non-electrical equipment are pressurisation (Ex p) and liquid immersion (Ex k). Pressurisation excludes hazardous atmospheres through the use of a positive pressure of inert gas, while immersion uses liquid to prevent contact with explosive atmospheres.

This article has given only a brief outline of the various ATEX classifications and ratings, and equipment specifiers should always seek out expert advice in assembling systems for use in hazardous environments. Softstart UK, for example, has significant expertise in the supply and assembly of control gear for use in hazardous environments.

Standard products include the HRVS-EX – the world’s first medium voltage soft starter certified for use with explosion-proof motors. In addition, Softstart UK can custom build complete panels, taking control components such as soft starters, thyristor power modules, fibre optic EPT, electronic control modules and user interfaces and building them onto a chassis which is then enclosed in an ExD housing.

These bespoke systems meet the requirements of challenging installation areas where, for example, space may be severely limited.

What we have presented here, however, is a basic overview of the ATEX directives which should at least equip designers and specifiers to look in the right direction and ask the right questions. Because nobody want to see another Piper Alpha.