Control panels built for use in hazardous areas have to be designed to meet the specific risks of the installation site. Stuart Harvey, managing director of Softstart UK, explains explosion protection for electrical and non-electrical equipment
ATEX 95 applies to equipment and protective systems used in or near potentially explosive atmospheres. The directive recognise and explosion danger where there is a possible ignition source (flame, spark, hot spot, etc) in combination with sufficient oxygen and fuel – gas, mist, vapour or dust, including methane, hydrogen, coal dust, sawdust, flour, cocoa powder, sugar dust and many others.
The regulations apply to all equipment intended for use in explosive atmospheres, whether electrical or mechanical, including protective systems.
They 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 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 is divided into two groups: Group I is for mines and quarries; Group II covers other hazardous environments and is further divided into Category 1, 2 or 3, corresponding with the zones of risk as above.
Gases are further categorised into temperature classes, T-Class T1 to T6 (not to be confused with the temperature classes applied to equipment). A gas is allocated a temperature class below its specific auto ignition temperature.
Equipment, on the other hand, is given a temperature rating that relates to the maximum surface temperature likely to contact gases.
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, as follows:
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
These classifications are in fact relatively easy to apply in any given situation. Now let’s consider the basic concepts of protection for different types of 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 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).
The first concept of dust protection is to have tight fitting enclosures, certified Ex tD.
The next concept 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 and by encapsulation.
The various concepts of protection for non-electrical equipment define ratings for both gas and dust.
The first concept relies on tight seals and tough enclosures and 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.
Where there is low 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).
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.
For more information, please visit www.softstartuk.com