This post is by Simon Barrowcliff, Safety Director, TRaC
Few people know that the UK pioneered ensuring the safety of equipment in potentially explosive atmospheres. In fact, much of the basis data for all Ex standards throughout the world was obtained by the Health & Safety Labs in Buxton in the late 1940s.
Hazardous atmospheres occur when there are flammable gases (including vapours and mists) or dusts present. The flammable material is the fuel that, when combined with oxygen from the atmosphere and an ignition source, can cause a dangerous explosion. This can happen in a wide variety of locations, such as oil refineries, chemical plants, saw mills, paint spray shops, mines and even custard power factories!
Due to the common heritage of the research at Buxton, standards around the world have many similarities. The UK’s ATEX certification and the international IECEx standards are very similar, with only minor points of difference.
Ex standards use a concept of zoning, where the atmosphere is classified based on the likelihood of an explosive atmosphere being present. ATEX and IECEx use 3 zones, with zones 0, 1 and 2 representing the presence of gas and zones 20, 21 and 22 covering dust. These zones are defined as:
• Zone 0/20: Flammable atmosphere is present continuously, or for a long period of time (typically >1000 hrs/yr)
• Zone 1/21: Flammable atmosphere is likely to be present during normal operation (10 to 1000 hrs/yr)
• Zone 2/22: Flammable atmosphere is not likely to occur during normal operation, and will exist for only a short time if it occurs (<10 hrs/yr)
The most commonly used standard in the US, however, the NEC article 500 system, only defines two zones. Even so, they map conveniently to the zones used in the ATEX standard, as the US Div 2 corresponds directly to Zone 2/22. Div 1 corresponds to Zone 1/21 and, as there is no direct equivalent for Zone 0,20 in the US standard, they are also included in Div 1.
It’s great to see that the UK’s lead in the important area of ensuring the safety of electrical equipment in potentially explosive atmospheres has not only put us at the forefront of preventing explosions, but has also resulted in similar standards across the globe. The level of consistency has made compliance easier for companies wishing to sell products world-wide, ensuring safe products are available in all countries.