The growth of complex industrial processes created a need for standards and certifications in potentially explosive environments. Different nations developed standards for equipment and installations in these environments and a patchwork of requirements evolved.

Today, the various classifications of hazardous locations continue to evolve toward harmonised standards. At present each geographic area of the world works under slightly different standards and classification guidelines. These areas can be loosely categorised as North America, Europe, Latin America, Asia-Pacific, Commonwealth of Independent States (CIS) and other international markets. Each has its particular certifying and product marking requirements and reciprocity agreements with the different agencies allowing local certifying bodies to test and/or certify to standards of another region.

What constitutes a hazardous location? An area can be classified as hazardous based on the following factors:

o The possible presence of an explosive atmosphere such as flammable gases.
o The likelihood that the explosive atmosphere is present when equipment is operating.
o The ignition-related properties of the explosive atmosphere that is present.
This approach is used by the United States (NEC), Canada (CEC), Europe (CENELEC) and the rest of the world (IEC).

Hazardous locations classification follows the same general structure for all the different classes of hazardous materials. The first level is the type of material: Class I (Gases, liquids, vapours), Class II (combustible dusts) or Class III (ignitable fibres and flyings). The next level is the division (or zone) depending on whether the explosive condition is very likely to occur (Division I) or not very likely to occur (Division II). Materials have different levels of ignitability so within each Class there are also Groups and Temperature Codes.

A variety of protection methods are available, but the most recognised methods and their designations are: flameproof (also called explosion proof) ‘d’, pressurisation ‘p’, powder filling ‘q’, oil-immersion ‘o’, increased safety ‘e’, intrinsic safety ‘i’,
non-incendive (or non-sparking) ‘n’, and encapsulation ‘m’. For optical encoders, the most common methods are explosion proof construction and intrinsic safety:
The equipment is contained in an enclosure that can withstand an internal explosion of the most volatile gas-to-air mixture that can penetrate into the interior of the enclosure. The enclosure must contain the explosion without damage and without allowing the flame to leave the enclosure through any joints or other openings.

This technique uses an apparatus limiting the maximum level of current and voltage (usually measured as energy in millijoules) that can be delivered into the hazardous location. This equipment ensures that even in a double fault condition, there will not be enough energy to ignite the gas or vapour in that area. Note that encoders that use this method must use energy limiting devices (commonly called Barriers) in their installation. An Intrinsically Safe encoder, installed without an Intrinsic Safety Barrier is not an Intrinsically Safe system.

When looking for equipment to operate in hazardous locations, ensure that it is of high quality, environmentally sealed and qualified to the appropriate industrial standards. Qualifying agencies include NEMA, UL, cUL, CSA, MSHA, CNELEC/ATEX
and CE.

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BEI Industrial Encoder is based in Goleta, CA, USA. www.beiied.com