Electrical equipment in hazardous areas
Sources of such hazards include gases, vapors, dust, fibers, and flyings, which are combustible or flammable.
Intrinsic safety designs equipment to operate using minimal energy, insufficient to cause ignition.
As world trade becomes more globalized, international standards are slowly converging, so that a wider range of acceptable techniques can be approved by national regulatory agencies.
Classification of locations, testing and listing of equipment, and inspection of installation, is typically overseen by governmental bodies.
In the US, the independent National Fire Protection Association (NFPA) publishes several relevant standards, and they are often adopted by government agencies.
Canada has a similar system with CSA Group standard C22.1, the Canadian Electrical Code, which defines area classification and installation principles.
Equipment is placed into protection level categories according to manufacture method and suitability for different situations.
Coal mines, grain mills, elevators, and similar facilities likewise present the risk of a clouds of dust.
In other cases, the atmosphere is normally non-hazardous, but a dangerous concentration can be reasonably foreseen—such as operator error or equipment failure.
Typical gas hazards are from hydrocarbon compounds, but hydrogen and ammonia are also common industrial gases that are flammable.
This has been replaced by a European numerical system, as set out in directive 1999/92/EU implemented in the UK as the Dangerous Substances and Explosives Atmospheres Regulations 2002.
Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight.
Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas.
Conversely if the same material is heated and handled above its flash-point, the area must be classified for proper electrical system design, as it will then form an ignitable mixture.
If equipment is marked, for example, Ex e II T4 then it is suitable for all subgroups IIA, IIB and IIC A list must be drawn up of every explosive material that is on the refinery or chemical complex and included in the site plan of the classified areas.
In the US, the National Electrical Manufacturers Association (NEMA) defines standards for enclosure types for a variety of applications.
Some manufacturers claim "suitability" or "built-to" hazardous areas in their technical literature, but in effect lack the testing agency's certification and thus unacceptable for the AHJ (Authority Having Jurisdiction) to permit operation of the electrical installation/system.
Some equipment in Division 2 areas do not require a specific label, such as standard 3 phase induction motors that do not contain normally arcing components.
This was accompanied by electrically initiated explosions of flammable gas such as fire damp (methane) and suspended coal dust.
The inductance of the signal bell coils, combined with breaking of contacts by exposed metal surfaces, resulted in sparks, causing an explosion.
