Electrical equipment in flammable atmospheres

The techniques to be adopted to prevent danger when using electrical equipment in the vicinity of potentially explosive or flammable atmospheres have changed over the years and present legal requirements are contained in regulation 6 of the EAW Regulations. 

The construction of electrical equipment to be used where a flammable or explosive atmosphere is likely to occur must be such as to prevent ignition of that atmosphere. The selection and installation of such equipment are detailed and specialised matters requiring expert knowledge. 

The relevant standards have been affected by the standard harmonisation process within the EU through the European Committee for Electrotechnical Standardisation (CENELEC) and through the International Electrotechnical Commission (lEC). Individual parts of BS EN 60079 deal with different aspects concerning electric apparatus for potentially explosive gas atmospheres, namely classification of hazardous areas, electrical installations in hazardous areas (other than mines) and inspection and maintenance of electrical installations in hazardous areas (other than mines). BS 50014 covers general requirements. 

The first step in the selection of appropriate equipment will be the classification of the hazardous area from the viewpoint of the likelihood of a flammable atmosphere being present. Other considerations will include temperature class or ignition temperature of the gas or vapour involved and external influences. A nurnber of different safeguards may be employed in the design and construction of the equipment to minimise the risk of ignition. Equipment is normally certified for use in a particular situation and marked accordingly. 

Currently certification in Great Britain is through the British Approval Service for Electrical Equipment in Flammable Atmospheres (BASEEFA). 

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UK BASEEFA British Approvals Service for Electrical Equipment In Flammable Atmospheres... 

ASME ASSE ASTM AWWA BACT BASEEFA Engineers American Society of Mechanical Engineers American Society of Safety Engineers American Society for Testing and Materials American Water Works Association Best Available Control Technology British Approval Science for Electrical Equipment in Flammable Atmospheres... 

British Approvals Service for Electrical Equipment in Flammable Atmospheres (BASEEEA) and Physikalische-Technische Bimdesanstalt (PTB), Germany, which, along with other European approvals bodies, provides certification to the European Committee for Electrotechnical Standardization (CENELEC) standards... 

Using explosion-proof electrical equipment in flammable atmospheres... 

Much emphasis is placed on ensuring that electrical equipment in flammable atmospheres is so constructed that it cannot act as a source of ignition, and... 

Historically, there has been no duty on users of electrical equipment in flammable atmospheres to select equipment that has been approved or certified for use in such environments. The duty on employers, under the Health and Safety at Work Act, section 2, and the Electricity at Work Regulations, Regulation 6, has been to select equipment that is safe, so far as is reasonably practicable. However, guidance published by the HSE and others is that the best way of ensuring compliance is to use equipment that has been approved or certified against published standards by an appropriate test body. Similarly, suppliers of such equipment have been advised to ensure that their products are tested by an independent third party against the relevant standards before placing them on the market. 

Some of these documents have become obsolete or little used, e.g. BS229, BS1259, Electrical Installations in Flammable Atmospheres (from ICI) whilst others have been up-dated several times. New standards have also been introduced. A similar situation exists with the international standards pertaining to the selection of equipment for hazardous areas, many more references could be quoted. 

The standards for electrical equipment for use in flammable atmospheres are affected by the harmonisation processes of the EEC, CENELEC (European Committee for Electrotechnical Standardisation) and the lEC (International Electrotechnical Committee). 

Electrical equipment for use in flammable atmospheres should be marked both with a group number, which relates to the relative ease with which flammable gases may be ignited by electrical sparking from the equipment, and by a temperature classification which relates to the maximum surface temperature of the equipment. 

The use of electrical equipment in potentially flammable atmospheres should be avoided as far as possible. However, there will be many cases where electrical equipment must be used and, in these cases, the standards for the construction of the equipment should comply with the Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations (known as ATEX) and details on the classification or zoning of areas are published by the British Standards Institution and the Health and Safety Executive. 

Before electrical equipment is installed in any location where flammable dusts vapours or gases may be present, the area must be zoned in accordance with the Dangerous Substances and Explosive Atmospheres Regulations and records of the zoned areas must be marked on building drawings and revised when any zoned area is changed. The installation and maintenance of electrical equipment in potentially flammable atmospheres is a specialized task. It must only be undertaken by electricians or instrument mechanics who have an understanding of the techniques involved. 

Where plugs and sockets are used for portable tools, sufficient sockets must be provided for all the equipment and adaptors should not be used. Many accidents are caused by faulty flexible cables, extension leads, plugs and sockets, particularly when these items become damp or worn. Accidents often occur when contact is made with some part of the tool which has become live (probably at mains voltage), while the user is standing on, or in contact with, an earthed conducting surface. If the electrical supply is at more than 50 volts ac, then the electric shock that a person may receive from such defective equipment is potentially lethal. In adverse environmental conditions, such as humid or damp atmospheres, even lower voltages can be dangerous. Portable electrical equipment should not be used in flammable atmospheres if it can be avoided and it must also comply with any standard relevant to the particular environment. Air operated equipment should also be used as an alternative whenever it is practical. 

The main purpose is to allow equipment to be made safe under particular circumstances, such as maintenance, when unsafe conditions occur, or when adverse conditions such as electrical equipment in a flammable atmosphere or wet conditions occur. 

Intrinsic Safety. Static electrical concepts such as minimum ignition energy do not directly apply when assessing the safety of electrical circuits such as radios, flashlights and instmmentation. Intrinsically safe electrical equipment is usually available which has been subjected to fault analysis and testing. The equipment must be certified for the flammable atmosphere in which it will be used (NFPA 497). Refer to texts on Intrinsic Safety such as [63]. 

While some video display screens such as liquid crystal, gas plasma or vacuum fluorescent displays do not present the same charged screen hazards as CRTs, this does not imply that they are safe for use in hazardous locations. This requires special design and certification for use with a given flammable atmosphere. Non-certified equipment used in locations classified as hazardous under Article 500 of NFPA 70 National Electrical Code require a purged or pressurized enclosure to control ignition hazards as described in NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment. The screen in this case is located behind a window in the enclosure. 

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