Electrical Equipment in Flammable and Explosive Atmospheres

Over the years a considerable amount of knowledge has developed on the means by which the risks can be controlled, so much so that serious incidents arising from electrical ignition are now infrequent. The purpose of this chapter is to explain the principles of the main techniques. The first section covers flammable gas/air mixtures, and later the risks associated with explosions in dust-laden atmospheres are considered. The chapter does not cover the broader issues associated with controlling risk from flammable and explosive atmospheres, such as substituting for non-flammable materials where possible and containing the flammable materials to avoid the formation of an explosive atmosphere the intention is to explain what needs to be done to control the risk of ignition from electrical equipment. 

A flammable liquid is defined, in the Highly Flammable Liquids and Liquefied Petroleum Gases Regulations, as one that gives off sufficient 

Some liquids which do not emit sufficient vapour at normal ambient temperatures to be classed as flammable are ignitable when sprayed in a fine mist Avtur, the paraffin used in jet engines, is an example. 

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The safe use of electricity can pose particular problems in adverse or hazardous work environments. Such environments may degrade equipment to the extent that it becomes unsafe or even develops a fault thus increasing the electric shock risk. Flammable and explosive atmospheres present special risks. 

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. 

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. 

Electrical equipment installed in flammable or dust-laden explosible atmospheres needs to be selected so that it cannot act as an ignition source. This requires hazardous areas to be classified according to the likelihood of a flammable or explosible atmosphere being present, and then suitable equipment being selected, installed and maintained by competent persons. Detailed information on the requirements is contained in Chapter 15. 

Establishing a procurement policy, which includes a pre-procurement risk assessment, will ensure that correct selection of plant and equipment for use in hazardous areas (where a flammable or explosive atmosphere has the potential to cause a fire or explosion) will minimise the risk relating to such equipment. BS EN 50020 2002, Electrical apparatus for potentially explosive atmospheres - Intrinsic safety i , specifies the construction and testing of intrinsically safe apparatus, intended for use in potentially explosive atmospheres and for associated apparatus, which is intended for connection to intrinsically safe circuits which enter such atmospheres. This document may well form the basis from which an organisation establishes its procurement policy for such equipment. 

Work in damp or wet conditions, including sewers, tunnels and quarries Where there is an increase to the designed/planned load Work in flammable and potentially explosive atmospheres Following design changes to electrical installation and equipment... 

Generally, it is more economical to prevent explosive atmospheres in rooms than to try to provide explosion-proof electrical equipment. Personnel should never be allowed to work in a hazardous atmosphere. Where such an atmosphere cannot be avoided through control of flammable liquids, gases, and dusts, access to the area involved should be limited and the area segregated by hoods or special ventilation. Electrical equipment on open, outdoor structures more than 8 m above-ground usually is considered free from exposure to more than temporary, local explosive mixtures near leaks (86). 

Methane is highly flammable and is therefore an explosion and fire hazard the lower explosive limit is 5-15% by volume. Extreme care must be taken to keep areas of high concentration free from ignition sources, such as sparks from static electricity. Explosion-proof equipment should be used in these areas. Many people believe that methane is an important greenhouse gas, and that the apparent threefold increase in atmospheric concentrations over the last 200 years affects the stratospheric ozone layer and the oxidizing capacity of the atmosphere. 

Section 5 - Locations with increased risk is concerned with potentially explosive atmospheres and the processing or storage of flammable materials. For the former, the precautions detailed in Chapter 15 of this book are advocated, and for the latter, separating the electrical equipment from the flammable materials as far as possible is advised and for the rest, in proximity to the material, taking special precautions to minimise the danger. Cables, with low emissions of smoke and corrosive gases, are recommended in places where people congregate or where there are escape problems. 

Any electrical system used as part of the ventilation or extraction systems must meet rigid electrical standards and as in the case of all such equipment be intrinsically safe. As discussed earlier, classifying hazardous or potentially explosive atmospheres is not only a legal requirement under DSEAR but has been widely used to determine the extent of hazardous zones created by flammable concentrations of vapours. 

These explosions in air are usually the result of the release of flammable gas and/or mists by leaks, rupture of equipment, or rupture of safety relierdng devices and release to the atmosphere, which become ignited by spark, static electricity, hot surfaces, and many other... 

HPM Storage Facilities. Explosion-proof electrical apparatus is required in rooms where a flammable atmosphere may occur. Electrical wiring and equipment located in HPM storage rooms shall be approved for Class I, Division 1 hazardous locations, and shall be in accordance with the Electrical Code. Exception When separate storage rooms are used, the storage rooms without flammable liquids or gases need not be approved for Class I, Division 1 hazardous locations (UFC 5108.2.8). 

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