Electricity and potentially explosive atmospheres
Introduction
UK legislation has contained requirements for electrical installations in adverse environments in the Electricity Regulations of the Factories Act since 1908. Over the years these have been given more detail as legislation has been updated. Potentially explo- sive atmospheres, otherwise known as hazardous areas or flammable atmospheres, have been given particular emphasis because of the possible major consequences of inadvertent ignition. The disasters at Flixborough and Abbeystead are examples of the many incidents which have occurred, and continue to occur, worldwide. There have been significant changes to the UK legislation in recent years, largely as a result of EU directives dealing with the safety and health of workers. This chapter concentrates on the specific problem of potentially explosive atmospheres, the legal requirements and the practices for meeting them.
EU directives
The two EU directives dealing specifically with potentially explosive atmospheres are 1999/92/EC (reference 15A), which deals with the safety and health of workers exposed to potentially explosive atmospheres, and 94/9/EC (reference 15B), which covers the equipment for use in such atmospheres. The Chemical Agents 98/24/EC Directive (reference 15C) is also relevant as it contains requirements for flammable materials.
Directive 1999/92/EC covers the assessment of explosion risks, technical measures for explosion protection, organizational measures for explosion protection, coordination duties and explosion protection documentation.
Directive 94/9/EC covers the certification process for hazardous area equipment and the role of Notified Bodies. It contains Essential Health and Safety Requirements (EHSRs) for the equipment and is intended to allow equipment to cross EU boundaries without discrimination on the grounds of safety. It requires CE marking of equipment, unlike earlier directives for hazardous area equipment, which signifies that the equip- ment complies with all relevant directives and it covers all equipment intended for use in hazardous areas. Directive 94/9/EC is a ‘New Style’ directive which, with the intro- duction of the EHSRs, does not require replacement as equipment standards change.
These directives are usually referred to as the ATmosphere EXplosible (ATEX) 137 and 100a Directives respectively. Equipment Categories were introduced in the ATEX
100a directive and the corresponding zones of use (see section 15.4) are defined in the ATEX 137 directive.
UK legislation
The ATEX 137 Directive, along with the parts of the Chemical Agents Directive deal- ing with fire and explosion, were enacted into UK law as the Dangerous Substances and Explosive Atmosphere Regulations 2002 (DSEAR) (reference 15D), and both the ATEX 137 Directive and DSEAR came into force on 1 July 2003. Workplaces which were already in use on this date have until 1 July 2006 to comply. A key feature of DSEAR is area classification, which zones workplaces according to the probability of a potentially explosive atmosphere being present. The Health and Safety Executive have published Codes of Practice (references 15E to 15J) giving guidance on compliance with DSEAR.
The ATEX 100a Directive was enacted into UK law as the Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996 (EPS) (reference 15K). They came into force on 1 July 2003 and compliance was optional during the transitional phase between 1996 and 2003. Although DSEAR speci- fies the use of equipment complying with the EPS regulations, there is no requirement to change equipment in workplaces which were already in use on 1 July 2003.
DSEAR is directed at employers and the EPS regulations are directed at equipment manufacturers. The link between them is in DSEAR, where the categories of equip- ment defined in the EPS regulations are allocated to the various zones defined in DSEAR.
Area classification
Area classification defines hazardous areas on the basis of the probability of a flammable atmosphere being present. It is a legal requirement under DSEAR for any employer whose business involves flammable materials in any form, and it results in the division of the workplace into one or more zones. The levels of probability used in area classi- fication are high, medium and low. Historically, dust and gas or vapour were dealt with separately which is reflected in different nomenclature for their zoning. Zones 0, 1 and 2 are for gas, vapour and mist; zones 20, 21 and 22 are for dust. The definitions for gas or vapour zones and dust zones are very similar and are paraphrased here for simplicity:
● Zone 0, Zone 20 – High probability (A flammable atmosphere is present continuously, for long periods or frequently)
● Zone 1, Zone 21 – Medium probability (A flammable atmosphere is likely in normal operation occasionally)
● Zone 2, Zone 22 – Low probability (A flammable atmosphere is unlikely in normal operation and, if it occurs, will exist only for a short time)
The zones do not reflect the ignition properties of the flammable materials; this is dealt with separately by their ignition characteristics. It should also be noted that the zones describe ‘atmospheres’, which are mixtures of flammable material with air under atmospheric conditions. Any other situations are not part of area classification but require consideration as part of the overall safety case for the workplace.
‘Normal operation’ is considered to be the actual, day-to-day operation of the workplace. It is not the idealized operation of the workplace as it was designed, and the area classification has to be reviewed regularly to accommodate any changes to the workplace and the work therein. ‘Not likely in normal operation’ raises the question of how unlikely an event does area classification consider. This is best illustrated by considering a pipeline with flanged joints carrying a flammable gas. For a properly designed, constructed and maintained pipeline, fracture of the pipeline would not be considered; this is considered as a catastrophic failure and dealt with by the overall risk assessment. Leakage at a flanged piping joint would, however, be considered.
There are circumstances where area classification is not appropriate and these will be revealed by the overall risk assessment. For example, small quantities of flammable materials in the workplace may represent such a small risk to workers that other control measures may suffice.