Intrinsically safe?

5 mins read

In the rush to protect electrical equipment from causing fires in explosive atmospheres, it's easy to forget the basics of risk assessment, as enshrined in DSEAR.

Pedantic, maybe. Nitpicking even, but a mistake commonly made by engineers and engineering managers alike is to assume that the ATEX Directives – ATEX 95 Equipment Directive 94/9/EC, covering equipment and protective systems, and the ATEX 137 workplace directive, which dictates minimum health and safety requirements for workers – together cover primary legislation where potentially explosive atmospheres are concerned. Yes they do, but in the UK it is DSEAR (the Dangerous Substances and Explosive Atmospheres Regulations 2002) that provides the underpinning regulatory framework and, just as important, the starting point for any project involving hazardous areas. It is DSEAR that articulates the definition of hazardous areas as "any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety of workers". DSEAR also mandates that hazardous area studies be carried out and documented, with plant or factory areas then classified in the form of zones. And it is DSEAR that effectively maps those zones to appropriate protection, with equipment categories then defined by the ATEX Equipment Directive – which is the Equipment and Protective Systems for Use in Potentially Explosive Atmospheres Regulations 1996 in UK law. HSE's information on 'Hazardous Area Classification and Control of Ignition Sources' ( makes the point that European electrical standards then set out the various protection concepts, "with subdivisions for some types of equipment, according to gas group and temperature classification". It also suggests that the best standard for determining a hazardous area classification is BS EN 60079 part 10-1 for flammable gases, vapours or mists, or BS EN 60079 part 10 – 2 for dusts (covering construction, installation and use of apparatus). Walk before you run That's because it relates likely releases and available ventilation to the relevant zone number. However, HSE also draws attention to the Institute of Petroleum Model Code of Practice (Area Classification Code, 2002 – aimed at refinery and petrochemical operations) and the Institution of Gas Engineers' Safety Recommendations SR25, 2001 (mainly for natural gas transmission, distribution and storage sites). Why does this matter? Well, it's important to remember that appropriate equipment and preventive and/or mitigating measures can only be considered once DSEAR's hazardous area classification studies have been carried out, taking into account sources of flammable gas, vapour and dust, their likelihood and duration, and the relevant auto-ignition temperatures. Additionally – although not under DSEAR – when major plant contains flammable materials, we might add that separate HAZOP (hazard and operability) studies also become essential. And don't forget that these diktats apply not only to new installations, but also when plant modification projects are involved. There is nothing 'fit and forget' about hazardous areas. So, when you're told that certain equipment is ATEX-approved, you just might just want to put that information into some sort of context. You might also want to think about its applicability to your particular plant's zone(s) and your preferred protection methods. And just to recap, Zone 2 (22 for dust) is the lowest risk (explosive gas atmospheres are not likely to occur in normal operation and, if they do, will only exist for a short time), moving up to Zone 1 (21 for dust) as a higher risk (explosive atmospheres are likely to occur in normal operation – for example, in coal mining operations), and Zone 0 (20 for dust) as the highest risk area (explosive atmospheres are present continuously or for long periods, such as the above the fluid level in a closed tank. As for protection methods, IS (intrinsically safe – based around limiting the available electrical energy, so that ignition sources cannot occur from a short circuit or failure) remains the preferred protection for low-voltage, low-current instrumentation (24V and 4-20mA, etc). That's for all potentially explosive atmospheres ('ia' for Zone 0, 'ib', Zone 1, and 'ic', Zone 2), primarily because of its ease of installation, commissioning and maintenance. Current technologies are zener barriers, galvanic isolators and IEC/EN 60079-11 standard FISCO (fieldbus intrinsically safe concept) equipment for fieldbus devices. However, if more power is required, the next step up can be flameproof (type 'd') for Zone 1 or non-incendive (type 'n', or in the US, explosion-proof XP), which is the default choice for higher voltage and higher current equipment in Zone 2. There are plenty of other protection concepts – ranging from increased safety (type 'e') for Zones 1 and 2, to encapsulated for Zones 1 (type 'mb') and 0 (type 'ma'), pressurised/purged for Zone 1 ('px' or 'py') and Zone 2 ('pz'), and then the general class, special protection (EX 's'), which is quasi-manufacturer certified, although coming into line under EN 60079-33. And you will see legends, such as 'Ex n IIB T2', where the second code (in this case 'n') applies to the protection type; the third defines the gas group hazard (here, ethylene); and the fourth, the auto-ignition temperature (in this case, 300ºC). Easing operations That said, when it comes to maintenance – and specifically of process monitoring and control equipment – in hazardous areas there are some practical steps that can make a difference. Robert Kuipers, product manager at Pentair Thermal Management (which supplies trace heating systems), suggests that, if automation of equipment like his is required, it may make sense to go for local control with centralised monitoring. Describing it as a 'best of both worlds' solution, he makes the point that such a configuration enables areas to be controlled locally and independently, but with all site data (in his case, covering temperatures, ground fault currents and operating currents) sent to a central location, away from the hazardous area. Quite apart from allowing low-cost daisy-chaining of power distribution wiring, this also means that "a significant part of the maintenance can be carried out from a central location, so that [engineers] do not have to go into the field as frequently". That's one approach. Terrington Data Management business development manager Tracey Marshall points to another, aimed at easing data gathering for Ex equipment inspections. Citing speciality chemicals manufacturer Innospec – the world's largest producer of fuel additives and a manufacturer of octane additives – she says plant managers there significantly reduced the annual cost of hazardous area inspections by using mobile computing and IndEx data capture software to manage the asset register and regulatory equipment regime. Site electrical engineer Paul Mills explains that Innospec has 7,000 items to inspect on a three-year cycle at its Ellesmere Port plant – the usual mix of lights, motors, valves and transmitters, with a 10% detailed strip-down carried out annually to comply with HSE requirements. "The IndEx software, coupled with the use of intrinsically safe hand-held devices pre-loaded with details of the items to be inspected, offers a faster, more accurate and cost-effective approach than the paper-based method previously in use," he confirms. "Controls within IndEx safeguard data integrity and provide a full audit trail. Process improvement efficiencies gained have also enabled us to streamline our Ex inspection regime, resulting in an 85% reduction in inspection and repair hours," he continues. "This represents a significant annual saving and provides a substantial return on investment." Mills explains that, as inspectors tour the site, the hand-helds prompt them on items to be inspected and the relevant checks required, in line with IEC 60079-17. "Engineers can carry out and record details of repairs at the time the fault is found, which is a real cost-saving benefit. Then, on their return to the office, the recorded information is downloaded to the database to provide a record of inspections carried out, with full traceability of when the inspection was made and who carried it out."