Protection of electrical equipment in hazardous areas

Hazardous areas, in which potentially explosive atmospheres exist, are encountered in a wide variety of industries. Thankfully the science of how to operate safely in such areas is now well understood, though knowing how best to comply with the requirements is something that still causes widespread confusion. Particular care has to be taken with electrical apparatus because of its potential for creating sparks and hotspots that could ignite a gas, vapour, mist or dust-laden atmosphere.

Such environments are encountered in everyday life, with petrol station forecourts being an obvious example, but industry sectors that are prone to hazardous areas are mining, chemical processing, petrochemicals, and oil and gas (Fig. 1). In addition, pharmaceutical production facilities often have areas where solvents are used, and specialty chemical plants often have hazardous areas. Flour mills, bakeries, sugar processors, timber processors, coal handling plant, paper mills and processors of metals such as aluminium and magnesium, for example, can all have areas where dust-laden atmospheres are potentially explosive, so it can be seen that the full spectrum of production and process plants that can contain hazardous areas is extremely broad.

Regulations, gas groups and zones

Here in the European Economic Area (EEA), hazardous gases are classified in EN 50014: 1997 (Electrical apparatus for potentially explosive atmospheres - General requirements), which is a harmonised standard under Directive 94/9/EC (Equipment and protective systems intended for use in potentially explosive atmospheres) or, as it is commonly known, the ATEX Directive. This standard is about to be superseded by EN 60079-0:2004, which is based on the IEC standard IEC60079-0:2004 (Electrical apparatus for explosive gas atmospheres Part 0: General requirements). EN 50014 divides potentially explosive gases into two groups: Group I relates to mines susceptible to fire damp (methane) and Group II relates to other places.

Because of the specialist nature of mining, this present article considers Group II areas only.Group II is further sub-divided into three to reflect the different flammability of gases (and vapours and mists). Group IIA is for the least flammable gases (such as propane), while Group IIB is for medium-flammability gases (such as ethylene) and Group IIC is for the most flammable (such as hydrogen).

However, it should be remembered that, in general, gases on their own are not flammable; they also need oxygen or another oxidant with which they can react in combustion. (The exception is acetylene, which can decompose explosively in the absence of oxygen into carbon and hydrogen.)Furthermore, specifiers of apparatus for hazardous areas need to know the likelihood of the explosive gas-air mixture being present, so three zones of risk are defined within the hazardous area. In Zone 0 the risk is greatest, with the hazard continuously present - usually due to a continuous source of release.

In Zone 1, the risk is lower but the hazard is likely to be present under normal operating conditions – normally due to a primary source of release. In Zone 2, however, the hazard is unlikely to be present and, if so, it will be present only for short periods or due to a fault condition (normally stemming from a secondary source of release). As a general rule, in Zone 0 the hazard will be present for more than 1000 hours per year and in Zone 2 the hazard will be present for less than 10 hours per year – though these figures are not laid down in any standard.

Protection to suit Zones

Having established the nature of the hazard and the level of risk associated with it, a protection method needs to be selected to suit (Fig. 2).For Zone 0 the preferred method is intrinsic safety type ‘ia’ (two-fault tolerant), though special protection can be employed if specifically certified for this use, and it is possible to use encapsulation in some limited circumstances.

Nonetheless, intrinsic safety is almost always the only practical option, especially for sophisticated apparatus such as instrumentation.There is a much wider choice for applications in Zone 1 areas. Intrinsic safety (type ia, which is two fault - or type ib - which is single-fault tolerant) is often the preferred method, though flameproof protection, increased safety and purge/pressurisation protection are also commonly used.

Less frequently encountered are sand/powder filling, oil immersion, encapsulation and special protection.Within a Zone 2 area any of the above methods may be used, or Type-n (non-incendive) protection - in which the apparatus is not capable of causing ignition through the creation of sparks or hot surfaces during normal operation (though fault conditions could potentially cause ignition).

In all cases the requirement is to provide the necessary level of protection at a reasonable cost, though specifiers should be aware that a lower purchase cost will almost always lead to a higher cost-of-ownership. For Zone 0 we have seen that the only practical option is intrinsic safety, and for Zone 2 the considerably lower purchase cost of Type-n protected apparatus will often make replacement more cost-effective than repair, so cost-of-ownership is less of an issue. However, the choice is not so straightforward for Zone 1.

Intrinsic safety