Electrical protection 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. 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 in the EEA, gas groups and zones of risk

In the European Economic Area (EEA), hazardous gases are classified in EN50014:1997 (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 is commonly known, the ATEX Directive.

This standard is about to be superseded by EN60079-0:2004, which is based on the IEC standard IEC60079-0:2004 (Electrical apparatus for explosive gas atmospheres Part 0: General requirements). EN50014 divides potentially explosive gases into two groups: Group I relates to mines susceptible to fire damp (methane) and Group II relates to other places.

Group II is further subdivided 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.

Suitable protection methods for each zone

Having established the nature of the hazard and the level of risk associated with it, a protection method needs to be selected to suit.

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 sur-faces during normal operation (though fault conditions could potentially causes 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.

Zone 1 protection: pros and cons of alternative protection methods

Intrinsic safety

Intrinsically safe apparatus is designed and constructed such that, even under fault conditions, the electrical energy within the circuits is less than the minimum ignition energy of the flammable atmosphere in which it is to operate. One of the main benefits of intrinsically safe equipment is that live maintenance within the hazardous area is permitted, which greatly reduces the cost of ownership.

If maintenance is required, the equipment can be left in place, which saves time and minimised plant down time. Furthermore, cables can be installed without additional mechanical protection, and this helps to reduce installation costs compared with using other protection methods. The, literally, intrinsic safety of this method of protection means many users view this as the safest option.

Issues to be aware of with intrinsic safety are the fact that some associated apparatus will always be required to connect the apparatus to the non-hazardous area (typically by means of a safety interface unit - such as a zener safety barrier or galvanic isolator - located in the non-hazardous area), and the purchase cost is often higher than for other protection methods due to the greater design effort, the need for low-power electronic components, and the higher degree of fault tolerance that is built in.

Flameproof protection

Flameproof protection essentially refers to the placement of all electrical apparatus within a special enclosure that is capability of containing an explosion that initiates inside. In some cases the enclosure has a complete and perfect metal-to-metal seal at all openings and the enclosure is capable of withstanding an internal explosion. Other designs use special wide flanges that enable any flame escaping through a joint gap to be quenched before it reaches the potentially flammable atmosphere outside the enclosure. Installation and maintenance clearly requires great care in order to ensure that the flameproof characteristics of the enclosure are not compromised. Similarly, cable glands and conduit must be correctly specified and installed to provide adequate physical and flameproof protection. Because of the nature of the components within the enclosure, live maintenance is forbidden, which can add significantly to the cost of ownership.

Moreover, the concept allows for the ingress of flammable gases into the enclosure from outside, and accepts that combustion will occur. Where flammable gas is continually supplied into the enclosure this creates the risk of the enclosure itself heating to the point that its external temperature may present an ignition risk even if the combustion is contained inside. For this reason, it is understandable why many users prefer the philosophy appertaining to intrinsic safety.

Increased safety

Another alternative protection method for Zone 1 areas is increased safety. This relies on safeguards applied during design and construction that ensure the apparatus con-tains no normally arcing or sparking devices or hot surfaces that could cause ignition.

Measures that are taken include the use of high-integrity insulation, the temperature derating of insulation materials, enhanced creepage and clearance distance, careful attention to terminal design, protection against the ingress of solids and liquids, high impact strength for the enclosure and the control of maximum temperatures. Increased safety is generally considered to be suitable for medium-power apparatus, with typical applications being small motors, luminaires and junction boxes.

Purge/pressurisation

The last of the popular protection methods for Zone 1 hazardous areas is over-pressurisation of the apparatus enclosure using clean air or an inert purge gas. Depending on the exact procedure employed, the region inside the cabinet becomes either a non-hazardous or a Zone 2 area, with resultant implications for the design and construction of the electrical equipment contained therein.

Purge/pressurisation is often used where the function of the electrical equipment makes it difficult to redesign it so that it is intrinsically safe. It is also an easy concept to understand, and it is human nature to trust what is readily understood. Another advantage of using the purge/pressurisation method is that the cost of the core apparatus is, relatively speaking, lower than the intrinsically safe equivalent.

However, the complexity and cost associated with a purge/pressurisation system should not be underestimated. If the purge gas is to be clean air, this may have to be piped from some considerable distance away, and there is a need to install the pumping system outside the hazardous area, plus protected pipework must be run through the hazardous area to the apparatus. Correct connection of the pipework to the apparatus enclosure is also essential. All of this hardware needs to be maintained and suitable fail-safe monitoring and alarm systems need to be in place to detect any failure of the pressurisation system and shut down the apparatus.

If, on the other hand, an inert purge gas is used, this can reduce the installation cost, but there is an additional ongoing cost associated with the consumable gas. Furthermore, whichever purge/pressurisation method is used, live maintenance of the apparatus itself is prohibited, which can add to maintenance costs and lead to significantly longer down time when the apparatus needs to be maintained.

Other methods

Other methods that can be used, such as oil immersion, encapsulation and filling with sand or powder, are only suited to a limited range of applications - in particular, those where maintenance is not likely to be required.

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