Explosive atmosphere zones

Table 5.1 Types of protection electrical apparatus for potentially explosive atmospheres (zone 1)...

Hazardous places are classified in terms of zones (divisions) on the basis of the frequency and duration of the occurrence of an explosive atmosphere (IEC 60079-10 200X, Classification of Hazardous Areas European Standard EN 50281-3 200X, Classification of Areas Where Combustible Dusts Are or May Be Present ). 

Due to the quite different structures of industrial plants equipped with electrical installations, there are different probabilities for the existence of potentially explosive atmospheres formed by gas-air, vapour-air or dust-air mixtures. With respect to economical reasons, the types of explosion protection for electrical apparatus and systems will depend strongly on the explosion risk at the installation point. It is very unusual to construct and build all explosion protected electrical apparatus in such a manner that they can operate permanently in a surrounding hazardous atmosphere with combustible gases, vapours and dusts in air. The common way is to classify different areas in industrial plants according to the probability for the existence of a hazardous atmosphere and to establish adequate types of explosion protection. In other words, industrial plants with potentially explosive atmospheres are divided and classified into zones. ... 

Ha - areas in which an explosive atmosphere is present permanently or for long periods zone 0 zone 0... 

An example of a two-zone concept for areas hazardous due to combustible substances is given in Table 2.2. It shows the traditional classification (in the USA and Canada) into classes and divisions according to the probability of forming an explosive atmosphere. 

In the same way as for zone 0, a national forerunner in Germany started work for mining apparatus intended for continuous operation in an explosive atmosphere. The work was done by members of the national committees focused on explosion protection and resulted in a decree stated by the national authority. On the basis of clause 30 of ElBergV [7], the following protection concepts are considered applicable for a continuous operation under a permanently existing explosive atmosphere ... 

Caution needs to be taken with local pressure variations of the environmental atmosphere. A well-known example is the outlet side of an external cooling fan of a surface-cooled motor or generator (see Fig. 6.15). At this point, the environmental potentially explosive atmosphere shows an increased pressure, which shall be overcompensated by an adequate internal overpressure. The p-standards ask for a minimum overpressure of 50 Pa (=0.5mbar) for Group I p-apparatus or Group II p-apparatus in zone 1. 

After the localization of places and areas with the possibility of a formation of an explosible atmosphere, the resulting hazard must be assessed. Corresponding to the probability of formation of an explosible atmosphere, which represents the endanger-ment, these localized areas are attributed zones . Within a building the boundaries of a zone are defined at concrete walls. The zone classification still in use in Germany was first published by the BG-Chemie. Some changes to this system are to be expected... 

The areas with a hazard of explosion are divided into zones according to frequency and duration of the occurrence of an explosible atmosphere [5, 37]. They determine the extent of the measures to be taken and are defined as follows ... 

Zone 20 is an area in which a hazardous explosive atmosphere in the form of a cloud of flammable dust contained in air is present permanently, frequently or for long periods. 

Intrinsic safety ia, ib, ic (note 2) EN 60079- 11 The equipment used in the explosion hazard zone only contains intrinsically safe circuits. A circuit is intrinsically safe if neither a spark nor a thermal effect can cause the ignition of a standard explosible atmosphere. The corresponding tests are carried out under standardized conditions, which comprise both normal operation and certain faulty states. Measurement and control devices, communication devices, sensors, actors... 

Pressurization P EN 61241-4 The occurrence of an explosible atmosphere inside a casing is prevented by maintaining a protective gas overpressure with respect to the surrounding atmosphere. If necessary the interior of the casing is permanently supplied with a protective gas so that the flammable mixture is diluted. Switchgear and control cabinets May only be used in zones 21 and 22... 

The top of the bell was classified as zone 2 corresponding to an explosive atmosphere which is not likely to be produced during normal operation or, if it does nevertheless arise, is only of short duration. 

Flameproof equipment is regarded as safe for use when exposed to the risk of explosive atmosphere for which certification has been given. Electrical apparatus, defined as flameproof, has an enclosure that will withstand an internal explosion of the flammable vapour or gas in question which may enter the enclosure. The joints of the enclosure which are designed with clearance gaps to prevent a build-up of internal pressure also prevent any internal explosion igniting vapour or gas surrounding the equipment. The surface temperature of the enclosure must be below the ignition temperature of the vapour or gas in question, hence the importance of the temperature classification referred to above. Flameproof enclosures are primarily intended for use in Zone 1 or Zone 2 classification but not in Zone 0. 

In many cases, identifying a standard that can be adopted to measure workplace performance is straightforward. For example. Regulation 7 of the UK Dangerous Substances and Explosive Atmospheres Regulations 2002 requires certain places at workplaces to be classified in relation to the subject of the regulations as per Schedule 2, and to further divide the places classified as hazardous into different hazard zones. By referring... 

It is generally assumed that a volume of 10 L of an explosive mixture in an enclosed space can cause damage—particularly to people. For this reason, any area in which such a volume of an explosive mixture can collect is described as a potentially explosive atmosphere. It is only in the range between the LEL and UEL that the mixture reacts explosively when ignited. So, from a safety point of view, it is better to keep this concentration around 20% below LEL. Also see Clause 3.4.1 for further explanations. LEL and UEL are defined in Fig. X/3.1.2-2. The LEL and UEL of a few important gases are also shown in Fig. X/3.1.2-1 with associated gas group as per zone system (and NEC 505). 

In line with the directive, employers need to classify into zones the areas in which explosive atmospheres may be present, and to ensure that the directive is observed. In the zone system there are clear divisions between gas and dust. For gas systems, zones are classified as zone 0, 1, and 2, whereas for dust they are designated as zone 20, 21, and 22, as detailed in Table X/3.23-1. 

Decision about extent of zone, that is, distance from source (in all directions) up to which an explosive atmosphere exists before the material is dispersed. Rate of release (velocity, size, and geometry of sources), ventilation, and relative density of flammable material (mainly gas) are important considerations. 

Zone systems are followed in area classification by ATEX and lEC, and even NFPA has included zone systems of classification in their standard. The zone classification system probably has the edge over the class/division system because many consider the zone classification system to be more flexible and safer in hazardous locations. However, zone systems do not take care of the consequence of release. In case the consequences are important, as per the advice of the HSE.UK, it is better to go for a more conservative equipment selection. Although not officially accepted yet, various sources have tried to place time limits on these zones. The most commonly used values are as shown in Table X/3.3.2-l based on probability and duration of explosive atmosphere. 

EC, on the other hand, looks after the minimum health and safety requirements for workplaces with a potentially explosive atmosphere. In ATEX 95, entire systems have been divided into groups and categories as already discussed. In Directive 137, workplace area has been divided into zones. Use of various categories of equipment in various zones is elaborated in Table X/3.2.3-3. The basic purposes of each of these directives are elaborated in Fig. X/3.3.3-1. This figure also shows the correlation between the two directives so readers can have better understanding of the directives with their distinctive features. 

Principles In this case the electrical apparatus cannot ignite potentially explosive atmospheres surrounding them in normal and defined abnormal situations. These are applied for zone 2 applications ... 

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. 

From 1 July 2003 the Regulations are fully in force with the exception that workplaces where explosive atmospheres may occur which were in existence at this date have until July 2006 to comply with the full zoning requirements. However, all modifications must fully comply immediately. 

Where an explosive atmosphere may occur, workplaces must be classified into hazardous and non-hazardous places. The following zones are specified by Schedule 2 of the Regulations (see Table 20.7). 

Zone 0 A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is present continuously or for long periods. 

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