Electrical Apparatus and Hazardous Areas

(Published 2007)

This book gives full information of both electrical and non-electrical hazardous area technology

and covers terminology, equipment marking, ATEX, DSEAR, Area Classification, Types of Protection etc.

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• The type of hazard
• The likelihood of the hazard being present in flammable concentrations
• The (auto) ignition temperature of the hazardous material

The hazard will be in the form of a gas, vapour, dust or fibre.

Gases and vapours are categorized in terms of their ignition energy or the maximum experimental safe gap (in respect of flameproof protection).  This categorization leads to the Gas Groups:

(The gases noted in the table are typical gases for each group.)

Group IIC is the most severe group.  Hazards in this group can be ignited very easily indeed.

Dusts and Fibres  (Group III)

Dusts and fibres are also defined in terms of their ignition properties including dust cloud ignition properties.

The likelihood of the hazard being present in flammable concentrations (Zones)

The likelihood of the hazard being present in flammable concentrations will vary from place to place.  A location very close to an open source of hazard will have a high likelihood of a flammable atmosphere.  On the other hand, outside a flanged pipe containing a flammable liquid, the likelihood of a flammable atmosphere being present is much lower since it will only occur if the flange leaks.  Rather than work with an infinite range of possibilities, three zones are defined.

Gases and Vapours

There are three zones for gases and vapours:

Dusts

There are three zones for dusts:

(The presence of dust layers does not automatically lead to the dust zone.  The likelihood of the dust layer being disturbed to create a cloud needs to be considered.  Dust layers also need careful consideration in terms of ignition temperature.  Because the dust layer can make the equipment under it hotter then normal, a factor of safety is applied to the layer ignition temperature.)

The (auto) ignition temperature of the hazardous material (Temperature Classes)

As well as considering the protection against electrical arcs and sparks igniting a flammable atmosphere, consideration needs to be given to the surface temperature of equipment.  (Most electrical apparatus dissipates some heat!)  Flammable materials are categorized according to their ignition temperature.  Again, rather than work with an infinite range, six temperature classes are defined as follows:

The Temperature classification will be marked on items of equipment. If the hazardous area in which you are installing equipment has gases or vapours with a low autoignition temperature then you will need equipment with a bigger T-Number so as to ensure that any hot surfaces on the equipment will not ignite the hazard.

For example, if a hazard has an autoignition temperature of 180°C, then it would be safe to use equipment which is marked T6 or T5 or T4.  It would not be safe to use equipment marked T3 or T2 or T1as this equipment could exhibit surface temperatures which are hot enough to ignite the hazardous atmosphere.

Don't forget that, unless the certification documents state otherwise (in which case there will be an addition to the T-Classification code on the equipment label such as T4 (60°C Amb)) the equipment is only certified in ambient temperatures up to 40°C. If exposed to higher temperatures there are two possible dangers. First the stated T-Class temperature may be exceeded and secondly safety components within the equipment could fail to an unsafe condition. If you expect equipment to be subjected to temperatures above 40°C (such as in direct sunshine or in a roof space) you should install equipment which is certified for a higher ambient temperature.

Protecting Electrical Apparatus to make it suitable for use in hazardous areas

Electrical apparatus for use in hazardous areas needs to be designed and constructed in such a way that it will not provide a source of ignition.  There are ten recognized types of protection for hazardous area electrical apparatus. Each type of protection achieves its safety from ignition in different ways and not all are equally safe.  In addition to the equipment being suitable for the Gas Group and the Temperature Class required, the type of protection must be suitable for the zone in which it is to be installed.  The different types of protection and the zones for which they are suitable are as follows:

Standards

There are various standards which give details of hazardous area requirements.  In the main, the IEC standards are now parallel voted by CENELEC (Europe). New standards are being introduced at a bewildering rate and it is always worth checking against, for example, the BSI website. (See Useful Links Page)

It is important for equipment designers to design to the latest edition of ATEX harmonised standards - the up to date list can be found on the EC ATEX website. (See Useful Links Page)

Gradually, the technical standards for the technology for gas/vapour hazards and those for dust hazards are being incorporated into the same series. For example IEC 60079-10 covers area classification IEC 60079-10-1 deals with gas/vapour hazards and IEC 60079-10-2 deals with dust hazards. Similar numbering will apply for other aspects such as installation.

Top level standard

EN1127-1    Explosive atmospheres: Explosion prevention and protection.  Part 1: Basic concepts and methodology

Standards for Equipment

 IEC 60079-      ELECTRICAL APPARATUS FOR EXPLOSIVE GAS ATMOSPHERES

Part 0       General requirements

Part 1       Construction and test of flameproof enclosures of electrical apparatus

Part 2       Electrical apparatus - type of protection ‘p’

Part 3       Spark test apparatus for intrinsically safe circuits

Part 4       Method of test for ignition temperature

Part 5       Powder filling ‘q’

Part 6       Oil-immersion ‘o’

Part 7       Increased safety ‘e'

Part 11     Construction and test of intrinsically safe and associated apparatus

Part 15     Electrical apparatus for explosive gas atmospheres, type of protection ‘n’

Part 18     Encapsulation ‘m’

Part 25     Intrinsically safe systems

Part 26     Special requirements for construction test and marking of zone 0 electrical apparatus

Part 27     Fieldbus intrinsically safe concept (FISCO)

Standards for Installation etc. (gases and vapours)

 IEC 60079-      ELECTRICAL APPARATUS FOR EXPLOSIVE GAS ATMOSPHERES

Part 10    Classification of hazardous areas

Part 14    Installation (and selection) of equipment in hazardous areas

Part 17    Inspection and maintenance of equipment

Note that the 60079 series of standards will gradually address dust hazards as well as gas/vapour hazards. The standard number will have an additional suffix 1 or 2. Thus, for example, 61241-14 (see below) will be replaced by 60079-14-2 [installation for equipment in dust hazards], with 60079-14-1 dealing with installations in gas/vapour hazards.

Standards for Dust Hazard Equipment and Installation

IEC 61241        ELECTRICAL APPARATUS FOR USE IN THE PRESENCE OF COMBUSTIBLE DUST

Part 0       General Requirements

Part 1       Protection by enclosure ‘tD’

Part 2-1     Methods for determining the minimum ignition temperature of dusts

Part 2-2     Method for determining the resistivity of dust in layers

Part 2-3     Method for determining minimum ignition energy of dust air mixtures

Part 4       Type of protection ‘pD’

Part 10     Classification of areas where combustible dusts are or may be present

Part 11     Protection by intrinsic safety ‘iD’

Part 14     Selection and installation

Part 17     Inspection and maintenance

Part 18     Protection by encapsulation ‘mD’