Electrical Equipment in Explosive Atmospheres/ATEX

INTRODUCTION

Explosive atmospheres in industrial premises can be caused by flammable gases, mists or vapours or by combustible dusts. Where such atmospheres are encountered it is of paramount importance that the correct attention is given to the selection, installation and maintenance of electrical equipment as a key risk control measure. Such considerations should be a main component of a Dangerous Substances and Explosive Atmosphere Regulations 2002 (DSEAR) risk assessment.

Electrical equipment in explosive atmospheres has become a complex subject, partly due to the many changes that have occurred over the years, brought about by changes in terminology and developments in standards and equipment. The purpose of this technical bulletin is to provide Consultants with an overview of the present situation and to define RSS survey and reporting strategy.

ATEX

ATEX is the name commonly given to the legal requirements for controlling explosive atmospheres and the suitability of electrical equipment and protection systems employed in such environments. The “ATEX Directive” as it is known consists of two EU Directives which came into force in 2006 with the objective of harmonising standards across a single market.

Details of these Directives are as follows:

1.   Directive 99/92/EC (also known as “ATEX 137” or the “ATEX Workplace Directive”). This concerns health and safety in workplaces where potentially explosive atmospheres are present and is implemented by the HSE under DSEAR 2002.

2.   Directive 94/9/EC (also known as “ATEX 95” or the “ATEX Equipment Directive”). This concerns the manufacture and supply of electrical and mechanical equipment and protective systems intended for use in potentially explosive atmospheres and includes:

• Equipment and protective systems for use within potentially explosive atmospheres;

• Devices for use outside potentially explosive atmospheres, but which are required for, or contribute to, the safe functioning of equipment and protective systems located inside such atmospheres; and

• Components relating to the above.

This Directive is implemented by the Department for Business, Innovation and Skills (BIS) under the Equipment and Protective Systems for Use in Potentially Explosive Atmosphere Regulations 1996 (EPS). These regulations and DSEAR are complimentary and are enforced by the HSE (and, in some cases local authorities, petroleum licensing authorities and others). DSEAR is concerned with the safe use of dangerous substances and requires employers to zone workplaces and select equipment and protective systems that meet the EPS Regulations. The EPS Regulations place a duty on manufacturers to supply suitable equipment for such purposes. Manufacturers may use the CE mark on their products to show compliance with this (and any other relevant) Directive.

When considering this Directive, it is important to recognise that this is not confined to main electrical circuitry and equipment, but extends to all electrical equipment including items such as fire and intruder alarms, instrumentation, clocks, paging systems, radios, inspection lamps, mobile phones and torches, and to lift trucks and other mobile plant.

CLASSIFICATION OF HAZARDOUS AREAS/ZONES

Schedule 2 of DSEAR classifies hazardous areas (referred to as “zones”) on the basis of the frequency and duration of the occurrence of an explosive atmosphere.

For gases, vapours and mists the zone classifications are:

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 or frequently.

Zone 1

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 likely to occur in normal operation occasionally.

Zone 2

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 not likely to occur in normal operation but, if it does occur, will persist for a short period only.

For dusts the zone classifications are:

Zone 20

A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently.

Zone 21

A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.

Zone 22

A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

Notes:

1. Layers, deposits and heaps of combustible dust must be considered as any other source   which can form an explosive atmosphere.

2. "Normal operation" means the situation when installations are used within their design parameters.

3. Although it is seldom, if ever, possible to establish the Zone number from the actual length of time the hazard will be present, the following values are commonly used as a guide:

• Hazard present for >1000 hours/year   =                   Zone 0

• Hazard present for >10 <1000 hours/year  =                   Zone 1

• Hazard present for <10 hours/year  =             Zone 2

DSEAR states that where necessary, areas classified as Zones 0 (20), 1 (21) or 2 (22) shall be marked at their points of entry as shown below. Signs may also contain reference to the particular zone.

For the purposes of illustrating the application of area classification, a small detached highly flammable liquids store would normally be classified as Zone 2. However, in circumstances where mixing or dispensing is conducted, the classification would be raised to Zone 1. Another common example might be that of a spray booth, in which the area within the extraction duct is likely to be classified as Zone 0, the area in the booth as Zone 1, and the immediate area from the face and perimeter of the booth to a specified distance and height, Zone 2. Areas beyond the Zone 2 perimeter would be defined as non-hazardous, sometimes referred to as “safe areas”.

As regards dust atmospheres, the following are examples of typical zone locations:

• Zone 20. Inside silos, hoppers, cyclones and other plant.

• Zone 21. Areas outside dust containment, such as those in the immediate vicinity of access doors and other access points which are subject to frequent opening or removal, and areas in the proximity of filing and emptying points.

• Zone 22. Locations near equipment which is opened infrequently; areas used for the storage of bagged products where there is a risk of the failure of bags during handling; areas that are normally classified as Zone 21, when measures are employed to prevent the formation of an explosive atmosphere, for instance, by exhaust ventilation.    

Equipment and protective systems for all places in which explosive atmospheres may occur should be selected on the basis of the requirements the EPS Regulations and the BS EN 60079 series, unless a risk assessment determines otherwise (this is mentioned later). This will require categories of equipment to be used in Zones as indicated overleaf:

• Zone 0(20) – ATEX category 1.

• Zone 1(21) – ATEX category 1 or 2.

• Zone 2(22) – ATEX category 1, 2 or 3.

Further details relating to equipment and protective systems are given later.

It should be remembered that the most effective solution to the control of potential ignition sources from electrical equipment (and, for that matter, other sources of ignition) is often to remove the equipment, such as switchgear, from the hazardous area, resulting in savings in installation and maintenance costs and an improvement in overall safety. However, where this is not possible, the above criteria should apply.

Determining the nature and extent of hazardous areas/zones is covered by the following Standards:

• BS EN 60079-10-1:2021 Explosive Atmospheres - Classification of Areas – Explosive Gas Atmospheres.

   

• BS EN 60079-10-2:2015 Explosive Atmospheres - Classification of Areas – Combustible Dust Atmospheres.

In each case, calculating the type and extent of zones can be a complex process, which for gas or vapour atmospheres involves due consideration and documentation of the following:

• The flammable materials that may be present;

• The physical properties and characteristics of each of the flammable materials;

• The source of potential releases and how they can form explosive atmospheres;

• Prevailing operating temperatures and pressure;

• Presence, degree and availability of ventilation (forced and natural);

• Dispersion of released vapours to below flammability limits;

• The probability of each release scenario.

As such, unless dealing with a very simple case, calculation and verification of hazardous area zoning should only be conducted by those Consultants with proven skills in chemical sites and similar high hazard occupancies. For other Consultants, reference to the DSEAR risk assessment, where this exists, may confirm whether a competent approach to zoning appears to have been taken.

It is important to recognise that DSEAR extends beyond the traditional application of electrical zoning, taking into account non-electrical sources of ignition and also mobile equipment.

Catastrophic failures such as a vessel or pipeline rupture would normally be excluded from an area classification study. Such abnormal events would normally fall within a hazard identification process such as a Hazard and Operability Study (HAZOP).

Once a plant has been classified and all necessary records made, it is important that no modifications to equipment or operating procedures are undertaken without consultation with those responsible for the area classification, preferably under a formal management of change policy.  It is necessary to ensure that all equipment affecting the area classification which has been subjected to maintenance is carefully checked during and after re-assembly to ensure that the integrity of the original design, as it affects safety, has been maintained before it is returned to service.

SELECTION OF EQUIPMENT

Having determined the hazardous area classifications/zones, the next stage in the process is to select the correct equipment appropriate to the zone in which it is to be employed. There are eight recognised concepts of protection within Europe relating to gases and vapours, which are detailed in BS EN 60079-14:2014 Explosive Atmospheres – Electrical Installations Design, Selection and Erection. These are shown in the following table, together with their application and design methodology:

As well as considering the protection against electrical arcs and sparks in igniting a flammable atmosphere, consideration needs to be given to the surface temperature of equipment. Flammable materials are categorised according to their auto-ignition temperature and six temperature classes have been defined as per the following table:

 Equipment should be selected with a maximum surface classification (T Number) which is below the auto-ignition temperature of the flammable material present. If several different flammable materials are evident within a particular area, the material that gives the highest classification dictates the overall area classification.

Finally, gases and vapours are categorised in terms of their ignition energy as defined below:

 Note: Equipment marked IIB is suitable for applications requiring Group IIA equipment. Similarly, equipment marked IIC is suitable for applications requiring Group IIA or Group IIB equipment.

EXPLOSIVE DUST ATMOSPHERES

With regards to equipment in dust atmospheres, the main protection considerations are to define and control the maximum surface temperature, to contain any ignition capable parts within a “dust tight enclosure” and that any electrical circuit has energy limitation such that sources of ignition will not occur.

The procedures for dust hazards are, in terms of recognised technical standards, not as advanced as those applicable to gas and vapour hazards and, as a result, continue to evolve.

Four generic types of protection are recognised, as per table below:

• Intrinsic safety “iD”

• Dust excluding enclosure “tD”

• Encapsulation “mD”

• Pressurisation “pD”

  
  

Dusts are separately categorised to the following hazard groups:

 Notes:

1. Equipment marked IIIB is suitable for applications requiring Group IIIA equipment. Similarly, equipment marked IIIC is suitable for applications requiring Group IIIA or Group IIIB equipment.

2. Combustible flyings are defined as solid particles, including fibres, greater than 500 µm in nominal size, which may be suspended in air, may settle out of the atmosphere under their own weight, can burn or glow in air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures. Examples of fibres and flyings include rayon, cotton (including cotton linters and cotton waste), sisal, jute, hemp, cocoa fibre, oakum, and baled waste kapok.

There are two considerations for the ignition of dusts from hot surfaces - the ignition of a dust cloud and the ignition of a layer of dust. Unlike gas or vapour, dust does not disperse, but accumulates in layers which can ignite at much lower temperatures than dust clouds.

Ignition data for dusts is given for clouds and layers, against which equipment must be selected based on its stated temperature rating. The equipment temperature rating must be suitable for the lower of either:

• two-thirds of the dust cloud ignition temperature, or

• the appropriate layer ignition temperature, adjusted in accordance to the thickness of the dust layer. There are two methods of arriving at the final temperature, which BS EN 60079-14 refers to as Practice A and Practice B.  

Determining the correct temperature classification can be an involved process, sometimes requiring laboratory investigation.

EQUIPMENT IDENTIFICATION

In accordance with the ATEX Directive, all equipment must be provided with the electrical equipment in hazardous atmospheres EC mark, together with other information as prescribed, as per the following example:

Notes:

1. The Ex marking for explosive gas atmospheres and explosive dust atmospheres shall be separate and not combined.

2. Equipment complying with earlier CENELEC standards will frequently bear the code EEx (as opposed to Ex).

3. For equipment designed for dust atmospheres, the surface temperature is limited to a given value; the T grouping prefix is not used.)

4. When identifying equipment for use in explosive atmospheres, confusion may arise when equipment is found to have an IP rating. This relates to the resistance of the enclosure to the equipment to the ingress of water and dust, and alone as has no direct bearing on the explosion-proof nature of the apparatus.

Some equipment will be encountered which pre-dates current identification protocols. For example, flameproof equipment may be designated with the letters FLP. All existing equipment which was on the market or in service prior to 1 July 2003, and which met safety requirements at the time can continue to be used, provided it remains fit for purpose.

Manufactures websites at which a wide range of explosion protected equipment can be viewed include:

Kempton Controls Ltd  – www.kempstoncontrols.co.uk/category/706/atex

Cooper Industries (EATON) – www.ceag.de

Ampcontrol UK Ltd – www.ampcontrolgroup.co.uk

Chalmit (part of Hubbell Incorporated) – www.chalmit.com

EQUIPMENT PROTECTION LEVELS - EPL

In recent years recognition has been given in the BS EN 60079 series to a more flexible risk assessment approach to the protection considerations in explosive atmospheres, as compared with the more traditional and relatively inflexible prescriptive approach linking equipment zones. To facilitate this, a system of equipment protection levels has been introduced to clearly indicate the inherent ignition risk of equipment, no matter what type of protection is used. Apart from major chemical and other high hazard sites, this is unlikely to be encountered in the field and therefore has not been included in this bulletin, other than by brief mention.

CABLES AND WIRING

Cables and wiring should be installed in positions that prevent them from being subject to mechanical damage, corrosion, chemical attack, heat and other detrimental environmental conditions. Selection of the wiring system and cable type must consider these influences and where exposure to such conditions are unavoidable, protective measures such as minimizing the risk of mechanical damage by the use of appropriate armoured cable types should be considered. Except for Zone 0 areas where special considerations apply, steel wire armoured cable is nowadays the most common cable employed having partly replaced the traditional use of steel conduit.

EQUIPMENT INSTALLATION AND MAINTENANCE

It is essential that the design, specification and installation of electrical equipment in explosive atmospheres is entrusted to responsible persons and operatives with the appropriate competency in this specialist field, including the appointment of specialist electrical engineers who are registered under either of the NICEIC, ECA or SELECT Hazardous Areas Schemes.

Such engineers should also undertake all routine electrical maintenance to ensure that it remains fit for purpose and continues to provide the designed level of protection. For example, flameproof equipment to perform as intended is dependent on there being a designed flame gap in the body of the equipment, which if disturbed for maintenance purposes needs to be correctly re-instated.

BS EN 60079-17:2014 Explosive Atmospheres – Electrical Installations Inspection and Maintenance advocates a maximum interval between periodic inspections of three years, although, in practice, more frequent inspection regimes can be encountered. Continuous supervision by skilled personnel is also recommended to enable the early detection of faults and their subsequent repair. When considering maintenance, fork lift trucks and other mobile plant must not be overlooked.

SURVEY EXPECTATIONS

As previously mentioned, determining and verifying hazardous areas can often be an involved and complex process, particularly when dealing with large manufacturing facilities and, as such, should only be handled by Consultants with proven expertise in this specialist area. 

For other members of the Surveys team, the expectation in circumstances where potentially explosive atmospheres are identified is summarised as follows:

• Reference to the DSEAR risk assessment (where this exists);

• Identification of dangerous substances in use and storage, and their physical properties;

• Assessment of how such materials are handled and the sources of release;

• Presence, degree and availability of ventilation (forced and natural);

• Prevailing operating temperatures and pressure where appropriate;

• Evidence of electrical zoning and appropriately protected equipment by visual inspection (this would normally be confirmed in the DSEAR risk assessment, if completed correctly);

• Presence of “non-approved” portable appliances;

• Correct electrical maintenance with suitable recording;

• Effective control of other ignition sources, including those of static electricity, hot work, etc..., as required under DSEAR.

Appropriate information concerning DSEAR/ATEX compliance is to be detailed in survey reports under Occupation/Processes, and risk improvements raised where applicable which, in the majority of cases, are to the submitted as Requirements.

NON-ELECTRICAL EQUIPMENT

Before concluding this Technical Bulletin, recognition should be taken that the ATEX Equipment Directive relates not only to electrical equipment, but also mechanical equipment which is the subject of BS EN ISO 80079-36: 2016 Non-electrical Equipment for Explosive Atmospheres. Taken directly from the standard; this European Standard specifies the basic method and requirements for design, construction, testing and marking of non-electrical equipment intended for use in potentially explosive atmospheres in air of gas, vapour, mist and dusts. Such atmospheres can also exist inside the equipment.

In addition, the external atmosphere can be drawn inside the equipment by natural breathing produced as a result of fluctuations in the equipment’s internal operating pressure, and/or temperature.

There are seven recognised concepts of explosion protection for non-electrical equipment as detailed in the following table:

The Protective measures shall be considered or applied in the following order:

1. Ensure ignition sources cannot arise (protection concept c)

2. Ensure ignition sources cannot become effective (protection concept g and b)

3. Prevent explosive atmosphere reaching ignition sources (protection concept fr, p and k)

4. Contains the explosion and prevent flame propagation (protection concept d)

It should be noted that the protection of non-electrical equipment as described above is a fairly recent innovation and technology in this area continues to evolve.

ADDITIONAL INFORMATION

By its very nature, equipment in hazardous atmospheres and ATEX is an involved subject on which, owing to its relevance, Consultants should have a level of awareness. A wealth of information can be found on the web for those members of the team who may wish to acquire further knowledge, in addition to which queries relating to a specific case should be directed to the RiskSTOP Technical Helpline.

Sira Test & Certification Ltd. www.siracsa.co.uk  and Chalmit -www.chalmit.com are examples of where additional information can be sourced.