Electrical equipment for use in areas where there may be combustible dusts

This document provides general technical guidance and explains Nova Weigh’s approach and policy to the supply of electrical and electronic equipment such as weighing instrumentation for use in locations where an explosion hazard may exist due to the presence of combustible dusts. 

European Directive

European Directive 94/9/EC ‘Equipment and protective systems intended for use in potentially explosive atmospheres’ details mandatory requirements for the supply and installation, within the EC, of hazardous area electrical equipment and systems.  Until 30 June 2003, compliance with this Directive is optional and either previous European Directives or National Requirements may be followed. 

Directive 94/9/EC is commonly referred to as the ATEX Directive.  (Atmosphere Explosive)  It defines two equipment groups: group I for use in mining applications and group II for surface industry.  These equipment groups are divided into categories: categories M1 and M2 for mining and categories 1, 2 and 3 for surface industry.   European Standard EN1127-1 states that category 1 equipment is applicable in zone 20, category 1 or 2 equipment may be used in zone 21, and category 1, 2 or 3 equipment may be used in zone 23.

As equipment suppliers begin to conform to the ATEX Directive, the equipment group and category will appear on equipment labels in addition to other marking, certification information, CE marking etc.

Hazardous Area Classification for Dusts

The safe selection and installation of electrical apparatus for hazardous areas depends upon the correct classification of the hazardous area.  The classification includes defining the applicable zones within a dust hazard location and detailing information relating to the dust flammability such as ignition temperature and, in some cases, ignition energy.

Area classification is normally the responsibility of the site operator and certainly is not an aspect which the equipment supplier can influence.  Nova Weigh will, as appropriate, advise on the suitability of, and supply equipment for hazardous areas but cannot become involved in the classification of the hazardous area.

 Zones

Hazardous areas where combustible dust is the risk should be defined in terms of zones.  For dust hazards there are three zones: zone 20, 21 and 22.  The definitions of these zones are given below and further information may be found in the applicable standards which are listed in this document.  It should be noted that some countries, including the UK, have previously defined two zones, zone Y and zone Z for dust hazards.  Since zones Y and Z may still be referenced on some existing plants, their definitions are also given.

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.

NOTE  In general these conditions, when they occur, arise inside containers, pipes and vessels etc. 

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.

NOTE  This zone can include, among others, places in the immediate vicinity of e.g. powder filling and emptying points and places where dust layers occur and are likely in normal operation to give rise to an explosive concentration of combustible dust in mixture with air. 

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.

NOTE  This zone can include, among others places in the vicinity of equipment, protective systems, and components containing dust, from which dust can escape from leaks and from dust deposits (e.g. milling rooms, in which dust escapes from the mills and then settles).

(Definitions for Zones 20, 21 and 22 are given in European Standard EN 127-1.[BS EN1127-1:1998]) 

Zone Z  An area in which a combustible dust is, or may be, present as a cloud during normal processing, handling or cleaning operations in sufficient quantity to be capable of producing an explosible concentration of combustible or ignitable dust in a mixture with air. 

Zone Y  Areas not classified as Zone Z in which accumulations or layers of combustible or ignitable dust may be present under abnormal conditions and give rise to ignitable mixtures of dust and air. 

(Definitions for Zones Y and Z are given in BS6467: Part 2: 1988.)

Dust Ignition Temperature

Dusts can frequently be ignited from contact with hot surfaces and thus to establish the maximum permitted surface temperature of electrical equipment intended for use in dust atmospheres, the ignition temperature of the dust needs to be established.

 In many cases, the worst (lowest) ignition temperature of a dust will exist when a hot surface is covered with a 5mm dust layer.  Clearly, depending on the nature of the dust (some dusts such as metal dusts may allow temperature dissipation more readily) and the dust particle size, the worst case layer thickness cannot be stated with certainty unless laboratory tests have been carried out.  Even then, the inevitable variability of variants such as dust particle size mean that obtaining accurate results is often impractical. 

Nova Weigh’s customer or the end user should specify the applicable dust layer ignition temperature which supplied equipment must meet.  It is normal for a safety factor to be applied to layer ignition temperature as far as electrical equipment is concerned.  Exact computations are given in EN50281-1-2, but in general, 75K should be subtracted from the minimum ignition temperature to established the maximum permissible equipment surface temperature.  It is important for Nova Weigh to know whether or not this factor has been included in any data provided by our customers.

 It is also worth noting that an equipment temperature classification (T6, T5 etc.) on equipment certified for gas and vapour hazards does not (depending on the type of protection employed) necessarily refer just to external surfaces.  However, because dust hazard equipment has additional requirements for equipment ingress protection (see below) which results in prevention of dust ingress, it is only the external temperature which needs consideration for equipment for dust hazard use.  Thus it is possible that, for example, increased safety (Ex e) equipment certified to EN50014 and EN50019 and marked, say, T3 (indicating a maximum ignition-capable surface temperature of 300°C) may have its hottest surface inside the equipment enclosure.  Providing the applicable IP rating is achieved to prevent ingress of dust, then only the external surface needs consideration for dust hazard use.  Quite possibly, the hottest external temperature of the equipment may be significantly less and could even meet T4 (max of 135°C) for dust applications.

In the absence of reliable data (normally a test report from a European Notified Body [test house such as EECS, BASEEFA, PTB etc.]) there is, however, no alternative but to assume that the stated temperature class refers to external temperature and to use this value for dust hazard applications.

Equipment Suitability

Most electrical apparatus for dust hazard environments is based on types of protection established for gas and vapour hazards.  However, unlike the established position for dealing with gas and vapour hazards where the type of protection such as flameproof (Ex d), intrinsic safety (Ex ia / EX ib), etc. is itself sufficient protection to enable safe installation and operation in a suitably defined hazardous area to be established, with dust hazard equipment additional emphasis is placed on preventing the dust entering the equipment.

In general, in addition to utilising equipment with an appropriate type of protection, an ingress protection level of IP 5X (commonly referred to as ‘dust protected’) or IP 6X (commonly referred to as ‘dust tight’) is required.  These ingress protection levels are not automatically achieved with some types of protection and this is one reason why equipment which is suitable for gas and vapour hazards is not necessarily suitable for dust hazards. 

Additionally, the need to consolidate minimum layer ignition temperature with the temperature classification of the apparatus, taking account of the factor of safety discussed previously, means that additional temperature considerations sometimes apply.

Matching Type of Protection to Zone

European standards defining the requirements for matching the type of protection to the intended zone of installation and use have not, at the time of writing, been completed.  However, the general principles set out in Directive 94/9/EC and in the European Standard EN1127-1 define the levels of safety and fault considerations applicable to the various zones.

In general, type of protection Ex n will only be suitable for zone 22 applications.  Intrinsic safety Ex ia should be used in zone 20 unless the equipment has been carefully assessed for suitability in this zone.  In zone 21, any method of protection apart from Ex n should be suitable.

Remember that the requirements for temperature and ingress protection are additional selection requirements which must be met irrespective of the type of protection used. 

Ingress Protection

Although there are exceptions for apparatus such as rotating machines, the normally applied requirement is that equipment to be installed in zone 20 or zone 21 should meet the requirements of IP 6X.  Equipment to be installed in zone 22 need only meet the requirements of IP 5X unless the dust is electrically conducting, in which case IP 6X is required.

 It should be noted that the normal types of protection for hazardous area electrical apparatus do not necessarily achieve the required ingress protection levels.  For example, intrinsically safe equipment (either certified or used under simple apparatus rules) may not necessarily achieve better than IP20 and some flameproof equipment may not achieve IP54.  This aspect should be given careful consideration when selecting equipment for dust hazards since if the required ingress protection levels are not achieved, the possibility of igniting layers of dust penetrating to surfaces inside the equipment cannot be ruled out.

 Installation of Equipment

Nova Weigh will not normally be responsible for the installation of equipment.  The installer should follow the requirements of the applicable European or other installation standard in addition to any installation instructions supplied by Nova Weigh.

Marking of Equipment

Nova Weigh equipment which complies with the ATEX Directive will be marked accordingly.  Prior to June 2003 equipment which has not completed full assessment to ATEX will continue to conform to and be marked in accordance with BS6467 or BS7535 as appropriate.

 Documentation

Nova Weigh’s policy is to supply information to at least comply with the applicable standards and, where applicable, European Directives.  Documentation will normally include reference to applicable standards, the applicable maximum surface temperature of the equipment and its ingress protection rating.

 Maintenance

Any maintenance undertaken must be completed by suitably trained and competent personnel in such a way that the maintained product and its installation fully complies with the original product specification and with the applicable standards and directives.  European standards specify the requirements for training of competent personnel and provide guidance on applicable maintenance and inspection requirements.

 Applicable Standards

 NOTE

·          European Standards produced by CEN or CENELEC are published by British Standards with the prefix BS.

Thus EN1127-1 = BS EN1127-1 etc. 

·          Many European standards now equate to International Electrotechnical Commission  (IEC) standards in the series IEC 79-XX and thus compliance with applicable European standards may define acceptability in countries outside Europe.  Equally, compliance to current editions of IEC standards may equate to the technical requirements stated in European Standards.  Certification by a European Notified Body may, however, still be required.

EN1127-1

Explosive atmospheres – Explosion prevention and protection.  Part 1. Basic concepts and methodology

EN50281-1-1

Electrical apparatus for use in the presence of combustibe dust.

Part 1-1: Electrical apparatus protected by enclosures. Construction and testing

Part 1-2: Electrical apparatus protected by enclosures.  Selection, installation and maintenance

Part 2-1: Test methods – Methods for determining the minimum ignition temperatures of dust 

BS6467

Electrical apparatus with protection by enclosure for use in the presence of combustible dusts

Part 1: 1985: Specification of apparatus

Part 2: 1988: Guide to selection, installation and maintenance

BS7535: 1992

Electrical apparatus complying with BS5501 or BS6941 in the presence of combustible dusts

BS5501

Electrical equipment for potentially explosive atmospheres

Note: This standard is identical to EN50014 and its sister standards relating to the different methods of protection.  Above edition 1 of the standard it is referred to only as a European Standard.

EN50014

Electrical equipment for potentially explosive atmospheres

(See also BS5501)

BS6941: 1988

Electrical apparatus for explosive atmospheres with type of protection N

EN50021: Publication awaited

Electrical apparatus for potentially explosive atmospheres – Type of protection “n”

Further Information and Waiver

This document is intended for general guidance and should not be regarded as an authoritative or comprehensive technical statement on electrical equipment for dust hazards.  The applicable technical standards and legal requirements in the country or state of use must be followed. 

Additional product and technical information may be obtained from Nova Weigh Limited.  Further general guidance on hazardous areas may be found in the publication ‘Electrical Apparatus and Hazardous Areas’ by Robin Garside. ISBN 0 9516848 0 9 Published by Hexagon Technology Ltd. Telephone +44 (0)1296 612310. 

P4026 dated 25.1.99