Safety Later, low voltage signalling bells were introduced into
the mines. It was believed that, since these bells operated on a very low voltage (12V or less), they would be safe. However, in the years 1912 and 1913, there were two disastrous mine explosions in England, which were traced to the signalling bells. Research showed (Mines Research Establishment, Buxton, England) that these low voltage circuits were capable of igniting mine gases, it also lead to new circuit designs in which the stored energy was reduced to a non ignition capable level. This technique was labelled ‘intrinsic safety’ and it was the beginning of a new era in safety methods for explosive hazardous areas.
Definitions and Codes
An area of plant is defined as a hazardous area if it contains flammable gases, vapours, combustible dusts and easily ignitable fibres and flyings which may be expected to be present in concentrations such as to require special precaution for the construction and use of electrical equipment in their presence. An explosive atmosphere consists of a mixture of flammable substances in air in such proportions that it can be exploded by excessive TEMPERATURES, ARCS OR SPARKS. The gases, vapours or mists will only explode when mixed with air between specific percentage mixtures, these are called:
LOWER EXPLOSIVE LIMIT (LEL) UPPER EXPLOSIVE LIMIT (UEL)
These mixtures will also have different: auto-ignition temperatures (AIT); minimum ignition currents (MIC - intrinsic safety test apparatus); maximum experimental safe gaps (MESG - relates to flameproof enclosures flame path), depending upon the substances contained within the mixture.
Examples of Explosive Mixtures Substance LEL (%) UEL (%) MESG mm MIC mA AIT (°C) Methane 5 Propane 2 Ethylene 2.7 Acetylene 3 Hydrogen 4
15
9.5 34 17
75.6
1.14 0.97 0.65 0.25 0.28
Figure 2. Examples of Explosive Mixtures
It is evident from the limited list shown in the table above that there are some natural groupings for the gases based on their MESG and MIC values.
These groups are divided into two groups; Group I for mines susceptible to methane.
Group II for explosive gases for locations other than coal mines; group II is further divided into three sub groups: IIA, for atmospheres containing propane or
gases of an equivalent hazard. IIB, for atmospheres containing ethylene or gases of an equivalent hazard. IIC, for atmospheres containing hydrogen or gases of an equivalent hazard.
(Extensive listings have been published which details thousands of different gases and their associated groupings.)
The natural grouping of the gases based upon the MESG and MIC values does not bear any relationship to the auto-ignition temperatures (AIT) of the various substances.
The auto-ignition temperature is the temperature, in °C, at which a gas will ignite spontaneously without another source of ignition. Because these temperatures do not correspond with the above groupings, a temperature code was established. The resulting temperature codes for the substances listed above (temperature classification) are shown in the table below.
TEMPERATURE CODES
The full list of temperature codes are listed in the table below:
Substance Methane Propane Ethylene
Acetylene Hydrogen
Temperature Classification T1 T1 T2 T2 T1
Figure 3. Full List of Temperature Codes
195 595 146 470 108 425 60 305 75 560
The gas groupings and the temperature codes are reflected in the markings that appear on electrical equipment, which has been certified for use in a hazardous area. The marking of the gas grouping and temperature code on the equipment identifies to the user the type of explosive atmosphere in which it can be safely installed (see Section 4 for further details).
Hazardous areas are further divided in zones, these zones relate to the predicted occurrence of when an explosive atmosphere may be present in the area. These zones are defined as being:
ZONE 0, where an explosive atmosphere is continuously present, or present for long periods.
ZONE 1, where an explosive atmosphere is likely to occur in normal operation.
ZONE 2, where an explosive atmosphere is not likely to occur in normal operation and if it does occur it will exist only for a short time.
Commonly Recognised Concepts of Protection
There are eight commonly recognised concepts of protection within Europe. These are detailed in the European EN60079 series of Standards; ‘electrical equipment for use in explosive atmospheres’. These methods of protection have, over the years, been added to and expanded to satisfy the new equipment designs that have appeared.
GENERAL REQUIREMENTS
BS EN 60079 series of standards has been written for manufacturers of electrical equipment for use in gas explosive atmospheres.
Unless modified by one of the parts in the IEC 60079 series, electrical apparatus complying with this standard is intended for use in hazardous areas in which explosive gas atmospheres, caused by mixtures of air and gases, vapours or mists, exist under normal atmospheric conditions of:
• temperature –20 °C to +60 °C;
• pressure 80 kPa (0,8 bar) to 110 kPa (1,1 bar); and • air with normal oxygen content, typically 21 % v/v.
The application of electrical apparatus in atmospheric conditions outside this range may need special consideration.
FLAMEPROOF
BS EN 60079 Part 1 Electrical apparatus for explosive gas atmospheres, Flameproof enclosures‘d’.
Requirements for the construction and testing of electrical apparatus with protection flameproof enclosure‘d’.
This part of EN 60079 contains specific requirements for the construction and testing of electrical apparatus with the type of protection flameproof enclosure‘d’, intended for use in explosive gas atmospheres.
This European Standard covers supplementary requirements for shafts and bearings, breathing and draining devices which form part of flameproof enclosures, fasteners, associated holes and closing devices, materials and mechanical strength of enclosures, verification and tests, and non-metallic enclosures and non-metallic parts of enclosures.
INTRINSIC SAFETY
BS EN 60079:2007 Part 11 Electrical apparatus for explosive gas atmospheres: Intrinsic Safety ia and ib
Specifies the construction and testing of intrinsically safe apparatus intended for use in an explosive gas atmosphere and for associated apparatus that is intended for connection to intrinsically safe circuits that enter such atmospheres.
This type of protection is applicable to electrical apparatus in which the electrical circuits themselves are incapable of causing an explosion in the surrounding explosive atmospheres.
This standard is also applicable to electrical apparatus or parts of electrical apparatus located outside the explosive gas atmosphere or protected by another type of protection listed in BS EN 60079-0, where the intrinsic safety
INCREASED SAFETY
BS EN 60079 Part 7 Electrical apparatus for explosive gas atmospheres, Increased safety ‘e’.
BS EN 60079-7 specifies the requirements for the design, construction, testing and marking of electrical apparatus with type of protection increased safety ‘e’ intended for use in explosive gas atmospheres. This standard applies to electrical apparatus where the rated voltage does not exceed 11kV rms a.c. or d.c. Additional measures are applied to ensure that the apparatus does not produce arcs, sparks, or excessive temperatures in normal operation or under specified abnormal conditions.
This design concept is reflected in the equipment marking by the symbol ‘Ex e’. Equipment designed to this concept is suitable for use in ‘Zone 1’ and ‘Zone 2’ classified hazardous areas.
Type N Protection (Non-sparking)
BS EN 60079 Part 15 Electrical apparatus for explosive gas atmospheres, Type of protection ‘n’.
A type of protection applied to an electrical apparatus such that, in normal operation, it is not capable of igniting a surrounding explosive atmosphere, and a fault capable of causing ignition is not likely to occur.
This design concept is reflected in the equipment marking by the symbol ‘Ex n’. Equipment designed to this concept is suitable for use in ‘Zone 2’ classified hazardous areas.
Note that in the long term the protection concepts used in this standard are being incorporated into the individual protection concept standards.
For e.g. Ex nL (energy limitation) is now identified as Ex
of the electrical circuits in the explosive gas atmosphere may depend upon the design and construction of such electrical apparatus or parts of such electrical apparatus. The electrical circuits exposed to the explosive gas atmosphere are evaluated for use in such an atmosphere by applying this standard.
BS EN 60079-11:2007 replaces BS EN 60079-11:2002 which will be withdrawn.
This design concept is reflected in the equipment marking by the symbols ‘Ex ia’ or ‘Ex ib’. Equipment designed to this concept is suitable for use in: ‘Ex ia’ ‘Zone 0’, ‘Zone 1’ and ‘Zone 2’; ‘Ex ib’ ‘Zone 1’ and ‘Zone 2’ classified hazardous areas.
PRESSURISATION
BS EN 60079 Part 2 Electrical apparatus for explosive gas atmospheres, Pressurised enclosures ‘p’.
BS EN 60079-2 contains the specific requirements for the construction and testing of electrical apparatus with pressurised enclosures, of type of protection “p”, intended for use in explosive gas atmospheres.
BS EN 60079-2 specifies requirements for pressurised enclosures containing a limited release of a flammable substance. The standard gives requirements for the design, construction, testing and marking of electrical apparatus for use in potentially explosive atmospheres in which:
a) a protective gas maintained at a pressure above that of the external atmosphere is used to guard against the formation of an explosive gas atmosphere within enclosures which do not contain an internal source of release of flammable gas or vapour; and, where necessary.
b) a protective gas is provided in sufficient quantity to ensure that the resultant mixture concentration around the electrical parts is maintained at a value outside the explosive limit appropriate to the particular conditions of use. The protective gas is supplied to an enclosure containing one or more internal sources of release in order to guard against the formation of an explosive gas atmosphere.
BS EN 60079-2 includes requirements for the apparatus and its associated equipment including the inlet and exhaust ducts, and also for the auxiliary control apparatus necessary to ensure that pressurisation and/or dilution is established and maintained.
This design concept is reflected in the equipment marking by the symbol ‘Ex p’. Equipment designed to this concept is suitable for use in ‘Zone 1’ and ‘Zone 2’ classified hazardous areas.
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June/July 2010
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