A hazardous area is defined as ‘An area in which an explosive gas atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of equipment.’
Hazardous areas are divided into three Zones which represent the risk in terms of the probability, frequency and duration of a release. Area classification assists in the proper selection and installation of equipment.
The three Zones, as defined in IEC60079-10-1 (Classification of hazardous area explosive gas atmospheres), are as follows:
Zone 0 – In this Zone, an explosive gas atmosphere is continuously present, or present for long periods or frequently.
Zone 1 – In this Zone, an explosive gas atmosphere is likely to occur in normal operation occasionally.
Zone 2 – In this Zone, an explosive gas atmosphere is not likely to occur in normal operation but if it does occur, will persist for a short period only.
In the IEC system, electrical equipment for explosive atmospheres is divided into groups. The group allocation for surface and mining industries are separate.
Group I is reserved for the mining industry (hazards are ‘firedamp’ & coal dust).
Group II is for surface industries with gas hazards, and is subdivided into IIA (propane), IIB (ethylene) and IIC (hydrogen).
Group III is for surface industries with dust hazards, subdivided into IIIA (flying’s), IIIB (non-conductive dusts) and IIIC (Conductive dusts).
The equipment group or sub-division will be marked on the equipment nameplate:
Equipment was classified of into one of six temperature classes, from 85°C up to 450°C.
Electrical equipment must be selected to ensure that the surface temperature produced by the equipment (indicated by the ‘T’ Class) will not exceed the ignition temperature of the flammable atmosphere which may be present around the equipment. Equipment temperature class marking (‘T’ Class) and its equivalent °C.
|Temperature Class||Maximum Surface Temperature|
Temperature classification is based on the maximum temperature which any relevant part of the equipment, which may be in contact with an explosive gas can reach, when operated within its normal designed rating and maximum ambient.
Ambient temperature range for Ex equipment is -20°C to +40°C, unless otherwise marked on the equipment.
T-class temperature are based on the maximum ambient for the equipment, and this is the temperature rise calculations.
Equipment for use in hot climates, such as the Middle Eastern countries, will usually require ambient ratings greater than 40°C. Equipment for use in colder (arctic) climates will require a much lower limit to the ambient temperature range, which may be as low as -50°C. Such equipment would special material and expensive testing for certification. Equipment group & T-class are not directly related.
In the table, for each material, the equipment T class is seen to be below the ignition temperature of the flammable material.
|Material||Group||Ignition Temperature °C||Required T-Class °C|
Enclosures of electrical equipment are classified according to their ability to resist the ingress of solid objects and water by means of system of numbers known as the ‘Ingress Protection’ Code. This consists of the letters IP + two numbers, e.g. IP56.
The first number, in the range 0-6, indicates the degree of protection against solid objects, and the higher the number the smaller the solid object that is prevented from entering the enclosure. Zero (0) indicates no protection, and 6 indicates the equipment is dust-tight.
The second number, range 0-8, identifies the level of protection against water entering the enclosure, from 0- ‘no protection’ to 8- ‘withstands indefinite immersion a specified at depth.
As well as having the degree of ingress protection called for in the various construction standards, enclosures for electrical equipment must pass an impact test.
In this test, a test weight of 1 kg with an impact head of hardened steel, in the form of a 25mm hemisphere, is dropped onto the completely assembled equipment from a height ‘h’. At the end of the test, there must not be enough damage to invalidate the type of protection, IP code etc.
The height ‘h’ in meters, is given in the standards and varies with the equipment Group and the risk of mechanical impact (high or low). The standard details the two levels of impact resistance appropriate to high and low risk of impact. If the equipment is only suitable for low impact, the certificate is suffixed ‘X’ to signify special conditions of use apply.
In addition to the impact test, portable (hand-held) electrical or electronic equipment must be subjected to a drop test, in which it is dropped at least four times onto a horizontal concrete surface from a height of one meter.
Again the acceptance criterion is that there is not enough damage to invalidate the type of protection, IP code etc.
Many industries are involved in processing materials which can give rise to potentially explosive atmospheres by the presence of flammable mixtures of gases and/or dusts in air. These include mining, chemical plants, oil refineries, gas terminals, offshore installations, sugar, grain and other foodstuff processing and storage facilities, metal finishing works etc. All these industries use electrical energy.
The safe of electrical energy in the hazardous locations of these industries requires tried and tested methods of explosion protection to be implemented. For this reason, the authorities involved in compiling standards, and in the testing and certification of equipment have an important role.
Standards have evolved as a result of careful research, often prompted by accidents. Construction of equipment to relevant standards coupled with testing by an independent expert test authority will ensure that the equipment is suitable for its intended purpose.
Explosion protected equipment is constructed in accordance with standards, but its’ integrity will only be preserved if it is properly selected, installed and maintained in accordance with the manufactures’ recommendations and recognized codes of practice.
UK Code of Practice BS 5345 provided guidance for the selection, installation and maintenance of explosion protected equipment for use in potentially explosive atmospheres but it has now been superseded by five separated by five separate international standards in the IEC60079 series.
These five documents, IEC60079 – 10, 14, 17, 19 & 20 are cover classification of hazardous areas, selection & installation, inspection & maintenance, repair and the data for flammable gases and dusts.
In the United Kingdom, manufacturing and testing standards are published by the British Standards Institute (BSI).
In the European Community (European Union, EU), the organization which publishes harmonized standards for its member nations is the European Committee for Electro technical Standardization (CENELEC).
With the ultimate aim of global harmonization, the International Electro-technical Commission (IEC) publishes standards for this purpose.
For explosion protected equipment, IEC and European (EN) standards are now technically identical, published in the UK as BS EN60079
The EU ATEX Directives are now in force in Europe and the certification authorities are known as Notified Bodies (NB). Each has their own unique NB number, which will be marked on the certification labels of ATEX approved equipment and systems.
Notified Bodies in the UK include Baseefa Ltd (NB Number 1180), SIRA Certification Service (NB 0518), and ITS Testing and Certification (NB 0359).
Within Europe, EU directives are concerned with the removal of technical barriers to trade. They outline essential technical and safety requirements with products must comply.
Any product sold in the EU must meet the requirements of any EU directive relevant to that product, and will carry the CE mark. For equipment and systems for use in hazardous areas, the ATEX Directive become mandatory within Europe from 30th June 2003
All equipment and protective systems for use in potentially explosive atmospheres marketed in the EU have to be manufactured in compliance with the product directive (ATEX 95).
Products must satisfy the Essential Safety Requirements (ESR’s) specified in the annexes of the directive, with regard to the inherent risks associated with the product, for the protection of the public. After a successful Conformity Assessment, the product will display the CE mark, provided that it complies with any other applicable directives (Low Voltage directive etc), as well as ATEX.
Conformity Assessment involves a series of basic modules which give the manufacturer routes through which he may have his product verified for compliance. Depending on the type and category of the product, these modules will have the involvement of a Notified body to a greater or lesser extent.
In order that the CE mark can be applied, the product must comply with the requirements of all relevant EU directives.
The directive 94/9/EC (commonly known as ATEX 95, the ‘product’ directive) was adopted to enable free trade of products between EU member states through alignment of technical and legal requirements. The directive applies not only to electrical equipment but also to mechanical equipment and protective systems used in the presence of potentially explosive atmospheres containing flammable gases/vapours or combustible dusts.
Equipment is defined as any item which is inherently ignition capable or is potentially ignition capable and requiring the inclusion of special design and installation techniques to prevent ignition of any surrounding flammable atmosphere. The ‘equipment’ may also be interfaces located in non-hazardous area which are part of an explosion protection or system. Protective systems include quenching systems, flame arrestors, fast-acting shut-off valves and pressure relief panels installed to limit damage due to an explosion or prevent the spread of explosions.
Directive 2014/34/EU is the new ATEX direct which supersedes Directive 94/9/EC, and will take full effect from 20th April 2016. The new directive has more stringent requirements for accreditation and auditing of Notified Bodies, and there will be changes in the format of the Type Certificate etc. The Essential Safety Requirements remain unchanged.
Although the ATEX Directives are European, many other countries recognize them. Equipment from manufactures outside the EU must meet ATEX 95 before it can be imported into Europe. Therefore, to maximize market penetration, must of the Ex equipment from around the world will have ATEX certification.
Prior to UK entry into the European Union, electrical equipment for hazardous areas was manufactured to British Standard BS 4683. Equipment build and certified to this standard wound display the mark ‘Ex’ on the label, which indicated that the equipment was explosion protected ( do not confuse with explosion-proof, which is entirely different). In additional, the label was also marked with a ‘crown’ symbol, which was the distinctive mark for the UK test house BASEEFA.
Because of the differences in standards, UK equipment could not be used in the other European countries, and vice-versa. European co-operation resulted in the development of harmonized standards, known as “Euronorms”, for which the first English version was published as BS5501. The second generation of the UK version, which replaced this, carried the Euronorm numbering in the EN500—series. Equipment marking for EN500—was ‘EEx’
International standards are now identical to the Euronorms and carry numbers in the IEC60079 series. The identical European standards European standards carry the EN60079 numbers, often preceded by the individual country prefix e.g. BS EN60079-1. The marking for equipment manufactured to IEC/EN60079 standard is ‘Ex’.
Equipment complying with the ATEX Directive carries, in additional to the “CE” mark, the hexagon with Ex inscribed within symbol.
The latest (at March 2015) standards are:
|IEC||Gas / Dust||Euronorms|
|IEC Number||Dated||Constructional standards (Type of Protection)||
|Old EN number|
|IEC 60079-0||2011-06||General Requirements||G & D||EN 60079-0:2012 H||50 014|
|IEC 60079-1||2014-06||Flameproof Enclosure ‘d’||G||EN 60079-1:2007 H||50 018|
|IEC 60079-2||2014-07||Pressurised Equipment ‘p’||G||EN 60079-2:2007 H||50 016|
|IEC 60079-5||2015-02||Powder Filling ‘q’||G||EN 60079-5:2007 H||50 017|
|IEC 60079-6||2015-02||Oil Immersion ‘o’||G||EN 60079-6:2007 H||50 015|
|IEC 60079-7||2015-06||Increased Safety ‘e’||G||EN 60079-7:2007 H||50 019|
|IEC 60079-11||2011-06||Intrinsic Safety ‘i’||G & D||EN 60079-11:2012 H||50 020|
|IEC 60079-13||2010-10||Pressurised room ‘p’||G|
|IEC 60079-15||2010-01||Type of Protection ‘n’||G||EN 60079-15:2010 H||50 021|
|IEC 60079-18||2014-12||Encapsulation ‘m’||G & D||EN 60079-18:2009 H||50 028|
|IEC 60079-25||2010-02||Intrinsic Safety electrical systems ‘i’||G & D||EN 60079-25:2010 H||50 039|
|IEC 60079-26||2014-06||Equipment with equipment Protection Level ‘Ga’||G||EN 60079-26:2007 H|
|IEC 60079-28||2015-02||Protection of optical radiation system||G||EN 60079-28:2007 H|
|IEC 60079-29-1,2,3,4||2007-15||Gas detector||
EN 60079-29:2007 H
EN 60079-29:2010 H
|IEC 60079-30 1&2||2007-01||Electrical trace heating||EN 60079-30-1:2007 H|
|IEC 60079-31||2013-11||Equipment dust ignition protection by enclosure “tD”||D||EN 60079-31:2014 H|
|IEC 60079-32 1&2||
|IEC 60079-33||2012-09||Special protection “s”|
|IEC 60079-35 1&2||
|Caplights for use in mines||EN 60079-35-1:2011|
|“Codes of Practice”|
|IEC 60079-10-1||2008-12||Classification of hazardous areas gases||G|
|IEC 60079-10-2||2015-01||Classification of hazardous areas dusts||D|
|IEC 60079-14||2013-11||Electrical installation in hazardous areas||G & D|
|IEC 60079-17||2013-11||Inspection & maintenance of equipment in hazardous areas||G & D|
|IEC 60079-19||2015-03||Repair, overhaul & reclamation||G & D|
|IEC 60079-20-1||2010-01||Data for flammable gases||G||EN 60079-20-1:2010 H|
Ex equipment is marked in accordance with the standards as illustrated below. Equipment complying with ATEX 95 will also be marked in this way, but will have additional markings to indicate that it conforms to the directive.
Equipment certified as providing a method of protection for use in hazardous locations are requested to display the following markings:
The symbol Ex, (or EEx if constructed to the EN500—standards)
The type of protection used – ‘d’, ‘e’, ‘n’, etc
The equipment group IIA, IIB or IIC.
The T-class – T1, T2, T3…T6
Ex d IIC T6 EEx d IIC T6
Ex e IIC T4 EEx e IIC T4
Equipment to build it the latest IEC standards will have additional marking to show the equipment protection level
Ex d IIB T4 Gb EEx d IIB T4 Gb
Ex e IIC T5 Gb EEx e IIC T5 Gb
Ex ia IIC T6 Ga EEx ia IIC T6 Ga
A detailed discussion of ATEX 95 is the requirements of the directive for the marking of explosion protected equipment is, however, relevant and it illustrated below. This is in addition to the marking requirements of the constructional standards that the manufacturer may have used to meet the ESR’s of the directive.
The lasted editions of the IEC 60079 standards have introduced Equipment Protection Levels (EPL’s), corresponding to the ATEX equipment categories.
The EPL’s are: Very High (“a”), High (“b”) & Enhanced (“c”)
This enables a risk assessment approach, taking into consideration the consequences of an explosion, to be implemented for the selection equipment. The EPL / Category required for each location will appear on the area classification drawings, as well as the zoning, gas group and T-Class. EPL’s are designed Ma, Mb, Ga, Gb, Gc, Da, Db & Dc. Equipment marked “M” is for use in mines (Group I), that marked <strong >“G” is for use in flammable gases (Group II) and that marked “D” is for use in combustible dusts (Group III).
“Ga” : Equipment for explosive gas atmospheres, having a ‘very high; level of protection, which is not a source of ignition in normal operation, expected faults, or when subject to rare faults.
“Gb” : Equipment for explosive gas atmospheres, having a ‘high; level of protection, which is not a source of ignition in normal operation, or when subject to faults that may be expected, though not necessary on a regular basis.
“Gc” : Equipment for explosive gas atmospheres, having an ‘enhanced; level of protection, which is not a source of ignition in normal operation and which may have some additional protection to ensure that it remains inactive as an ignition source in the case of regular expected occurrences, for example, failure of a lamp.
Flameproof protection is one of the original methods of explosion protection developed for use in the mining industry. It has a wide range of applications, typically junction boxes, lighting fittings, electric motors etc. The letter ‘d’ comes from the German word ‘druckfeste’ (kapselung) which means ‘pressure tight’ (enclosure).The construction standard IEC 60079-1 defines flameproof as: ‘During an internal explosion of an explosive mixture, and which prevents the transmission of the explosion to the explosive atmosphere surrounding the enclosure’.
Increased safety Ex e concept was invented in Germany. The letter ‘e’ comes from the German phrase ‘Erhohte Sicherheit’ which means ‘increased security’. This method of protection has been widely used for many years and has a good safety record, comparable with the other methods of protection. Typical application are induction motors, lighting fittings and junction boxes. “A type of protection applied to electrical apparatus in which additional measures are taken to give increased security against the possibility of excessive temperatures, and of the occurrence of arcs and sparks in normal service or under specified fault conditions,.
Type ‘n’ protection applied to electrical equipment such that in normal operations and in certain specified regular expected occurrences, it is not capable of igniting a surrounding flammable gas atmosphere. “Restricted breathing ‘nR’ “ : A technique, mainly used in type ‘n’ lighting fittings, in which entry to the interior of the equipment by a flammable gas or vapour is restricted by virtue of good sealing at all joints and cable entries.
Cable entry glands for ExnR equipment normally utilize a restricted breathing washer, supplied by the equipment manufacturers, in place of normal IP sealing washers.
ExnR equipment shall be installed in a way that allows easy access to any test port. Equipment should be provided with a test port to enable testing of restricted breathing properties to be carried out after installation and during maintenance, with exemptions for some types of luminaires given in the standard.
Intrinsic Safety (I.S) is a low energy signaling technique that prevents explosion by ensuring that the energy transferred to a hazardous area is well below the energy required to initiate ignition. It can only be used for very low power applications, typically in control and instrumentation.
The safe area equipment limits the available energy into the hazardous area to below that which could ignite the most easily ignited gas/air mixture. The field equipment surface temperature will not cause ignition, even under specified fault conditions.
Intrinsic safety is a system concept and applies to the whole system – not just too individual parts of the system.
Ex “i” : A type of protection based on the restriction of electrical energy within equipment and of interconnecting wiring exposed to the explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects.
Intrinsically safe circuit: A circuit in which any spark or thermal effect, under normal operation and in specified fault conditions, is not capable of causing ignition of a given explosive atmosphere.
Intrinsically safe equipment: Electrical equipment in which all the circuits are intrinsically safe circuit.
References: Assets Training & Technical Services Pte Ltd Copyright © March 2015 & Copyright © ABB