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Historical Background of explosion protection The first hazardous area was discovered in the early coal mines. This area held a double hazard: methane gas (firedamp) and coal dust. Methane gas is absorbed in the pores of coal. When the coal is mined the methane is evolved, a process that takes a relatively short time. To be completely free of methane, coal has to be store for a period of up to 1000 hours. When miners worked an 8-hour shift pattern, the mined coal would be left in the shaft until the next day, during which time the methane would start to be evolved into the air in the shaft. The methane would collect in pockets at the roof of the mine and form an explosive layer. The mines returning for the next shift would carry with them the means of igniting the gas, hat mounted candles, and hand carried oil lanterns. The resulting ignition of the methane would in itself not necessarily be fatal for the miners. It was the secondary ignition of coal dust, throw up into a cloud by the methane explosion, that resulted in a more violent and deadly detonation. The first method used to remove the methane hazard was to have a person crawl along the mine floor holding a lighted torch in their outstretched hand. This procedure would ‘safely!' ignite the methane layer and burn it off before the miners started work. The person performing this task was known as the ‘fireman' and it was soon found that there were very few volunteers for this hazardous job. This resulted in prisoners being offered short jail terms if they would volunteer for the position. With the advent of forced ventilation in the mines, the hazards were reduced by the dilution of the methane with fresh air so that it was below its explosive limit. When electrical equipment was first introduced into the mines, there were some explosions due to electrical sparking. However, it was discovered that totally enclosed motors were able to contain explosions without transmitting it to the surrounding external atmosphere. This concept was transferred to the design of other electrical equipment; fitting it inside substantial cast iron enclosures with tight fitting joints. Later, low voltage signaling 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 signaling bells. Research showed 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 labeled ‘intrinsic safety' and it was the beginning of a new era in safety methods for explosive hazardous areas. Definitions and Codes A hazardous area is defined as an area in which explosive atmospheres, or may be expected to be, present in quantities such as to require special precaution for the construction and use of electrical equipment. An explosive atmosphere consists of a mixture of flammable substances with air in the form of GAS, VAPOUR OR MIST 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: Ø LOWER EXPLOSIVE LIMIT (LEL) Ø 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
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; Ø Group I for mines susceptible to methane. Ø Group II for explosive gases for locations other than mines; group II is further divided into three sub groups: Ø IIA, for atmospheres containing propane or gases of an equivalent hazard. Ø IIB, for atmospheres containing ethylene or gases of an equivalent hazard. Ø IIC, for atmospheres containing hydrogen or gases of an equivalent hazard. 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: FULL TEMPERATURE CODES LIST
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: Ø ZONE 0, where an explosive atmosphere is continuously present, or present for long periods. Ø ZONE 1, where an explosive atmosphere is likely to occur in normal operation. Ø 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 recognized concepts of protection There are eight commonly recognized concepts of protection within Europe. These are detailed in the European EN50 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. FLAMEPROOF European Harmonized Standard EN50 018. A method of protection where the equipment is contained within an enclosure which will withstand an internal explosion of a flammable gas or vapor that may enter it, without suffering damage and without communicating the internal flammation to the external explosive atmosphere, through any joints or structural openings in the enclosure. The enclosure will be designed for a particular gas grouping (I, IIA, IIB or IIC). This design concept is reflected in the equipment marking by the symbol ‘Ex d'. Equipment designed to this concept is suitable for use in ‘Zone 1' and ‘Zone 2' classified hazardous areas. Intrinsically Safety – Apparatus or System European Harmonized Standard EN50 020. A protection technique based upon the restriction of electrical energy within the apparatus and in the interconnecting wiring, exposed to a explosive atmosphere, to a level below that which can cause ignition by either sparking or heating effects. Because of the method by which intrinsic safety is achieved it is necessary that not only the electrical apparatus exposed to the explosive atmosphere, but also other (associated) electrical apparatus with which it is interconnected, is suitably constructed. The concept is divided into two sub types, which are dependent upon the number of allowable fault conditions. The symbols ‘ia' and ‘ib' denote the sub types 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. PRESSURIZATION European Harmonized Standard EN50 016 A method of protection using the pressure of a protective gas to prevent the ingress of an explosive atmosphere to a space that may contain a source of ignition and, where necessary, using continuous dilution of an atmosphere within the space that contains a source of emission gas, which may form an explosive atmosphere. 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. INCREASED SAFETY European Harmonized Standard EN50 019 A method of protection by which additional measures are applied to an electrical apparatus to give increased security against the possibility of excessive temperatures and of the occurrence of arcs and sparks during the life of the apparatus. It applies only to an electrical apparatus, no parts of which produce sparks, arcs, or exceeds the limiting temperature of the materials, upon which safety depends, that are used in its construction. 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) European Harmonized Standard EN50 021 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. OIL IMMERSION European Harmonized Standard EN50 015 A method of protection where the electrical apparatus is made safe by oil-immersion. In the sense that an explosive atmosphere above the oil or outside the enclosure will not be ignited. The oil presents a barrier between the explosive atmosphere and the electrical apparatus. This design concept is reflected in the equipment marking by the symbol ‘Ex o'. Equipment designed to this concept is suitable for use in ‘Zone 1' and ‘Zone 2' classified hazardous areas. POWDER/SAND FILLING European Harmonized Standard EN50 017 A method of protection where the enclosure of the electrical apparatus is filled with a mass of granular material such that, if an arc occurs the arc will not be liable to ignite the external explosive atmosphere. This design concept is reflected in the equipment marking by the symbol ‘Ex q'. Equipment designed to this concept is suitable for use in ‘Zone 1' and ‘Zone 2' classified hazardous areas. ENCAPSULATION European Harmonized Standard EN50 028 A type of protection in which parts that could ignite an explosive atmosphere by either sparking or heating are enclosed in a compound in such a way that the explosive atmosphere cannot be ignited. The compound provides a barrier between the electrical apparatus and the explosive atmosphere. This design concept is reflected in the equipment marking by the symbol ‘Ex m'. Equipment designed to this concept is suitable for use in ‘Zone 1' and ‘Zone 2' classified hazardous areas. Electrical Equipment Marking The electrical equipment that has been assessed and tested and, found to be in compliance with the relevant European Harmonized Standard is marked with the certification coding as described in the aforementioned standards.
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