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However, for example, in some domestic light switches you can see the arc inside when the switch is being operated, especially in the case of Fluorescent Lighting . In a chemical factory or refinery the electrical equipment obviously must be made to a much more robust industrial standard and under no circumstances must arcs or sparks be visible to the outside atmosphere because of any explosive gases, vapours or dusts that might be present around the equipment. The above industrial electrical equipment that is to be put into the hazardous area has to be made to a certain standard and given a ''certificate'' and ''stamp of approval''. The UK body that oversees all of these standards is an organisation called BASEEFA which is part of the factory inspectorate. ZONES (VAPOURS & GASES) If you look at the size of a Refinery or Chemical Factory and the amount of liquids and gases that circulate the various processes in that plant there must be a certain amount of risk of leaks and other hazards. In some cases the gas, vapour or dust is present all the time or for long periods. Refineries and chemical complexex should thus be divided into areas of risk of release of gas, vapour or dust known as zones. ;Safe area:A domestic domain such as a house would be classed as safe area where the only risk of a release of explosive or flammable gas would be the propellant in an Aerosol Spray . The only explosive or flammable liquid would be paint and brush cleaner. These are classed as very low risk. ;Zone 2 area:This is a step up from the safe area. In this case it has been decided that in this zone the gas, vapour or dust would only be present under abnormal conditions (most often leaks). Unwanted substances should only be present under 10 hours/year or 0–0.1% of the time. ;Zone 1 area:These areas are where special or classified electrical equipment must be used. It is expected that the gas, vapour or dust will be present or expected for long periods of time under normal running. This is defined as 10–1000 hours/year or 0.1–10% of the time. In this case there must be no sparks at all that can ignite these mediums. ;Zone 0 area:This is the worst scenario as gas or vapour is present all of the time (over 1000 hours/year or >10% of the time). Although this is the worst case it is very rare that a zone 0 area will be in the open. Usually this would be the vapour space above the liquid in the top of a tank or drum. ZONES (DUSTS) In the case of Dust s there is still a chance of explosion. In this case the areas are divided into two zones. ;Zone Z:This is an area where combustable dusts may be present in the form of a cloud under normal operation in sufficient quantity to be capable of causing an explosion with the correct mixture of air. ;Zone Y:Areas that do dot fit into the Zone Z description and deal more with layers of combustable dust which may be capable of causing an explosion with the correct mixture of air. The boundaries and extent of these zones would be decided by a committee set up by the factory. There must be a site plan drawn up of the factory with the zones marked on. The UK two zone system has now been largely replaced by the three zone European system: ;Zone 20:Dust cloud present continuously or nearly so. ;Zone 21:Dust clouds may be present in normal operation. ;Zone 22:Dust Cloud unlikely. The full definition is defined in EN61241-10. GAS GROUPS Each chemical gas or vapour used on the refinery or chemical works comes under a certain gas group. For this industry there are three: ; Propane :Categorised as gas group IIA. ; Ethylene :Categorised as gas group IIB. ; Hydrogen / Acetylene :Categorised as gas group IIC. If a piece of equipment has just II and no A, B, or C after then it is suitable for any gas group. A list must be drawn up of every chemical gas or vapour that is on the refinery/chemical complex and included in the site plan of the classified areas. The above groups are formed in order of how volatile the gas or vapour would be if it was ignited, IIC being the most volatile and IIA being the least. TEMPERATURE CLASSIFICATION Another important consideration is the temperature classification of the electrical equipment. One thing that must never be allowed to happen is the surface temperature of the electrical equipment rise beyond the auto-ignition temperature of the gas or vapour that it is put into. The temperature classification on the electrical equipment label will be one of the following (in Degree Celsius ): The above table tells us that the surface temperature of a piece of electrical equipment with a temperature classification of T3 will not rise above 200°C at an ambient temperature of 40°C. T5 will not exceed 100°C, etc. Auto-ignition temperatures (vapours & gases) The auto-ignition temperature of a liquid, gas or vapour is the temperature which the substance will ignite automatically by itself without any external heat source. Such temperatures for common substances are: ; Hydrogen :560°C ; Ethylene :425°C ; Acetylene :305°C ; Naptha :290°C ; Carbon Di-sulphide :102°C Auto-igition temperatures (dust) The auto-ignition temperature of a dust is usually higher than that of vapours & gases. Examples for common materials are: ; Sugar :460°C ; Wood :340°C ; Flour :340°C ; Grain dust:300°C ; Tea :300°C TYPE OF PROTECTION To ensure safety in a given situation, equipment is placed into categories according to manufacture method and suitability for different situations. Dual Protection Many items of equipment use dual protection, so a label could be 'de' and you might find that the case of the equipment is made to EEx 'e' and any switches that are a part of the equipment are EEx 'd'. An example of this would be a socket outlet. THE 'IP' CODE The IP Code is the ability of the equipment to keep out unwanted liquids and solids. The protection comes in the form of two numbers, for example: IP54. The first number (0-6), in the previous example '5', is the degree of protection against solids, and the second number (0-8) is the degree of protection against liquids. Solid protection ;0:No protection from ingress of solids. No personnel protection from live parts. ;1:No protection from large solids. ;2:Protection against medium sized solids and foreign objects the thickness of a finger. ;3:Protection against small solids and foreign objects greater than 2.5mm thick. ;4:Protection against small solids and foreign objects greater than 1mm thick. ;5:Protection against dust in such amounts to interfere with operation. ;6:Complete protection against dusts and complete personnel protection. Liquid protection ;1:No protection from ingress of water. ;2:Protection against droplets of water if equipment is stood correct way up. ;3:Protection against droplets of water if equipment is tilted 15° or more. ;4:Protection against liquids splashed from any direction. ;5:Protection against water jets. ;6:Protection against equiment on a ship in heavy seas. ;7:Protection against being immersed in water at certain pressure and time. ;8:Protection against indefinate immersion in water at certain pressure and time. Eventually there will be a third number which will deal with impacts and the amount of joules of impact that any particular enclosure can stand. |
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