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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 designed not to create an ignition risk 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 conform to one of a set of detailed standards, parts of BS 60079 and in some cases, certified as meeting that standard. Independent test houses (known as Notified Bodies)are established in most European countries, and a certificate from any of these will be accepted across the EU. The DTI appoint and maintain a list of Notified Bodies within the UK, of which Baseefa is the most well known. 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. Typical gas hazards are from hydrocarbon compounds. ;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 of causing an explosion and are more of a fire risk (although on rare occasions gas explosions in domestic property are known to occur). Safe area on chemical and other plant are present where the hazardous gas is diluted to a concentration below 0.25% of its lower explosive concentration limit. ;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 under abnormal conditions). As a general guide, unwanted substances should only be present under 10 hours/year or 0–0.1% of the time. Explosion safety compliant equipment, should be used. ;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. As a guide this can be defined as 10–1000 hours/year or 0.1–10% of the time. Explosion safety equipment that has a higher safety level than Zone 2 equipment must be used. ;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. An old system of area classification to a british standard used a system of letters to designate the zones. This has been replaced by a European numerical system, as set out in directive 1999/92/EU implemented in the UK as the Dangerous Substances and Explosives Atmospheres Regulations 2002 The boundaries and extent of these three dimensional zones should be decided by a competent person. There must be a site plan drawn up of the factory with the zones marked on. The zone definitions are: ;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. ;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. ;Zone 22: a place in which an explosive atmosphre in the form of a cloud of combustible dust is not likely to occur in normal operation, but if it does occur will persist for a short period only Guidance on assessing the extent of zones is given 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 :Representative gas for group IIA. ; Ethylene :Representative gas for group IIB. ; Hydrogen / Acetylene :Representative gas for 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. The groups also indicate how much energy is required to ignite the gas by spark ignition, Group IIA requiring th emost energy and IIC 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 Disulfide :102 °C Auto-ignition 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 protection level categories according to manufacture method and suitability for different situations. Category 1 is the highest safety level and Category 3 the lowest. Although there are many types of protection, a few are detailed below. 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 (Ingress Protection) 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. Eventually there will be a third number which will deal with impacts and the amount of joules of impact that any particular enclosure can withstand. 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 2.5 mm thick. ;4:Protection against small solids and foreign objects greater than 1 mm 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 equipment on a ship in heavy seas. ;7:Protection against being immersed in water at certain pressure and time. ;8:Protection against prolonged immersion in water under defined pressure and time. HAZARDOUS LOCATIONS CLASSIFICATIONS DESCRIPTIONS - NORTH AMERICAN TERMS Class 1, Div. 1 - Where ignitable concentrations of flammable gases, vapors or liquids can exist all of the time or some of the time under normal operation conditions. Class 1, Div. 2 - Where ignitable concentrations of flammable gases, vapors, or liquids are not likely to exist under normal operating conditions. Class 2, Div. 1 - Where ignitable concentrations of combustible dusts can exist all of the time or some of the time under normal operation conditions. Class 2, Div. 2 - Where ignitable concentrations of combustible dust are not likely to exist under normal operating conditions. Class 3, Div. 1 - Where easily ignitable fibers or materials producing combustible flyings are handled, manufactured or used. Class 3, Div. 2 - Where easily ignitable fibers are stored and handled. |
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