Group I for which the typical or representative gas is methane , is reserved for mining application olny and is therefore of interest olny to mining electrical engineers .
Group 11A is for gases with properties similar to propane and require more than 200 mili joules of energy to ignite .
Group IIB is for gases with properties similar to ethylene ( >60 mili joules to ignite ).
Group IIC (> 20 mili joules to ignite is for the most haardous gases , of which the typical one is hydrogen . These categories relate to the minimum ignition energy in milijoules required to cause ignition , at the most volatile gas air mixture .
It will be noted that the zones are numbered 0-2 in decreasing order of risk wherea the groups are numbere I-IIC in increasing order of risk . These classificication deal with the magnitude and type of risk .
Equipment designed for use in hazardous areas is itself classified according to the method used for achieving protection . Each types of protection is referred to by a letter .
Types d refers to equipment with a flameproof enclosure . The principle adopted with this types of protection is that a spark inside equipment should not cause fire outside it , It is not practicable to design equipment so that no air or vapour can get inside it . It is , however , possible to design it so that the air gaps between inside and outside are so narrow and so long that any flame starting inside will be extinguished before it has travelled to the outside . This is the method used for type d equipment .
Type e is a method of protection which applies olny to non-sparking equipment . The design of the equipment is such as to keep temperatures low and givee increased protection against mechanical damage which could cause an electrical fault . ZTHis is achieved by such features as non-sparking cable terminations , additional insulation , increased creepage and clearance distances , and , in the case of liminaires , special lampholdrs . The requirement of low internal temperature makes it inapplicable to heavily rated machines .
Type N is similar to type e but has a reduced level of protection . Consequently , whereas type e equipment can be used in Zone 1 , type N equipment can be used olny in zone 2 .
With type p protection , the enclosure of the equipment contains air or an inert gas at a pressure sufficient to prevent the surrounding vapour entering the enclosure . Since no enclosure is completely vapour -tight , there must be some eleakge out of the enclosure . Equipment suitable for this method of operation must be capable of withstanding the necessary internal pressure , and must be connected to a network of compresed air or gas which contains a low-pressure switch to disconnect the electrical supply in the event of loss of pressure .
Intrinsically safe equipment limits the energy available for causing ignition . The maximum current which may flow depends on the voltage but , in general , most intrinsically safe equipment is designed for operation on extra low voltages . It is permissible to limit the current by means of a barrier diode . This type of protection is designaed type i ; it may be designated type ia or type ib according to the number of faults it can sustain during testing .
Table 1.1 is based on BS EN 60079-14:1997 : part 1 , Electrical apparatus for explosive gas atmospheres . Electrical instalations in hazardous areas ( other than mines ) show which type of equipment may be used in which zone . Within a zone in which it is permitted , type e , N , or p equipment may be used with gas of any group . EQUIPMENT WITH TYPE d or i protection is further subdivided according to the group for which it is safe .
These provisions deal with the risks of ignition arising from operation of equipment under normal or fault conditions . It may also be necessary to limit surface temperatures . The safe temperature in an area does not necessarily depends on the magnitude or type of risk , and an additional classification is usual . Table 1.2 is based on BS 4683 : Part 1/ BS EN 50021 Electrical apparatus for potential explosive atmospheres . Types of protection 'n' , show the classes which are used to designate the maximum permitted surface temperature . It is possible for equipment having any type of protection to have any temperature classification , although one would not expect intrinsically safe equipment to have a lower class than T5 or T4 while it is difficult for other types of equipment to achieve classes T5or T6 . It should be noted that there is no real relationship between minimum ignition temperature and maximum surface temperature permitted . Hydrogen is a class IIC gas but its .
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