1、BRITISH STANDARD Electrical apparatus for use in the presence of combustible dust - Part 2-1: Test methods - Methods of determining minimum ignition temperatures The European Standasd EN 50281-2-1:1998 has the status of a British Standard ICs 29.260.20 BS EN 50281-2-1:1999 NO COPYING WITHOUT BSI PER
2、MISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW STI3.BSI BS EN 50201-2-II-ENGL 1799 II Lb24bb7 0777b7 872 W direction of the Electrotechnical No. Sector Committee, was published under the authority of the Standards Committee and comes into effect on 15 May 1999 BS EN 50281-2-1:1999 D - present to the r
3、esponsible European committee any enquiries on the - monitor related international and European developments and promulgate interpretation, or proposals for change, and keep the UK interests informed them in the UK. A list of organizations represented on this committee can be obtained on request to
4、its secretary. Cross-references The British standasdc which implement international or European publications referred to in this document may be found in the BSI Standards Catalogue under the section entitled “Internationai Standards Correspondence Index“, or by using the “Find“ facility of the BSI
5、Standards Electronic Catalogue. A British Standard does not purport to include all the necessary prcrvisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary
6、of pages This document comprises a front cover, an inside front cover, the EN title page, pages 2 to 24, an inside back cover and a back cover. O BSI 05-1999 I l ISBN O 580 32398 6 STD*BSI BS EN 50281-2-1-ENGL 1799 1b24bb7 0777b70 574 E EUROPEAN STANDARD EN 50281-2-1 NORME EUR0PEE”E EUROP - Method B
7、 Dust cloud in a furnace at a constant temperature (clause 5). Method A determines the minimum ignition temperature of a dust layer on a prescribed heated surface. Method B determines the minimum ignition temperature of a dust cloud within a prescribed heated funiace. The test methods are of a gener
8、al nature, and may be used for purposes of comparison, but in certain industrial situations further tests may be necessaxy The methods for determining the minimum ignition temperatures are not suitable for use with recognized explosives, for example, gunpowder, dynamite, or mixtures of substances wh
9、ich max under some circumstances, behave similarly. Where there is doubt, an indication of the existence of a hazard due to explosive properties may be obtained by testing a very sma quantity of the dust on a surface at 400 “C or higher, located remotely from the operator. 1 Scope This European Stan
10、dard specifies two test methods for determining the minimum ignition temperatures of dust for the purpose of selecting electrical apparatus for use in the presence of combustible dust in accordance with EN 50281-1-2:1998 and constructed in accordance These methods are not suitable for use with subst
11、ances having explosive properties. Method A (clause 4) is applicable to the determination of the minimum temperature of a prescribed hot surface which will result in the decomposition and/or ignition of a layer of dust of a specified thickness deposited on it. The method is particuiarly relevant to
12、industnal equipment with which dusts are present on hot surfaces in thin layers exposed to the atmosphere. Method B (clause 5) is applicable to the determination of the minimum temperature of a prescribed hot surface which will result in the ignition of a cloud of given sample of dust or other parti
13、culate solid. The test is intended to be carried out as a complementary test after determining the minimum ignition temperature of a dust layer by method A of this European Standard with EN 50281-1-1:1998. Page 3 EN 50281-2-11998 NOTE 1 Concerning method B because the method of operation of the furn
14、ace gives short residence times for dust particles within it, this method of test is applicable to industrial equipment where dust is present as a cloud for a short time. This method of test is of small scale and the results are not necessarily representative of all industnal conditions. NOTE 2 Conc
15、erning method B the method is not applicable to dusts which may, over a longer period of time than provided for in the test method, produce gasses from deposits generated during pyrolysis or smouldering. 2 Normative references EN 50281-1-1, Electrical apparatus for use in the presence of combustible
16、 dust - Part 1-1: Electrical apparatus protected by enclosures - Construction and testing. EN 50281-1-2, Electrical apparatus for use in the pesertce of combustible dust -Part 1-2: Electrical apparatus protected by enclosures - Selection, installation and maintenance. IS0 565, Test sieues - Metal wi
17、re cloth, or b) a temperature of 450 OC is measured or c) a temperature rise of 250 K above the temperature of the heated plate mgure 3C), is measured. With regards to items b) and c) above, ignition shall not be considered to have occurred if it can be shown that the reaction does not propagate to
18、glowing or flaming. The temperature shall be measured by thermocouple (see 4.2.2). It wil usually be found that, provided the temperature of the heated surface is high enough, the temperature in the layer will slowly increase to a maximum value which may be in excess of the temperature of the heated
19、 surface and then slowly fail to a steady value below the temperature of the heated surface (Figure 3B). NOTE 2 Some materials exhibit more than one stage of self-heating, and it may sometimes be necessary to prolong the test in order to fully explore this possibility. With organic dusts, combustion
20、 will usually take the form of charring followed by the appearance of smouldering with glowing which will progress through the layer and leave a residue of ash. With dust layers composed of certain divided metals, ignition may be chmtensed by the relatively sudden appearance of highly incandescent s
21、mouldering combustion progressing rapidly through the layer. In the determination of the minium ignition temperature for a layer of given thickness, repeated trials shall be carried out, using a fresh layer of dust each time and with up-anddown aciJustments to the temperature of the heated surface u
22、ntil a temperature is found which is high enough to cause ignition in the layer but which is no more than 10 K higher than a temperature which fails to cause ignition. The highest temperature at which ignition fails to occur shail be confirmed by continuing the test long enough to establish that any
23、 self-heating is decreasing in rate; that is, the temperature at the point of measurement in the layer is decreasing to a steady value lower than the temperatwe of the heated surface. 4.3.2 Method The apparatus shall be set up in a position free from draughts, and preferably under a hood capable of
24、extracting smoke and fumes. The temperature of the heated surface shall be acjjusted to the desired value and shall be allowed to become steady within the prescribed limits of 4.2.lb). A metal ring of the required height shall be placed centrally on the heated surface and this ring shall be frlled w
25、ith the dust to be tested and leveled off with a period of 2 min. The recorder for the dust layer thermocouple shall then be started. The test shall be continued until it is ascertained either that the layer has ignited, either visually or by the thermocouple record, or has self-heated without ignit
26、ing and is subsequently cooling down. If, after a period of 30 min, no seif-heating is apparent the test shall be terminated and repeated at a higher temperature. If ignition or self-heating occurs the test shall be repeated at a lower temperature, if necessary, prolonging the test beyond 30 min. Te
27、sting is continued until a temperature is found which is high enough to cause ignition or self-heating in the layer, but which is no more than 10 K higher than a temperature which fads to cause ignition or self-heating. 4.3.3 Results Tests shall be repeated with fresh layers of dust until a minimum
28、ignition temperature has been determined This shall be the lowest temperature, rounded down to the nearest integrai multiple of 10 OC, at which ignition occurs in a layer of given thickness. Where ignition has been deemed to occur, from readings of the test thermocouple (4.3.1), the minimum ignition
29、 temperature shall be the lowest such temperature, rounded down to the nearest integral multiple of 10 OC, less 10 K The hghest value of temperature at which ignition does not occur, or is deemed not to occur, shall also be recorded This temperature shall not be more than 10 K lower than the minimum
30、 temperature at which ignition does occur, or is deemed to occur, and it shall be confinned by at least three tests. For the purposes of this European Standard, the tests shaii be discontinued if ignition of a dust layer does not occur below a heated surface temperature of 400 OC. This fact shall be
31、 reported as the result of the test. O BSI 05-1999 Page 6 EN 58281-2-1:1998 Surface temperature “C 180 170 160 160 160 150 Times to obtain ignition, or times to the maximum temperature reached in the case of no ignition, shall be measured to the nearest 5 min from the time of placing the dust layer
32、on to the heated surface, and shall be reported. Where a dust layer fails to ignite at a temperature of less than 400 OC, the maximum duration shall be reported. 4.4 Test acceptance criteria Results obtained by the same operator on different days and results obtained in different laboratories shall
33、be considered unsatisfactory if they give ignition temperatures differing by more than 10 K in either case. Where validity of test results may sometimes be poor for reasons associated with the physical nature of the dusts and the behaviour of layers during test. This shall be reported (see 4.5) and
34、all results shall be accepted as equally did. The test report shall then include a brief description of the nature of the combustion following ignition, noting especially behaviour such as unusually rapid combustion or violent decomposition. Factors likely to affect the significance of the results s
35、hall also be reported these include difficulties in the preparation of layers, distortion of layers during heating, decrepitation, melting, and evidence of flammable gas generated during heating of the dust. 4.6 Reporting of results The test report shall include the name, source and description (if
36、not implicit in the name) of the material tested, the date and identification of the test, the ambient temperature and the density of the material as tested (4.2.6). The report shall state that the determination of minimum ignition temperature of the dust layer has been carried out in accordance wit
37、h this European Standard. The ignition tests shall be reported in the manner shown in the following table (showing results in descending order of surface temperature rather than in the order in which tests were performed). The igrution temperature shall be recorded in accordance with 4.3.3 for each
38、depth of layer. Tests in which the heated surface temperature differed by more than SO K from the recorded minimum ignition temperature need not be reported. Result of test Ignition Ignition No ignition No ignition No ignition No ignition Depth Time to ignition or to reach the highest layer value of
39、 temperature without ignition min 16 36 40 38 42 62 NOTE In the example given in the above table the minimum ignition temperature for the 5 mm layer would be recorded as 170 “C. The ignition temperature shall be recorded in accordance with 4.3.3 for each depth of layer. Tests in which the heated sur
40、face temperature differed by more than SO K from the recorded minimum ignition temperature need not be reported. 4.6 Application of results The values of minimum ignition temperatiire determined in accordance with method A of this European Standard apply to layers having the thicknesses used in the
41、tests. Although for some materials it is possible to estimate the minimum temperatures of a heated surface for the ignition of layers of a given dust of intermediate or greater thickness, by hear interpolation or extrapolation of the test results plotted as the logarithm of the thickness versus the
42、reciprocal of the minimum ignition temperature in kelvins, it is preferable to test with the required thiclmess. NOTE 1 The above is the simplest predictive procedure which has some theoretical justification. More elaborate treatment, based on thermal explosion theory will permit estimates for ignit
43、ion of layers in other configurations, such as layers on curved surfaces. However, if it is desired to make accurate predictions for ignition under widely different conditions of exposure, in particular exposure to a symmetrical high temperature environment rather than to an unsymmetrical environmen
44、t as on a hot plate, it is preferable to use results obtained for an experimental procedure matching the different environment more closely - such as ignition in an oven. NOTE 2 When extensive prediction is intended, it is desirable to determine ignition temperatures for more than two thicknesses of
45、 layer and with an emphasis on thicker layers. 5 Method B: dust cloud in a furnace at a constant temperature 6.1 Preparation of dust sample The sample shall be prepared so as to be homogeneous and representative of the dust received for consideration. The dust sample to be tested shall, in general,
46、be able to pass through a woven metal wire cloth or a square hole perforated plate test sieve with a nominal size of aperture of 71 pm (supplementary sizes. IS0 565). If it is necessary to test a coarser dust, passing a test sieve with a nominal size of aperture up to 500 Fm, the fact shall be state
47、d in the report of the test. Any apparent changes noted in the properties of the dust during preparation of the sample, for example, by sieving or owing to temperature or humidity conditions, shall be stated in the report of the test. O BSI 05-1999 5.2 Test apparatus Details of construction of the t
48、est apparatus are shown in Figures B.1 to B.lO, and in Table B.l. The heated silica tube of the furnace is vertical, and is open to atmosphere at its lower end. The upper end connects, by a glass adaptor, to the dust holder. Dust is dispersed into the furnace by opening a solenoid valve, which relea
49、ses compressed air from the reservoir. The furnace is mounted on a stand, enabling the lower end of the fumace tube to be readily observed. A mirror is placed below the tube to enable the interior of the funiace tube to be viewed. The thermocouples used are calibrated on a regular basis so as to maintain temperature measurements above 500 OC to fl %, and measurements below 300 OC f 3%. After assembly of the test apparatus, its accuracy shall be compared with results obtained elsewhere for a powder such as lycopodium. 5.3 Procedure 5.3.1 Setting up The apparatus shd be set up in an en
