Health and safety research 1975 : an account of the executive's research testing and laboratory services incorporating the annual report of the Safety in Mines Research Establishment.

  • Health and Safety Executive
Date:
1976
Catalogue details

Licence: Open Government Licence

Credit: Health and safety research 1975 : an account of the executive's research testing and laboratory services incorporating the annual report of the Safety in Mines Research Establishment. Source: Wellcome Collection.

    21/66 (page 11)
    compliance with sFA 3012. The remaining four items were environmental-monitoring systems which were accepted for use underground under the terms of letters of no objection or of experimental approval. 85 In addition, sixteen minor variations to previously certified equipment were considered and accepted by letter. 86 The total number of systems examined represents a marked increase over the past few years; the only time this number was surpassed was in 1962 when 161 systems were considered. Flameproof equipment Maximum experimental safe gap (MESG) 87 The 1974 Annual Report (p 26) referred to a computer program that was developed to follow the progress of chemical reaction of a static flammable mixture. Some experimental results for the ignition of methane—-oxygen—argon mixtures in a shock tube have now been compared with corresponding results gen- erated by the computer program. The two groups of workers who obtained the experimental results (Seery and Bowman, 1970; Liftshitz and others, 1971) fitted lines to their data. These lines are shown in fig 5; the function of pre-ignition time and initial oxygen and methane concentrations are those that these workers found to give the best fit to their experimental results. 0-5 0-6 1027-1 sakk 0:8 i 10? 107! 10° 107% 1078 SSM 5 it 10 +1 T =pre-ignition time : s [ ] = initial concentration : mo!/ml Lines show experimental results from (a) Seery and Bowman (1970) (b) Lifshitz and others (1971) Dots show results of SMRE computer ‘experiments’ Fig 5 Ignition in methane-oxygen—argon mixtures in shock tubes. Electrical equipment 11 The points shown on the figures are the results of RLSD computer ‘experiments’ covering a wide range of initial temperatures, pressures and mixture composi- tion. As can be seen, the experimental and computer results agree fairly well. The computer results are from the same series of computer experiments, but use different definitions for the end of the pre-ignition period corresponding to those used by the two groups of workers; as mentioned in the 1974 Annual Report (p26), one object of the work is to define the end of the pre-ignition period in an MESG experiment. Hazards from the volatilisation of insulating materials in flameproof equipment 88 Work described in previous Annual Reports on problems associated with electrical arcing across the surface of organic plastic insulating materials in flame- proof enclosures has now been concluded. It must be emphasised that all these materials, if they are sub- jected to a severe electrical fault, which causes surface tracking or arcing, are capable of producing sufficient flammable gas to burst a flameproof enclosure or to create an ignition or a fire hazard outside the enclosure. 89 They can, however, be divided into three groups, according to their suitability for use as insulators. With the first group of materials, an arc initiated under our standard test conditions (1972 Annual Report, p 30) would self-extinguish. These materials, which include certain types of glass reinforced polyester-bonded ‘dough moulding compounds’, are the most suitable for use as insulators. The second group contains materials (e.g. urea-formaldehyde) that, while they allow the test arc to burn for the duration of the applied voltage, do not cause it to restrike after the current has been momentarily switched off. The third group contains almost all the phenolic materials and these cause the arc to restrike after the current has been momentarily switched off. The materials in this last group should be avoided for insulating purposes in flameproof enclosures whenever possible, and especially if subject to electrical stress at high prospective fault levels. 90 The results of this programme of research have been published (Lord and Barbero, 1975). 91 Work on the development of a bi-stable tempera- ture-sensitive device that can detect a short-circuit arc in a flameproof enclosure and make or break an electric circuit in less than two seconds was described in last year’s Annual Report (p28). Tests on the device have been completed, with satisfactory results, and it is now being examined by NCB. 92 The apparatus used for the plastics experiments and the low power level of the arc, together with the