BS 5958-1-1992 Code of practice for control of undesirable static electricity - General considerations《有害静电控制惯例 第1部分 总则》.pdf

1、BRITISH STANDARD BS 5958-1: 1991 Code of practice for Control of undesirable static electricity Part 1: General considerationsBS5958-1:1991 This British Standard, having been prepared under the directionof the General Electrotechnical Standards PolicyCommittee, was publishedunder the authority ofthe

2、 Standards Board and comesinto effect on 31January1992 BSI 12-1998 First published July 1980 Second edition January 1992 The following BSI references relate to the work on this standard: Committee reference GEL/118 Draft for comment 90/24919 DC ISBN 0 580 20017 5 Committees responsible for this Brit

3、ish Standard The preparation of this British Standard was entrusted by the General Electrotechnical Standards Policy Committee (GEL/-) to Technical Committee GEL/118, upon which the following bodies were represented: British Cable Makers Confederation British Plastics Federation British Textile Tech

4、nology Group Chief and Assistant Chief Fire Officers Association ERA Technology Ltd. Engineering Equipment and Materials Users Association General Council of British Shipping Health and Safety Executive Institute of Petroleum Institution of Electrical research on the subject continues and new inform

5、ation is still coming to light. 1) BS CECC 00015-1 is in preparation and will supersede BS 5783:1987.BS5958-1:1991 iv BSI 12-1998 A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compl

6、iance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, pages 1 to 46, an inside back cover and a back cover. This standard has been updated (see copyright date) and may hav

7、e had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS5958-1:1991 BSI 12-1998 1 Section 1 Section 1. General 1 Scope This Part of BS 5958 gives basic information on the generation of undesirable static electricity in solids, liquids and gases, and a

8、lso on persons, and describes how the charges produced give rise to discharges which may cause ignitions or electric shocks. It gives the methods available for minimizing such generation and for safely dissipating the charges produced. Section 4 deals with the principles of earthing and bonding as a

9、pplied to avoid static electricity hazards. Appendix A gives guidance on the means of measuring or estimating the parameters which need to be taken into account when deciding upon the action to be taken to control static electricity. NOTEThe titles of the publications referred to in this standard ar

10、e listed on the inside back cover. 2 Definitions For the purposes of this British Standard, the following definitions apply. 2.1 antistatic; conductive used as an adjective to indicate that a material is, by virtue of its low resistivity, incapable of retaining a significant electrostatic charge whe

11、n in contact with earth NOTEThe terms “antistatic” and “conductive” are frequently used as synonyms, hence the common definition. However, in contexts where they have specific meanings, e.g. in relation to footwear, this definition is qualified in the appropriate clauses of this Part of BS 5958. 2.2

12、 antistatic additive a substance added to a liquid or a solid in order to increase its conductivity so that it is incapable of retaining a significant electrostatic charge when in contact with earth 2.3 bonding the use of an additional independent connection between conductors to provide electrical

13、continuity when this cannot otherwise be ensured 2.4 breakdown strength the minimum electric field strength in a medium at which electrostatic discharges can occur in a given situation 2.5 conductivity the reciprocal of resistivity 2.6 conductor a material possessing an electrical conductivity suffi

14、ciently high to render it incapable of retaining an electrostatic charge unless insulated from earth 2.7 earthing the electrical connection of a conductor to the main body of the earth to ensure that it is at earth potential 2.8 flammable capable of being ignited 2.9 flammable material a gas, vapour

15、, liquid, dust or solid that can react continuously with atmospheric oxygen and that may therefore sustain fire or explosion when such reaction is initiated by a suitable spark, flame or hot surface NOTE 1Many liquids and solids, though regarded as flammable, nevertheless do not normally burn. The a

16、pplication of heat to such materials serves to release vapour that may burn with atmospheric oxygen. The heat of the subsequent reaction serves to release further vapour for combustion. Flame may propagate through suspensions of dusts by this mechanism. NOTE 2In normal usage “gas” and “vapour” are s

17、ynonymous. 2.10 flammable mixture; explosive mixture a mixture with air of a flammable material, in the form of a gas, droplets or dust, which is within the flammable range and therefore capable of being ignited 2.11 flammable range the range of concentrations in air of a flammable material, in the

18、form of a gas, droplets or dust, between the lower and upper flammable limits 2.12 hazardous area an area in which flammable or explosive gas-air mixtures are, or may be expected to be, present in quantities such as to require special precautions against ignition hazardous areas are classified into

19、the following three zones (see BS 5345-1) a) Zone 0: zone in which an explosive atmosphere is continuously present, or present for long periods. b) Zone 1: zone in which an explosive atmosphere is likely to occur in normal operation. c) Zone 2: zone in which an explosive atmosphere is not likely to

20、occur in normal operation, and if it occurs it will exist only for a short time.BS5958-1:1991 2 BSI 12-1998 Section 1 2.13 incendive capable of igniting a prescribed flammable mixture 2.14 inert gas a gas or a mixture of gases incapable of supporting the combustion of a flammable material 2.15 ion a

21、n atomic or molecular particle carrying electrical charge 2.16 lower flammable limit the concentration in air of a flammable material, in the form of a gas, droplets or dust, below which there is insufficient of the material to support and propagate combustion 2.17 mass charge density the nett quant

22、ity of charge carried by unit mass of a material 2.18 minimum ignition energy the smallest quantity of energy that can ignite a mixture of a specified flammable material with air or oxygen, measured by a standard procedure 2.19 non-conductor a material possessing an electrical resistivity sufficient

23、ly high to enable it to retain enough charge to give rise to electrostatic effects 2.20 relaxation the dissipation of an electrostatic charge by conduction 2.21 relaxation chamber a part of a liquid flow system which provides sufficient residence time for the charge on the liquid to be reduced to a

24、safe level 2.22 relaxation time the time required for the charge or potential on a liquid or a solid to decay to 1/e of its original level (e is the base of natural logarithms) 2.23 surface charge density the nett quantity of charge per unit area of the surface of a solid or a liquid 2.24 surface re

25、sistivity (of a substance) the resistance between the opposite edges of a square 2.25 upper flammable limit the concentration in air of a flammable material, in the form of a gas, droplets or dust, above which there is insufficient of the air to support and propagate combustion 2.26 volume resistivi

26、ty (of a substance) the resistance at unit length and unit cross-sectional areaBS5958-1:1991 BSI 12-1998 3 Section 2 Section 2. Fundamentals of static electricity 3 Electrostatic charging 3.1 General The primary source of electrostatic charge is contact electrification, where two dissimilar material

27、s are brought into contact and then separated, carrying equal and opposite charges. Conducting objects may also become charged by the process of induction from another charged object or objects in the vicinity. Objects can also receive charge by direct transfer either from other objects or by the im

28、pingement upon them of a stream of ions. 3.2 Contact electrification Contact electrification can occur at solid/solid, liquid/liquid or solid/liquid interfaces. Gases cannot be charged in this way, but if a gas has solid particles or liquid droplets in suspension these may be charged by contact, so

29、that such a gas can carry an electrostatic charge. In the case of dissimilar solids, initially uncharged and at the same, normally earth, potential, a small amount of charge is transferred from one material to the other when they make contact. The two materials are therefore oppositely charged and t

30、here is a potential difference between them which is of the order of 1V. If the materials are then separated, work has to be done to overcome the attraction between the opposing charges and the potential difference between them therefore increases. This higher potential difference tends to drive cha

31、rge back across the interface whilst any contact remains. In the case of two conductors the recombination of charges is virtually complete and no significant amount of charge remains on either material after separation. If one material, or both, is a non-conductor, the recombination cannot take plac

32、e completely and the separating materials hold part, at least, of their charges. Because the clearance between the surfaces when in contact is extremely small, the potential generated on separation can easily reach many kilovolts despite the small amount of charge involved. For reasons not fully und

33、erstood the charging is enhanced if the contact and separation involves rubbing. Contact electrification in liquids depends on the presence of ions or sub-microscopic charged particles (the latter are usually less important). Ions (or particles) of one polarity are adsorbed at the interface and they

34、 then attract ions of opposite polarity which form a diffuse layer of charge close to the surface. If the liquid is then moved relative to the interface, it carries away this diffuse layer, thereby bringing about separation of the opposing charges. As in the case of solids a high voltage is generate

35、d because of the work done to bring about the separation, provided that the liquid is sufficiently non-conducting to prevent recombination. Such processes can occur at both solid/liquid and liquid/liquid interfaces. 3.3 Charging by induction There is an electric field around any charged object. A co

36、nductor introduced into this field changes the distribution of potential in the field in its vicinity and at the same time there is a separation of opposing charges in the conductor under the influence of the field. If it is insulated the conductor takes up a potential, dependent upon its position i

37、n the field, and is said to be charged by induction. By virtue of its potential, coupled with the separated charges that it carries, the conductor is capable of an electrostatic discharge. If, while it is in the field, the conductor is momentarily earthed, its potential is reduced to zero, but an im

38、balance of charges remains on it. Removal of the conductor from the vicinity of the original charged object, however, makes the remaining charge available to provide a spark. This type of induction sparking can be hazardous, for example in the case of an insulated person moving about near electrosta

39、tically charged materials. 3.4 Charge transfer Whenever a charged object makes contact with one that is uncharged, the charge is shared between them to the extent that their conductivities allow. This is a potent source of electrostatic charging when charged sprays, mists or dusts impinge upon a solid object, for example by settling. A similar transfer of charge can also take place when a stream of gaseous ions is incident upon an initially uncharged object.