BS 4444-1989 Guide to electrical earth monitoring and protective conductor proving《电气接地监测和保护导线检验指南》.pdf

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1、BRITISH STANDARD BS 4444:1989 Guide to Electrical earth monitoring and protective conductor proving UDC 621.316.99:621.396.66BS4444:1989 This British Standard, having been prepared under the direction of the Power Electrical Engineering Standards Policy Committee, was published under the authority o

2、f the Board of BSI and comes into effect on 31December1989 BSI 03-1999 First published May1969 Second edition The following BSI references relate to the work on this standard: Committee reference PEL/11 Draft for comment 88/26607 DC ISBN 0 580 17882 X Committees responsible for this British Standard

3、 The preparation of this British Standard was entrusted by the Power Electrical Engineering Standards Policy Committee (PEL/-) to Technical Committee PEL/11, upon which the following bodies were represented: ASTA Certification Services Association of Control Manufacturers (TACMA) (BEAMA Ltd.) Britis

4、h Lawn Mower Manufacturers Federation Consumer Policy Committee of BSI Department of Trade and Industry (Consumer Safety Unit, CADivision) Electrical Installation Equipment Manufacturers Association (BEAMA Ltd.) Electricity Supply Industry in England and Wales Electronic Components Industry Federati

5、on Health and Safety Executive Institution of Electrical Engineers National Inspection Council for Electrical Installation Contracting Portable Electric Tool Manufacturers Association Amendments issued since publication Amd. No. Date of issue CommentsBS4444:1989 BSI 03-1999 i Contents Page Committee

6、s responsible Inside front cover Foreword ii 0 Introduction 1 1 Scope 1 2 Definitions 1 3 Earthing proving systems 2 4 Earth monitoring systems 3 5 Practical arrangements 3 6 Cables 4 7 Plugs, socket outlets, cable couplers and appliance connectors 4 8 Multi-outlet systems 6 Appendix A Devices for e

7、arth fault protection 7 Appendix B Data processing equipment 7 Figure 1 Basic circuit for earth proving unit 2 Figure 2 Basic circuit for earth monitoring unit 5 Figure 3 Basic circuit for earth proving and monitoring unit 5 Figure 4 Typical arrangement of earth monitoring units 7 Figure 5 Typical a

8、rrangement of functional earthing system 8 Publications referred to Inside back coverBS4444:1989 ii BSI 03-1999 Foreword This British Standard has been prepared under the direction of the Power Electrical Engineering Standards Policy Committee. It supersedes BS4444:1969 which is withdrawn. This stan

9、dard has been revised to take account of the latest developments in the field of electrical earth monitoring. 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. Compliance with a British

10、 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 and ii, pages1 to 8, aninside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incor

11、porated. This will be indicated in the amendment table on theinside front cover.BS4444:1989 BSI 03-1999 1 0 Introduction The possibility of damage to, disconnection from, or a rise in impedance of, any protective conductor, cannot be eliminated by periodic inspection and, in any event, accidental da

12、mage could occur between inspections. For example, trailing cables supplying portable or transportable equipment such as conveyors, pumps and generators, are particularly vulnerable to faults in the protective conductor due to mechanical damage or excessive strain. Earth monitoring may be applied in

13、 certain special applications in fixed installations where the integrity of the protective conductor is vital to the safety of the system and where protective conductors are particularly vulnerable. Earth monitoring devices are intended to maintain a continuous check on the integrity of the protecti

14、ve earth conductor. They do not of themselves ensure protection against earth fault currents which would normally be provided by overcurrent protective devices or by some forms of earth fault circuit breaker. If the earth connection is severed or deteriorates and if the appliance or equipment develo

15、ps an earth fault then the overcurrent protective device used to give protection against indirect contact will not operate and the user may receive an electrical shock which, if not fatal in itself, may cause a fall from a height or against moving machinery. The various devices covered by this guide

16、 provide methods of checking the integrity of the earth connections and establish whether the earth return paths are in a healthy state to carry earth fault currents. High resistance earth fault currents due to insulation deterioration are not normally detected by overcurrent protective devices such

17、 as fuses and miniature circuit breakers. In these cases a residual current device should be used in conjunction with the earth monitoring or proving device to provide protection. 1 Scope This British Standard explains the principles involved in the design and construction of earth monitoring, earth

18、 proving and insulation monitoring devices, together with the requirements for their safe use and application. Systems of earth fault protection are given inAppendix A and special requirements for protection of data processing equipment are described inAppendix B. NOTEThe titles of the publications

19、referred to in this standard are listed on the inside back cover. 2 Definitions For the purposes of this British Standard the following definitions apply. 2.1 earth proving system a system for providing a means of maintaining a high degree of confidence in the continuity of a protective conductor wi

20、thin the installation 2.2 earth monitoring system a system for providing a means of maintaining a high degree of confidence in the measured impedance of a protective conductor forming part of the earthing arrangements of an electrical system 2.3 functional earthing system a system of earthing provid

21、ed in an installation where earth currents flow due to the normal function of load apparatus NOTEThis system is additional to the protective earthing arrangements. 2.4 residual current protection system a system for providing a means of detecting and interrupting the supply in the event of unintenti

22、onal residual currents flowing and thereby giving additional protection against dangerous electric shock and fire hazards caused by the flow of such currents NOTEThese residual currents are usually much smaller than those required to blow a fuse or trip a circuit breaker. 2.5 voltage operated earth

23、fault system a system which uses a voltage sensing device and is designed to minimize the risk of dangerous electric shock by interrupting the supply if the apparatus casing reaches a potential substantially different from that of its surroundings NOTEA.C. voltage-operated earth leakage circuit brea

24、kers are specified in BS842. 2.6 earth leakage current a current which flows to earth, or to extraneous conductive parts, in a circuit which is electrically sound NOTEThis current may have a capacitive component including that resulting from the deliberate use of capacitors. 2.7 fault current a curr

25、ent resulting from an insulation failure or the bridging of insulationBS4444:1989 2 BSI 03-1999 2.8 earth fault current a fault current which flows to earth 3 Earth proving systems An earth proving system provides a means of establishing a degree of confidence in the continuity of the protective con

26、ductor from the proving unit to remote protected apparatus. No provision is made for impedance measuring but designs centred around the use of specialized relays afford a coarse method of proving. The proving unit may be connected anywhere between the origin of the installation and the apparatus to

27、be protected. Detection of short circuits between pilot and protective conductors may be achieved by the use of d.c. sensitive relays as part of the proving system. Continuity may be proved with a shunt connected voltage sensing device or a series connected current sensing device. In either case the

28、 protective conductor should be a substantial earth path of low impedance. The impedance of the source and detector combination Z 1(seeFigure 1) should appear in series with the pilot earth path. The total impedance of the monitored loop will be the vector sum of the impedance Z 1and the impedance o

29、f the rest of the loop Z 2 . A value of Z 2in excess of about107 should prevent the load apparatus being energized. Furthermore, after the apparatus has been energized, if the monitored loop becomes open-circuited or increases in impedance beyond about207 the apparatus should become isolated. NOTETh

30、e impedance values quoted above are only typical and need to be considered in the light of specific applications. The proving current circulates in a loop circuit which contains the protective conductor to the apparatus, a section of the apparatus casing and a suitable return path. It is essential t

31、hat any plug and socket and flexible cable therefore provides not only the main protective path but also a return path which is usually known as the pilot conductor. Key E 1 E 2 P S V Z 1 Z 2 Protective earth terminal Earth terminal for pilot circuit (this may be internal or external) Pilot conducto

32、r terminal Sensing device Extra low voltage source Impedance of source Impedance of the loops NOTEIf a diode is not used then a link has to be provided for a direct connection between terminals P and E 2 . Figure 1 Basic circuit for earth proving unitBS4444:1989 BSI 03-1999 3 It is essential that th

33、e proving unit itself be properly earthed in accordance with the Regulations for Electrical Installations 1)published by the Institution of Electrical Engineers. The requirement that the earth loop contains a section of the apparatus casing, necessitates the provision of two independent earth termin

34、als on the casing, otherwise there would be a risk of both conductors becoming detached from the casing whilst still maintaining the monitored loop. It is worth noting that although the main and pilot paths are monitored in series, they function in parallel as far as actual earth-fault currents are

35、concerned. 4 Earth monitoring systems An earth monitoring system provides a means of maintaining a high degree of confidence in the impedance level of the protective conductor from the monitoring unit to the remote protected apparatus. The monitoring unit may be connected between the source of energ

36、y (if accessible) and the apparatus to be protected. NOTEThe source of energy may be, for example, a generator or a transformer. Earth monitoring depends upon the continuous circulation of a small current at a voltage not exceeding12V derived from a safety extra low voltage source. An interruption o

37、f this current is arranged to bring about disconnection of the protected apparatus from the supply. The monitoring current circulates in a loop circuit which contains the protective conductor to the apparatus, a section of the equipment casing and a suitable return path. It is therefore essential th

38、at any plug and socket and flexible cable provides not only the main protective path but also a return path which is usually known as the pilot conductor. It is worth noting that although the main and pilot paths are monitored in series, they function in parallel as far as actual earth-fault current

39、s are concerned. It is essential that the monitoring unit itself be properly earthed in accordance with the Regulations for Electrical Installations published by the Institution of Electrical Engineers. The requirement that the earth loop contains a section of the apparatus casing, necessitates the

40、provision of two independent earth terminals on the casing, otherwise there would be a risk of both conductors becoming detached from the casing whilst still maintaining the monitored loop. The impedance of the protective and pilot conductors forms part of a balanced bridge network (seeFigure 2). Th

41、e balance will be disturbed by variations in the parameters of the loop (e.g.open circuit or short circuit) and this can initiate a signal or disconnection. Typical impedance sensitivities of the balance bridge network( 10C to+40C) are: a) for domestic installations, 4 2 7; b) for industrial install

42、ations, 8 2 7. 5 Practical arrangements NOTEThere are several methods by which a protective conductor proving or monitoring unit may be used, depending on where the unit is sited and on how it is connected. The methods covered in this guide are described in5.1 to5.3. 5.1 Method A Method A proves or

43、monitors the protective conductor and pilot conductor loop in a flexible trailing cable supplying a transportable or mobile apparatus, the proving or monitoring unit being arranged to disconnect the supply to the apparatus at the point of connection of the trailing cable to the wiring installation.

44、This method is the more usual. A wall mounted protective conductor proving or monitoring unit is directly connected to a section of the fixed electrical installation and is arranged to feed the flexible trailing cable, which may be connected to the unit either by means of a plug and socket or by a p

45、ermanent connection. When connected in this manner both the trailing cable as well as the apparatus will be disconnected when the unit operates in the event of failure of the monitored loop. 5.2 Method B Method B proves or monitors the protective conductor and pilot conductor loop in a flexible trai

46、ling cable supplying a transportable or mobile apparatus, the proving or monitoring unit being arranged to disconnect the supply to that apparatus at the point of connection of the trailing cable to the apparatus. 1) Available from IEE, Savoy Place, London WC2R 0BL.BS4444:1989 4 BSI 03-1999 This met

47、hod may be employed where the fixed wiring installation is not provided with a proving or monitoring unit and where there is a need to be able to connect a transportable or mobile apparatus. In this case a protective conductor proving or monitoring unit is mounted on the apparatus itself, its purpos

48、e being to prove or monitor the protective conductor of the supply cable. In effect, the protective conductor proving or monitoring unit has been transferred from a fixed site to a mobile one and the flexible cable is now on its supply side instead of its load side. It should be noted that, in such

49、a case, the protective conductor proving or monitoring unit disconnects the load but cannot necessarily isolate the trailing cable which is supplying it; as well as disconnecting the load, the unit can be arranged to give an alarm. Any protective conductor fault occurring on the trailing cable has to be cleared by other means. It is important that a plug and socket with separate protective conductor and pilot connections be used to carry the loop through the fixed portion of the wiring and it is recommended that these connections be made in the manner shown in