1、BRITISH STANDARD BS 7197:1990 Specification for Performance of bonds for electric power cable terminations and joints for system voltages up to 36 kV UDC 621.315.687.1:621.3.016:621.3.027.4/.7:006.3/.8BS7197:1990 This British Standard, having been prepared under the directionof the Cables and Insula
2、tion Standards Policy Committee, was published underthe authority of the BoardofBSI and comes intoeffecton 29June1990 BSI 10-1999 The following BSI references relate to the work on this standard: Committee reference CIL/20 Draft for comment 88/27188 DC ISBN 0 580 17788 2 Committees responsible for t
3、his British Standard The preparation of this British Standard was entrusted by the Cables and Insulation Standards Policy Committee (CIL/-) to Technical Committee CIL/20, upon which the following bodies were represented: Aluminium Federation Association of Consulting Engineers Association of Manufac
4、turers of Domestic Electrical Appliances British Approvals Service for Electric Cables British Cable Makers Confederation British Plastics Federation British Railways Board British Shipbuilders British Steel Industry British Telecommunications plc Department of the Environment (Property Services Age
5、ncy) Department of Trade and Industry (Consumer Safety Unit, C A Division) Electricity Supply Industry in England and Wales Engineering Equipment and Materials Users Association ERA Technology Ltd. Institution of Electrical Engineers London Regional Transport The following bodies were also represent
6、ed in the drafting of the standard, through subcommittees and panels: Association of Manufacturers Allied to the Electrical and Electronic Industry (BEAMA Ltd.) Department of Transport Electrical Installation Equipment Manufacturers Association (BEAMA Ltd.) Institution of Lighting Engineers Ministry
7、 of Defence Amendments issued since publication Amd. No. Date CommentsBS7197:1990 BSI 10-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Definitions 1 3 Bonds 2 4 Performance 3 Appendix A Guidance on bonding 6 Appendix B Performance test methods 6 Appendix C Bo
8、nd cross-sectional areas 7 Figure 1 Test circuit for continuity bonds used in cable terminations 11 Figure 2 Test circuit for cable joint earth continuity bonds 12 Table 1 Physical constants for bonding materials 5 Table 2 Solid copper bond cross-sectional areas for 1 s fault duration 8 Table 3 Soli
9、d aluminium bond cross-sectional area for 1 s fault duration 9 Table 4 Solid steel bond cross-sectional areas for 1 s fault duration 10 Publication referred to Inside back coverBS7197:1990 ii BSI 10-1999 Foreword This British Standard has been prepared under the direction of the Cables and Insulatio
10、n Standards Policy Committee. Power cables used for the distribution of electricity are generally provided with a metallic envelope. This envelope, generally known as a screen, functions as the protective conductor of the cable. It is designed to have an adequate current rating to enable it to carry
11、, without damage, any fault current that may flow during the time taken for the devices protecting the cable to operate. Therefore in a cable joint, provision has to be made to connect together screens of the cables in the joint and/or terminations by means of bonds. This specification not only spec
12、ifies the general requirements of such bonds but also gives recommended cross-sectional areas of copper, aluminium and steel bonds. An appendix gives guidance on various forms of bonding. There is currently no published International Standard for bonds covered by this standard. It is assumed in the
13、drafting of this British Standard that the execution of its provisions is entrusted to appropriately qualified and experienced people. 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.
14、Compliance 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 and ii, pages1 to 12, an inside back cover and a back cover. This standard has been updated (see copyright date) and ma
15、y have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS7197:1990 BSI 10-1999 1 1 Scope This British Standard specifies requirements for bonds for connecting the metallic envelopes (protective conductors) of electric power cables at terminations
16、and joints for system voltages up to36kV for the different types of cables covered by British Standards. The standard describes performance test methods, tables cross-sectional areas of solid copper, aluminium and steel bonds calculated in accordance with a given formula and gives guidance on matter
17、s which affect the following. a) Provision of a circuit in joints between cables and between a cable and its terminal equipment for the transmission of currents which may arise under system fault conditions. b) Provision of bonding for neutral conductors (CNE only). c) Provision of overall protectiv
18、e conductors at joints to provide for the passage of a fault current in the event of mechanical damage or failure of the insulation and to contain the electric field around high voltage conductors. Guidance on bonding is given in Appendix A. The standard specifically excludes additional safety preca
19、utions necessary when jointing live cables. NOTEThe title of the publication referred to in this standard is given on the inside back cover. 2 Definitions For the purposes of this British Standard, the definitions given in BS 4727-2:Group08apply, together with the following. 2.1 low voltage a system
20、 voltage normally exceeding50V but not exceeding600/1000V a.c. 2.2 high voltage a system voltage exceeding 600/1000V a.c. 2.3 SNE (separate neutral earth) a cable in which the neutral load carrying conductor is separate from and insulated from the earth conductor 2.4 CNE (combined neutral earth) a c
21、able in which the neutral load carrying conductor is also the earth conductor 2.5 straight through joint a connection between two identical or substantially similar cables 2.6 termination a device fitted to the end of a cable to ensure electrical connection with other parts of the system and to main
22、tain the insulation up to the point of connection 2.7 branch joint a connection with three cable entry ports used to connect one cable (designated branch) to a through cable (designated main), the cables being of comparable size NOTEThe geometric arrangement may be “T” or “Y” form and the main cable
23、 may be cut or uncut. 2.8 transition joint a connection between cables having different types of insulation 2.9 service joint similar to a branch joint but with the tap off cable (designated service) considerably smaller than the main cable and generally not greater than35mm 2 2.10 earth conductor t
24、he metallic conductor that may be in the form of a metallic screen, lead or aluminium sheath, armour or a separate conductor cable which is intended to form a continuous electrical connection to earth 2.11 earth bond a connection between the earth potential components of cable, joint or termination
25、to the cable earth conductor 2.12 earth continuity bond 1) A connection between the earth conductors of cables within a joint which maintains earth continuity throughout the jointed cable. 2) A connection between the earth conductor of a cable and a suitable earthing point at a termination. 2.13 ear
26、th screen a metallic layer over or within a joint the principal function of which is to provide a protective screen against electric fieldsBS7197:1990 2 BSI 10-1999 2.14 earth fault current the prospective short time current (generally less than3s) that can flow to earth under system phase to earth
27、fault conditions without permanent system damage 2.15 three phase symmetric fault current the prospective short time current (generally less than3s) that can flow between phases under system phase to phase fault conditions without permanent system damage 2.16 continuous load current the maximum cont
28、inuous current that can flow in the system under normal operating conditions for an unrestricted time and without system damage 2.17 protective conductor a conductor used for some measures of protection against electric shock, and intended for connecting together any of the following parts: exposed
29、conductive parts; extraneous conductive parts; the main earthing terminal; earth electrode(s); the earthed point of the source, or an artificial neutral. 3 Bonds 3.1 General NOTE 1Power cables used for the distribution of electricity are generally provided with a metallic envelope(s) protective cond
30、uctor(s). The purpose of this envelope is: a) to contain the electric field around the cores where there is no dielectric screen; b) to ensure that in the event of a failure of the insulation of the cable, a circuit is provided to enable current to flow through the fault to the neutral point of the
31、system supplying the cable which is usually connected to the mass of earth; c) to carry fault current, derived from a failure of insulation in electrical plant that is connected to the cable, to the neutral point of the system supplying the cable; d) for use, in certain designs of low voltage cable,
32、 as a neutral conductor which will carry the neutral current of the circuit and is rated accordingly. The metallic envelope may be made from steel, copper or aluminium wires that are equally spaced around the circumference of the cable or cores and wound in a helix or waveform along the length of th
33、e cable or cores. Screens can also be made from lead or aluminium that has been extruded over the cable to form a metallic tube. In addition tapes of copper or aluminium may be wound in a helix around the cable or cores, or applied longitudinally along the cable or cores, and folded around the circu
34、mference of the cable to provide a complete screen. The metallic envelope is designed to have an adequate current rating to enable it to carry, without damage, any fault current that may flow during the time taken for the protective devices to operate. Provision shall be made in a cable joint to con
35、nect together the metallic envelope(s) of the cables in the joint by means of bonds. Collectively the bonds need to have the same fault current rating as the envelope which in turn shall be not less than the earth fault rating of the cable. At cable terminations, connections shall be included as par
36、t of the termination to provide means whereby the envelope(s) of the cable can be connected to: 1) the metallic enclosure of an item of electrical plant; 2) any other electrical circuit that will enable fault current to return to the neutral point of the supply system. The connections shall be rated
37、 to carry a current no less than the earth fault rating of the cable. NOTE 2Different types of envelope will require different methods of connection to provide adequate earth continuity bonds. 3.2 Construction 3.2.1 General Each bond shall consist of the following: a) a metallic conductor with, if n
38、ecessary, protection against corrosion; and b) a means of connection to the cable protective conductors by either mechanical clamps, soldering or compression with, if necessary, protection against corrosion. Alternatives, including the use of the cable protective conductor(s) itself as the joint or
39、termination earth continuity bond, are acceptable providing they comply with this standard. NOTEIf the cable protective conductor is used its rating within a joint or at a termination might be lower than its rating within the cable. In particular, bunched armour or screen wires may have only about50
40、% of the rating of the same wires applied helically in the cable. 3.2.2 Additional requirements for joints that require screening by a protective conductor In joints that incorporate a metallic screen surrounding the insulated conductors, the metallic screen shall cover the full length of the joint
41、from one metallic envelope to the other. The metallic screen of the joint shall be bonded to the metallic envelope of the cable and it is permitted to form all or part of the bond. Screens in joints and terminations for use on systems up to and including12kV shall not have any aperture greater than8
42、mm width. 3.3 Avoiding mechanical damage to cable components The bond shall be of a suitable form to allow effective connection without damaging the cable protective conductor or other cable components.BS7197:1990 BSI 10-1999 3 NOTEThe following recommendations should be taken into account. a) If of
43、 the mechanical type, the bond should not impose undue pressure on the cable insulation. b) If of the soldered type, the bond should not require heating of the earth continuity conductor during installation in a manner liable to damage the cable. c) If of the compression type, the bond should not re
44、quire the use of tooling liable to damage the cable components. 3.4 Avoiding corrosion The bond shall be constructed of materials, compatible with the components of the cable and joint or termination, which may include dissimilar metals, or be suitably protected, to prevent corrosion. 3.5 Performanc
45、e in single core cables In single core circuits, bonding at both ends of the sheath may result in circulating currents of the same order of magnitude as the current in the cable conductor. The bonds shall be capable of carrying this current continuously without overheating or progressive deteriorati
46、on arising from normal and short circuit operational conditions. 3.6 Neutral protective conductor bonding 3.6.1 General Bonds for use with neutral/protective conductors shall be of such design that they may be fitted without at any time losing continuity of the neutral protective conductor. 3.6.2 Lo
47、ad current requirements The bond shall be constructed in such a way that its temperature when carrying the required neutral load current does not exceed the design operating temperature of the cable neutral/earth conductor, and/or that of the joint. NOTE 1Mechanical considerations. Under normal serv
48、ice conditions mechanical, tensile or compression stress can be induced in the cables as a result of thermal movement under load cycling conditions or ground subsidence. The neutral/earth bond should be designed such that it cannot be subjected to excessive mechanical stress under these conditions.
49、NOTE 2Thermal considerations. Under single phase fault conditions the phase and neutral/earth conductors may rise to comparatively high temperatures. The neutral/earth continuity bond should not be more liable to thermally induced failure than the phase conductors. 3.7 Current equalization Some designs of CNE cable incorporate a neutral/protective conductor comprising a number of individual wires which are not in contact with each other throughout the cable length. For a number of reasons, e.g. localized corrosion of some of the wires, or incorporation of service j
