1、THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE (C.C.I.T.T.) EARTHING OF TELECOMMUNICATION INSTALLATIONS Published by THE INTERNATIONAL TELECOMMUNICATION UNION Geneva 1976 THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE (C.C.I.T.T.) EARTHING OF TELECOMMUNICATION INS
2、TALLATIONS Published by THE INTERNATIONAL TELECOMMUNICATION UNION Geneva, 1976 ISBN 92-61-00301-X 48b2591 Ob79008 $50 = Printed in Switzerland ISBN 92- 61 -00301 -X = 4862591 0679009 097 -1- TABLE OF CONTENTS INTRODUCTION . 1 SECTION 1 . Definitions . 3 1.1 Earth electrode 1.2 Earth conductor . 1.3
3、Earth collector 1.4 Earth distribution system 1.5 Earthing system . 1.6 Service earthing system 1.7 Protective earthing system . 1.8 Separate earthing system 1.9 Earthing 1.10 Contact voltage . 1.1 1 Potential equalizing interconnection 3 3 3 3 3 4 4 4 4 4 4 SECTION 2 . General considerations . 5 2.
4、1 Main factors affecting earth potential . 2.2.1 Earthing of metal structures. etc . 2.2.2 Crosstalk . 2.2.4 Protection against lightning 2.2.5 Earthing of signalling circuits 2.3 Subdivision of earthing systems . 2.3.1 Service earthing systems 2.3.2 Protective earthing systems . 2.2 Main purposes o
5、f earthing telecommunication installations . 2.2.3 Danger and disturbances due to electric lines . 2.4 Service earthing system requirements . 2.4.1 Earthing requirements for switching equipment 2.4.3 Earthing requirements for D.C. power supplies 2.4.2 Earthing requirements for transmission equipment
6、 . 2.5 Protective earthing system requirements 2.5.1 Earthing requirements for racks and metal structures 2.5.1.1 Connection of racks to the earthing system 2.5.1.2 Connection of telecommunication cable sheaths, water pipes. gas pipes. centrai . heating pipes. etc . to the earthing system 2.5.1.3 Co
7、nnection of concrete reinforcement to earthing systems 2.5.2.1 Earthing of coaxial cabling of carrier equipment racks 2.5.2 Earthing requirements for exchange cabling 2.5.2.2 Earthing of the screens of symmetrical pair cabling of carrier equipment racks . . 5 5 6 6 6 6 6 6 7 7 7 7 8 8 8 8 8 9 9 9 9
8、9 4Bb2591 Obi90110 809 - II - Page 2.5.2.3 Earthing of the screen of sound programme circuit cabling . 9 9 2.5.3 Earthing requirements for equipment fed from electricity mains 2.5.4 Earthing requirements for lightning protection purposes 9 2.5.4.1 Subscribers premises 9 2.5.4.2 Buildings with lightn
9、ing arrester 10 2.5.4.3 Protection of equipment against lightning damage to the publlic mains 2.6 Systems with common electrodes versus systems with separate electrodes . 10 . 10 SECTION 3. Distributionsystems, earth collectors andearth conductors 11 3.1 Characteristics, advantages and disadvantages
10、 of star, mesh and mixed star-mesh earth dis- tribution systems . 12 3.1.1 Star distribution . 12 3.1.2 Mesh distribution 12 3.1.3 Mixed star-mesh distribution 12 3.2 Earth collectors . 12 3.3 Dimensioning of conductors . 13 3.3.1 Dimensions of distribution system conductors . 13 3.3.1.1 Dimensions
11、of service conductors . 13 3.3.1.2 Dimensions of protective conductors 13 3.3.2 Dimensions of earth collectors . 14 3.3.3 Dimensions of earth conductors . 14 SECTION 4. Earfh electrodes . 15 4.1 General 15 4.2 Soil resistivity 15 4.3 Practical aspects . 15 4.4 Electrical characteristics of earth ele
12、ctrodes . 17 4.4.1 Resistance . 17 4.4.1.1 The resistance of the electrode 17 4.4.1.2 The contact resistance between the electrode surface and the adjacent soil 17 4.4.1.3 The resistance of the volume of soil surrounding the electrode 17 4.4.1.4 The impedance of earth electrodes . 17 4.5 Various ear
13、th electrode configurations and their earth resistance values 17 4.5.1 Trench electrodes 18 4.5.1.1 Description 18 4.5.1.2 Installation 18 4.5.1.3 Resistance of trench electrodes 18 4.5.2 Earth spikes (driven rods) . 19 .4.5.2.2 Installation of earth spikes 20 4.5.2.3 Earth resistance of earth spike
14、s 21 4.5.3 Plate electrodes . 21 4.5.3.1 Description 21 4.5.3.2 Installation of plate electrodes 22 4.5.3.3 Earth resistance of plate electrodes 22 4.5.2.1 Description 19 4Bb2592 0679033 745 - III - Page 4.6 Method of construction of earth electrodes 23 4.6.1 General 23 4.6.2 Suitable materials for
15、earth electrodes 23 4.6.3 Minimum cross-sections of earth electrodes 23 4.6.3.1 Trench electrodes 23 4.6.3.1 a) Trench electrodes of galvanized steel 23 4.6.3.1 b) Trench electrodes of copper-clad or copper-plated steel 4.6.3.1 c) Trench electrodes of copper . 24 4.6.3.2 a) Earth spikes of galvanize
16、d steel 24 4.6.3.2 b) Earth spikes of copper-clad or copper-plated steel . . 23 4.6.3.2 Earth spikes (driven rods) 24 24 24 4.6.3.3 Plate electrodes 24 . 4.6.3.2 c) Earth spikes of copper 4.6.4 Connections 24 Appendix 1 (to Section 4) Diffusion resistance of an earth electrode . 25 Appendix 2 (to Se
17、ction 4) Earthing resistance with impulse currents 29 1. General . 29 1.1 Without breakdown of the ambient soil . 29 1.2 With breakdown of the ambient soil 33 . Appendix 3 (to Section 4) The use of long earth-pins as earth electrodes for lightning protection pur- poses . 35 SECTION 5. Measurement of
18、 earth electrode resistance 31 SECTION 6. Soil resistivivis, 39 6.1 General 39 6.2 Factors determining the resistivity of the soil 39 6.3 Resistivity of different types of soil 39 6.4 Measurement of soil resistivity 41 6.4.1 Nature of the problem 41 6.4.2 The four-electrode method (four probes) . 41
19、 6.4.3 The Wenner method . 43 6.4.4 The Schlumberger method . 43 6.4.5 Other methods . 44 6.4.6 Factors affecting the accuracy of measurements 44 6.4.7 Measuring the instruments commonly used 45 6.4.8 Processing of measurements 46 4862591 Ob79012 bL . IV . SECTION 7 . Maintenance of earthing install
20、ations . 7.2 Frequency of checks 7.1 Need for periodic measurement of earthing installations APPENDIX A (to Handbook) List of symbols . Page 49 49 49 51 m 4862591 Ob79013 518 m 1 INTRODUCTION A great deal of research has been done on earthing problems in connection with telecommunications and severa
21、l technical papers have been published as a result. Although opinions and practices differ, there is also a large measure of unanimity on certain aspects. This first edition of a handbook on earthing, prepared by the C.C.I.T.T, is an attempt to set out those principles on which a measure of agreemen
22、t has been reached, and, in those cases where there are different possibilities, to compare their merits. Attention is directed to factors that should be considered in the study of earthing problems. The first edition of the handbook does not claim to cover the subject completely or to present answe
23、rs to all the problems that may be encountered. Also, the contents should not be considered as formal C.C.I.T.T. recommendations. It is recognized that existing national regulations concerning electric power supplies must be respected in the various countries and that earthing for telecommunication
24、purposes is inevitably influenced by these regulations. In a future edition of this handbook, it may be possible to indicate individual practices in greater detail and to show why these practices have been adopted. It seems unlikely, however, that there can be a high degree of unanimity on such matt
25、ers of detail. 9 4862591 Ob79014 454 5 L L - - I 5 SECTION 1 DEFINITIONS 1.1 Earth electrode Conducting element or group of conducting elements in electrical contact with the ground designed or used for dispersing electrical currents through the ground. 1.2 Earth conductor Conductor or group of cond
26、uctors connecting the earth electrode to the earth collector (see 1.3). In the case of partly buried connections, this definition is valid only for the sections which are electrically insulated from the ground, the sections in contact with the ground forming part of the earth electrode. 1.3 Earth co
27、llector Conductor in the form of a bar or ring to which the earth conductor(s) and the earth distribution system are connected. 1.4 Earth distribution system Group of conductors connecting the various parts which have to be earthed to the earth collector. 1.5 Earthing system An installation comprisi
28、ng the earth electrode, the earth conductor(s), the earth collector and the earth distribution system (see Fig. l(1). FIGURE i(1) Previous page is blank. = 9862593 Ob79015 390 4 1.6 Service earthing system Earthing system used in the operation of telecommunication equipment. 1.7 Protective earthing
29、system Earthing system used to limit accidental excess voltages to non-dangerous levels. 1.8 Separate earthing system Earthing system with independent earth electrodes which do not affect one another appreciably during operation (from point of view of danger or operation). 1.9 Earthing Connection of
30、 telecommunication equipment to an earthing system. 1.10 Contact voltage Voltage to which the human body may be subjected as a result of contact with accessible metallic elements which are usually not live but which may become live through an insulating fault or for other reasons. 1.1 1 Potential eq
31、ualizing interconnection Interconnection of independent metallic parts by a conductor of suitable cross-section in order to eliminate differences of potential between the structures, in cases when it might constitute a danger to personnel or equip- ment. 4862571 067901b 227 5 SECTION 2 GENERAL CONSI
32、DERATIONS 2.1 Main factors affecting earth potential Before being able to judge the efficiency of the earthing methods used in telecommunications installations, it is necessary to have some fundamental information about the way in which the relatively stable potential, called “earth potential”, is e
33、stablished. In general it depends on three factors: - the soil resistivity in the area concerned (see Section 4); - the earth electrode (see Section 4); - the earth electrode current. Soil resistivity is of great importance, and though nothing can be done to change it, except over very limited areas
34、, a knowledge of the value of the resistivity (and hence methods for accurately measuring the value) is essential. Methods of measuring soil resistivity have already been described in “Recommendations for the protec- tion of underground cables against corrosion”, Chapter X, paragraph 10.3.3. A fulle
35、r treatment is given in Sec- tion 6 of this handbook. Effective connection with the earth (soil) is achieved by means of earth electrodes, the various forms of which are described in Section 4 of this handbook. Each of the above factors introduces resistance into the common earth path of all circuit
36、s connected to the earthing system and, as a consequence, the voltages developed across this resistance can give rise to crosstalk, interference, noise, signalling difficulties and reduction of protection. Also the potential of the soil is subject to variation from a number of causes such as “natura
37、l” earth currents of low intesity, earth currents from traction systems, earth currents due to lightning discharges and current arising from a power line fault. These factors give rise in some cases to dangerous potentials, and may also cause noise and interference. it is to be concluded from the fo
38、regoing that although the earth forms a convenient mass for helping to provide a stable potential, variations in the potential are often great enough to give rise to difficulties necessitating special precautions to eliminate or overcome them. 2.2 Main purposes of earthing telecommunication installa
39、tions Generally speaking, when telecommunication installations are earthed, it is to ensure: - safety of operating and maintenance staff and users in the presence of dangerous voltages; - protection of equipment from dangerous voltages; - compliance with legal requirements; - reduction of crosstalk
40、and noise in telecommunication circuits to prescribed values; - use of the earth as a return path for one of the conductors of a telecommunication circuit. Information on some of these factors is given in certain C.C.I.T.T. publications. So much has been published on the subject elsewhere that it is
41、 impossible to give a select short list, far less a complete one. The specifications established by Administrations or operating agencies may, however, be made available to interested users on request. 4862593 0679037 lb3 H 6 2.2.1 Earthing of metal structures The objective in the earthing of racks,
42、 metal frameworks, covers, screens, etc. is to maintain at a sufficient- ly low level the contact voltages which occur in the event of a fault in the earthing of the electrical system and to ensure the rapid triggering of cut-out devices. Safety requirements in respect of the ?protective earthing? o
43、f the electrical equipment of buildings is usually specified in national regulations and is outside the scope of the C.C.I.T.T. 2.2.2 Crosstalk Reduced insulation resistance to earth on subscriber lines may cause crosstalk because some of the speech current in one line can find a path to another lin
44、e through the surrounding soil. If one pole of the exchange battery is earthed, part of the leakage speech current will flow through the soil to the earthed pole of the battery, thereby reducing the crosstalk level. The reduction obtainable depends on the effectiveness of the earthing of the battery
45、. Several investigations have shown that it is possible to keep crosstalk within reasonable limits if the earth resistance of the battery terminal earthing is kept below 20 R; but this limit must not be regarded as generally acceptable or necessary, since it varies from one telephone system to anoth
46、er and may also depend on the dimen- sions of the line plant, insulation standards, etc. 2.2.3 Danger and disturbances due to electric lines Questions of earthing relating to the safety of personnel and protection of equipment from the effects of dangerous induction caused by electric lines, or from
47、 high potential gradients in the earth in the vicinity of electric installations, and questions of disturbance are dealt with in the Directives and in the C.C.I.T.T. Recommendations. One of the main purposes of earthing the metallic sheaths of cables or of screening wires is to lower the induced vol
48、tage. Effective earthing is also necessary for the proper functioning of protective devices. 2.2.4 Protection against lightning The special precautions to be taken to protect telecommunication installations against lightning are dealt with in another C.C.I.T.T. publication.* Instructions for the lig
49、htning protection of buildings, on the other hand, are laid down in national regulations. 2.2.5 Earthing of signalling circuits Several signalling systems use an earth return for the signalling current; this method is mainly confined to The possible consequences of using an earth return for these purposes are dealt with in the Directives, local networks and to comparatively short circuits. Chapter V, and in C.C.I.T.T. Recommendation K.4. 2.3 Subdivision of earthing systems The main subdivisions into which earthing systems for telecornmunication equipment can be classified are: - Service e
