GT/TDINT100 Issue: 1 Date: SEP 1993 Page 1 of 11

Transcription

1 British Railways Board Group Standard Page 1 of 11 Part A Synopsis This Directive gives the general requirements of telecommunications earthing systems, and encompasses the requirements for the earthing and equipotential bonding of all telecommunications equipment to an earthing system. Earthing and Equipotential Approval and Authorisation Signatures removed from electronic version DISTRIBUTION (SEE TIGEN0002 FOR DETAILS OF CODES) INCLUDE AHQ AA1 AA3 BH9 BA13 BA14 BA15 EXCLUDE This document is the property of the British Railways Board. It shall not be reproduced in whole or in part, nor disclosed to a third party, without the written permission of the Director, Engineering Standards. Copyright Sept 1993 British Railways Board Published by Group Standards

2 Page 2 of 11 Issue/Revision Record Issue Date Comments Approved at S&T Subject committee subject to minor editorial change. Responsibilities and distribution Controlled copies of this Standard will be distributed by Group Standards to the following as a minimum. S&T Engineer, InterCity S&T Engineer, NSE S&T Engineer, Regional Railways S&T Engineer, TLF S&T Engineer, RfD Director, TQM, BRT Ltd Managing Director, Rail Express Systems E&M Manager, CrossRail Director TQM, Central Services Engineering Manager, Union Railways Ltd The above recipients must arrange for controlled copies of this Standard to be made available to anyone responsible for the design, installation and testing of telecommunications earthing systems. The Engineering Standards Manager, BRT will arrange for controlled distribution of this Standard to those within the above businesses who hold TD folders in accordance with the distribution code identified on the front page. Implementation The provisions of this standard are mandatory for all telecommunications earthing systems. Staff who are not issued with a personal copy of this standard but whose duties require access to a copy, must be briefed about this standard. Implementation date: 1st January 1993 Supply Controlled copies of this standard may be obtained from: Telecomms Quality Group, Barbican, 137 Aldersgate Street, London, EC1A 4JA. Telephone: ; facsimile:

3 Page 3 of 11 Part B 1. PURPOSE 1.1. The Health and Safety at Work Act: 1974, places a responsibility on BR for the safety of both the users of its telecommunications system and to the personnel who are required to work on it. It is essential to maintain induced voltages at safe levels, therefore preventing the incidence of dangerous voltages on cables and equipment that might cause injury to personnel or damage to telecommunications equipment The proper operation of certain telecommunications systems requires that the susceptibility to, and generation of, electromagnetic interference by these systems is controlled Earthing and equipotential bonding is a necessary procedure to achieve the objectives of 1.1 and This standard also covers the mandatory requirements concerning telecommunications installations which are connected directly or indirectly to the public telecommunication system under Section 22(6) of the Act SCOPE 2.1 Anyone involved in the design, maintenance or testing of a telecomms earthing system shall comply with the contents of this Group Standard. 3. DEFINITIONS bonding is any arrangement used in connection with telecommunications cables and equipment for one or more of the following purposes:- a) To ensure effective and rapid operation of protective equipment in the event of fault currents in an electrical supply system which might otherwise cause fire and to protect against the danger to life from electric shock due to non-current carrying equipment and metal work rising to dangerous potentials. b) To provide a means for the earthing of self screened cables and separate screening conductors in order to provide electromagnetic screening by the inductive generation of screening currents. c) To provide a direct connection to earth for discharge tubes, lightning protectors, etc. d) To provide a telecommunications functional earth. Bonding Conductor Earth Earth Electrode Earth Electrode Resistance A protective conductor providing equipotential bonding. The conductive mass of the earth, whose electric potential at any point is conventionally taken as equal to zero. A conductor or group of conductors in intimate contact with and providing an electrical connection to earth. The resistance of an earth electrode to earth.

4 Page 4 of 11 Earthing Conductor Earthing System Equipotential Bonding Exposed Conductive Part Extraneous Conductive Part Fast Transient Earth Functional Earth Main Earthing Terminal Master Earth Electrode Neutral Conductor Protective Conductor A protective conductor connecting a main earthing terminal of an installation to an earth electrode or other means of earthing. One or more earth electrodes with their earthing conductors, capable of being connected to a main earthing terminal. Electrical connection maintaining various exposed conductive parts and extraneous conductive parts at substantially the same potential. A conductive part of equipment which can be touched and which is not normally a live part but which may become live under fault conditions. A conductive part of equipment liable to assume an arbitrary, but generally earth potential, and not forming part of the electrical installation. Shall comprise of two earth rods forming one vertically driven earth electrode providing an effectual connection with the general mass of earth for the operation of discharge tubes and lightning protectors, etc. Connection to earth necessary for proper functioning of telecommunications equipment. The terminal or bar provided for the connection of protective conductors, including equipotential bonding conductors and conductors for functional earthing if any to the means of earthing The master earth electrode, is the earth electrode which is closest to the installation to be earthed. Both the protective conductor connecting the various earth electrodes together and the Earthing conductor, terminate on the master earth electrode. A conductor connected to the neutral point of a system and capable of contributing to the transmission of electrical energy. A conductor used for some measure of protection against electric shock and intended for connecting together any of the following parts: exposed conductive parts; extraneous conductive parts; the main earthing terminal; earth electrode(s); the earthed point of the source, or an artificial neutral. System An electrical system consisting of a single source of electrical energy and an installation. For certain purposes of the IEE Wiring Regulations for Electrical Installations, types of system are identified as follows, depending upon the relationship of the source and of exposed conductive parts of the installation, to earth, (see Figure 1 at the end of this document):- (T, C and S are abbreviations for terra earth, combined and separate, respectively. TN system A system having one or more points of the source of energy directly earthed, the exposed conductive parts of the installation being connected to that point by protective conductors.

5 Page 5 of BRIEFING NOTES TN-C System in which neutral and protective functions are combined in a single conductor throughout the system. TN-S System having separate neutral and protective conductors throughout the system. TN-C-S System in which the neutral and protective functions are combined in a single conductor in part of the system. TT system A system having one point of the source of energy directly earthed, the exposed conductive parts of the installation being connected to earth electrodes electrically independent of the earth electrodes of the source. IT system A system having no direct connection between live parts and Earth, the exposed conductive parts of the installation being earthed. 4.1 This TD has been written to ensure that common standards are applied to the design, maintenance and testing of telecomms earthing systems. The designated maintenance engineer shall ensure that a record, including test certificates, is maintained of the earthing systems within his geographical area. 5. REQUIREMENTS FOR EARTHING SYSTEMS 5.1 GENERAL The responsibility for the safe operation of a normally installed a.c. power supply rests with the approved tester who is usually associated with the relevant Electricity Board or BR Power Engineer. Supplies should be regularly checked by the provider The responsibility for the functional design of a telecommunications earthing system resides with the appointed Project Engineer, with the assistance as required from the Engineer (Systems) at BRT Headquarters The design of all earthing systems shall conform to the requirements of the British Standard Code Of Practice for Earthing: BS This document applies only to earthing systems provided for telecommunications purposes. These shall only use earth rods and shall be installed in accordance with the relevant TI Where the Project Engineer is to be responsible for the detailed design of the earthing system, a plan shall be provided for the installers which shall give the following details:- a/ Location of earthing system including National Grid reference.

6 Page 6 of 11 b/ The purpose for which the earthing system is to be used and the maximum earth resistance value required. c/ The planned layout of the earth electrodes shall be shown on a detailed site map which shall include a diagram as per Figure 2 below, with the master electrode, designated A0. d/ Installations that require more electrodes than shown in Figure 2 shall be designated in a consistent manner. e/ The depth to which the earth rods are to be driven will be given along with the spacing between adjacent rods. f/ Details of the course of action to be taken if the completed earthing system does not meet the design requirements. E3 D3 C3 B3 A3 E2 D2 C2 B2 A2 E1 D1 C1 B1 A1 E0 D0 C0 B0 A0 Figure Where the Project Engineer is not responsible for the detailed design of the earthing system then this shall devolve to the appropriate installer who shall be supplied with the following information by the Project Engineer:- a/ Location of earthing system including National Grid reference. b/ Details of any physical site constraints. c/ The purpose for which the earthing system is to be used and the maximum earth resistance value to be met Earthing systems provided by contractors shall conform to the requirements of GT/ES EARTHING SYSTEM CATEGORIES The purpose for which an earthing system is provided determines the maximum permitted value of the earthing systems resistance. The purposes are categorised as follows: Category 1; earthing systems are provided for the protection of line-side equipment from the effects of transient potentials induced by lightning discharges or power distribution system faults only Category 2; earthing systems are provided for the limitation of induced potentials in line-side plant due to the proximity of electric traction equipment.

7 Page 7 of Category 3; earthing systems are provided mainly to facilitate the correct operation of telecomms systems in limiting background noise. They may also provide safety protection in special cases of operational power supply distribution. CAT Earthing System Application Maximum Earthing System Resistance (Ohms) Testing Periodicity (Months) 1 Voltage Surge Protection F.T.E Cable Sheath / Separate Screening Conductor Safety / Functional Earth RESTRICTED LOCATIONS FOR EARTHING SYSTEMS Whilst in many circumstances there will be only a limited choice of sites for an earthing system, the following locations are to be specifically excluded from consideration: Locations within 100 metres of electricity generating stations, substations or BR traction feeder stations Locations within 50 metres of track sectioning cabins or return conductor/rail connections. In addition, cable sheath earthing systems shall not be located within 25, but preferably 50 metres of telecommunications accommodation Between any lines of railway designated as running lines Locations that fall within the area of effect of a cathodic protection system, whether of the sacrificial or impressed current type Earth electrodes should not be sited on areas of made up ground (eg. embankments) unless this is unavoidable as these areas almost invariably have a soil resistivity that is less stable than the underlying soil No attempt shall be made to improve local resistivity conditions by chemical means, as this will only have a temporary effect Care should be taken during the design of the earthing system to ensure that earth electrodes will not be driven in on top of, or close, to underground services. 6. EARTHING SYSTEM APPLICATIONS 6.1. Earthing systems must ensure safe and electrically quiet operation of telecommunications systems All fixed telecommunications equipment shall normally be protected from a.c. overcurrent by fuses. There shall be no use of residual current circuit breakers All a.c. power supply sockets shall be protected by residual current circuit breakers earthing systems provided for safety must have some spatial diversity, such that testing can be achieved without disconnection of system protection. For this

8 Page 8 of 11 purpose the earthing conductors of separate earthing systems shall terminate at opposite ends of the main earthing terminal Earths provided for purely functional purposes can be singly provided All exposed conductive parts at a particular telecommunications location shall be bonded to a single common earthing system. In no circumstances must conductive parts bonded to different earthing systems be within a touchable distance of 2.5 metres of each other. Only one common earthing system shall exist within a telecommunications equipment room Where telecommunications cables link locations having separate earthing systems there shall be no direct interconnection of the earthing systems by the telecommunications cable, except in special cases such as immunisation The telecommunications system will normally be presented with live, neutral and earth terminals, of which the live will be fused, from either a TN-C-S or TN-S supply. The earth terminal shall be connected to the main earthing panel using a protective conductor. In this situation a category 3 earthing system shall be provided Where the incoming a.c. power supply is referenced to the traction return rail the Power engineer will install an isolation transformer to supply an IT system. In this case two category 3 earthing systems shall be provided and one side of the isolated supply shall be bonded to the main earthing terminal to form the neutral conductor Where a.c. power is needed for trackside telecommunications systems, the Signalling 650V a.c. supply, is normally used. This supply shall be locally isolated. This is an IT system. Where the lineside location is a building then two category 3 earthing systems shall be provided, but for lineside equipment cases a category 3 and category 1 earth system shall suffice. One conductor of the secondary of the isolated supply shall be bonded to the earthing system, forming a neutral conductor Where no a.c. power is required at a lineside telecommunications installation only a single category 1 earthing system is required Detailed earthing and equipotential bonding requirements for most installation situations are contained in the following earthing documentation:- TI TCAB1257 areas. TI TCAB1258 TI TGEN1259 Lineside equipment and cables in d.c. electrified and non electrified Lineside equipment and cables in a.c. electrified areas. equipment rooms No exposed conductive surfaces such as lineside location cases shall be bonded to the traction return rail unless they are within 2.5 metres of other traction rail bonded metalwork.

9 Page 9 of Where equipment is located within the 2.5 metre limit quoted above then it is not necessary to provide a bond to the traction return system if an insulated barrier can be provided between the exposed conductive parts connected to different earthing systems. The barrier shall prevent simultaneous human contact to these conductive parts at all times. 7. TESTING OF EARTHING SYSTEMS 7.1. The testing of earthing systems shall be carried out in accordance with Telecomms Instruction TGEN A maintenance engineer shall be designated to maintain a record, including test certificates of the earthing systems within their geographical area The designated engineer shall be responsible for ensuring that the periodic testing required by TI TGEN0153 is undertaken at the stipulated intervals. These periods are chosen to ensure that the testing does not occur at the same season in successive years If the maximum earth resistance value of an earthing system exceeds its design value then the designated engineer shall take immediate action to ensure that the resistance value of the earthing system is restored The designated engineer shall ensure that all earthing system installations undertaken within their geographical area, conform to the requirements of TI TGEN END -

10 Page 10 of 11 APPENDIX A DEFINITIONS a.c. BS d.c. F.T.E. IEE PME TI Alternating current. British Standard. Direct current. Fast Transient Earth. Institute of Electrical Engineers. Protective Multiple Earthing. Instruction. APPENDIX B REFERENCES BS 7430 Code of Practice for Earthing. BS 6701 Installation of apparatus intended for connection to certain telecommunications systems. EHQ/SP/D/101 Specification for Earthing and Bonding for a.c. Electrified Lines. IEE Wiring Regulations Sixteenth edition. Health and Safety at Work Act The Act GT/ES1683 Specification for Earthing Systems for telecommunications purposes. TI TGEN0153 Testing of Earthing Systems. TI TGEN1256 Earthing Systems. TI TI TCAB1257 Lineside Cables, Screening Conductors and Fixed Lineside in Areas with D.C. Electric Traction or No Electric Traction. TCAB1258 Lineside Cables, Screening Conductors and Fixed Lineside in Areas with A.C. Electric Traction. TI TGEN1259 bonding of Rooms.

11 Page 11 of 11 APPENDIX C Figure 1 Source of energy Distribution supply cable Consumers terminal Final Circuit Point of utilization Current using equipment L TN-C N & E Combined protective and neutral conductor throughout system L TN-S N E L TN-C-S N E Combined protective and neutral conductor between source and consumers terminals only L TT N E L IT Neutral point of source unearthed or earthed through high impedance N E The IT system as shown shall not be connected directly to telecommunications equipment (see paragraphs 3.9 & 3.10 TN TT and IT systems