Low Voltage Power Supplies in Electrified Areas

Transcription

1 Low Voltage Power Supplies in Electrified Areas Synopsis This document mandates requirements for control of return and stray currents and the management of earthing for low voltage power supplies in electrified areas. Copyright in the Railway Group documents is owned by Rail Safety and Standards Board Limited. All rights are hereby reserved. No Railway Group document (in whole or in part) may be reproduced, stored in a retrieval system, or transmitted, in any form or means, without the prior written permission of Rail Safety and Standards Board Limited, or as expressly permitted by law. RSSB Members are granted copyright licence in accordance with the Constitution Agreement relating to Rail Safety and Standards Board Limited. Content approved by: Plant Standards Committee on 16 October 2008 Authorised by RSSB on 20 November 2008 In circumstances where Rail Safety and Standards Board Limited has granted a particular person or organisation permission to copy extracts from Railway Group documents, Rail Safety and Standards Board Limited accepts no responsibility for, and excludes all liability in connection with, the use of such extracts, or any claims arising therefrom. This disclaimer applies to all forms of media in which extracts from Railway Group documents may be reproduced. Published by: Rail Safety and Standards Board Evergreen House 160 Euston Road London NW1 2DX Copyright 2009 Rail Safety and Standards Board Limited

2 Issue record Issue Date Comments One February 2009 Original document Supersedes GI/RT7007 The three new requirements have been marked by a vertical black line. Superseded documents The following Railway Group documents are superseded, either in whole or in part as indicated: Superseded documents Sections superseded Date when sections are superseded GI/RT7007, issue 1, June 2002 Low Voltage Electical Installations GI/GN7607, issue 1, June 2002 Guidance for Low Voltage Electrical Installations All 04 April 2009 All 04 April 2009 Supply Controlled and uncontrolled copies of this may be obtained from the Corporate Communications Department, Rail Safety and Standards Board, Evergreen House, 160 Euston Road, London NW1 2DX, telephone or enquiries@rssb.co.uk. s and associated documents can also be viewed at Page 2 of 18 RAIL SAFETY AND STANDARDS BOARD

3 Contents Section Description Page Part 1 Purpose and Introduction Purpose Introduction 4 Part 2 Earthing Requirements General earthing requirements Earthing requirements for LV electrical installations in ac electrified lines areas Earthing requirements for LV electrical installations in dc electrified lines areas Residual current devices providing additional protection against direct contact in dc electrified lines areas Guidance for Part 2 6 Part 3 Other Requirements Segregation Interlocking of power supplies Consultation with affected parties Guidance for Part 3 7 Part 4 Temporary LV Electrical Installations Temporary installations 8 Part 5 Application of this document Application infrastructure managers Application railway undertakings Health and safety responsibilities 9 Appendices 10 Appendix A Guidance for Part 2 of this standard 10 Appendix B Guidance for Part 3 of this standard 16 Definitions 17 References 18 RAIL SAFETY AND STANDARDS BOARD Page 3 of 18

4 Part 1 Purpose 1.1 Purpose This document mandates requirements for the management of earthing to avoid hazardous potentials, and for the management of segregation to control stray currents and minimise current transfer between different electrical power supply systems These requirements are additional to those normally required for low voltage (LV) power supplies (for example. compliance with BS 7671) and are needed to avoid hazardous touch potentials arising from differences in two independent power supply systems (LV power supplies and traction power supplies) while ensuring fault and stray currents are effectively controlled This document excludes power supplies to signalling and telecommunications (S&T) equipment, for which specific earthing arrangements are made This document excludes installations which do not form part of the main line railway, such as traction substations and depots. 1.2 Introduction Infrastructure managers need to co-operate to design, install and maintain LV electrical installations such that the earthing and segregation arrangements are not compromised. Page 4 of 18 RAIL SAFETY AND STANDARDS BOARD

5 Part 2 Earthing Requirements 2.1 General earthing requirements Touch potentials Where it is possible for a person to simultaneously touch exposed conductive parts that are connected to the equipotential bonding system of a LV electrical installation and the exposed conductive parts of the equipotential bonding system of another electrical system or rail mounted vehicle, the LV electrical installation shall be designed to ensure the touch voltage values do not exceed those specified in BS EN Circuit protective conductor Metallic conduit, trunking and cable trays shall not be used as a circuit protective conductor where high currents associated with other electrical systems could affect the integrity of the circuit protective conductor. 2.2 Earthing requirements for LV electrical installations in ac electrified lines areas LV electrical equipment directly bonded to the traction return circuit Where LV electrical equipment is either a) in contact with a conductive structure directly bonded to the traction return circuit, or b) is directly bonded to the traction return circuit, then the LV electrical equipment circuit protective conductor and any other earthed metallic conductor (for example screen wire or armouring) shall not be connected to the LV electrical equipment Guidance for is set out in A.1.2 and Figure A LV electrical equipment indirectly bonded to the traction return circuit Where LV electrical equipment is not directly bonded to traction return circuit as set out in 2.2.1, then the LV electrical equipment circuit protective conductor shall be connected to the LV electrical equipment The LV electrical equipment and circuit protective conductor shall be rated to carry the prospective traction fault current, unless the equipment is not at risk of carrying this current under fault conditions Where indirect bonding is used, a bond between the LV electrical installation supply main earth and the traction return circuit shall be provided Guidance for is set out in A.1.3 and Figure A Utility services and LV electrical installation bonding Where the LV electrical installation is bonded to another electrical system, the bonding and segregation arrangement of the utility services (for example gas, water etc) shall avoid hazards arising from touch voltages and the transfer of current between the systems and the utility services under operating and fault conditions. 2.3 Earthing requirements for LV electrical installations in dc electrified lines areas Protection against dc leakage Where a socket outlet is located such that an electrical appliance could be used on or near an object in contact with the running rail or other exposed conductive parts of the traction return circuit, then the design of the LV electrical installation shall prevent the adverse effects of dc leakage in the earth wire. RAIL SAFETY AND STANDARDS BOARD Page 5 of 18

6 The design of the LV electrical installation shall prevent adverse effects from dc leakage entering the neutral conductor of any other equipment or electrical distribution system because of earth faults between the earth conductor and neutral conductor The detection system of earth monitoring or earth proving devices shall be protected against the effects of dc leakage Guidance for is set out in A Residual current devices providing additional protection against direct contact in dc electrified lines areas In dc electrified lines areas dc leakage can affect the operation of the residual current devices (RCD) used to provide additional protection against direct contact. Therefore, Network Rail shall produce and maintain a list of approved RCDs that are immune up to declared levels of dc leakage and shall make this list available to others on request. 2.5 Guidance for Part Guidance for Part 2 is set out in Appendix A. Page 6 of 18 RAIL SAFETY AND STANDARDS BOARD

7 Part 3 Other Requirements 3.1 Segregation High voltage (HV) and traction systems shall be segregated from LV earth systems in order to avoid extraneous connections. 3.2 Interlocking of power supplies Interlocking of power supplies shall be provided where it is necessary to prevent paralleling of independent LV electrical power sources. 3.3 Consultation with affected parties Before making any change to a LV electrical installation the infrastructure manager shall consult with any other infrastructure manager that could be affected to ensure that the design, installation, testing and maintenance of the LV electrical installation maintains the separation and ensure the values of touch voltages specified in BS EN are not exceeded. 3.4 Guidance for Part Guidance for Part 3 is set out in Appendix B. RAIL SAFETY AND STANDARDS BOARD Page 7 of 18

8 Part 4 Temporary LV Electrical Installations 4.1 Temporary installations Temporary LV electrical installations shall comply with the arrangements set out in Part Temporary LV electrical installations shall not affect existing permanent earthing arrangements or segregation arrangements. Page 8 of 18 RAIL SAFETY AND STANDARDS BOARD

9 Part 5 Application of this document 5.1 Application infrastructure managers Scope The requirements of this document apply to all new and existing LV electrical installations, including all cabling and equipment from the main incoming power supply, through distribution and sub-distribution boards to ring mains, and discrete power supplies to fixed and portable equipment Where it is known, or becomes known, that existing LV power supplies do not comply with the requirements of this document, action to bring them into compliance is required as follows: a) When a LV power supply is modified b) When a LV power supply is renewed as a whole c) When any major component of a LV power supply is replaced The requirements of this document apply to all work that affects LV power supplies, whether new or altered Exclusions from scope There are no exclusions from the scope specified in for infrastructure managers General compliance date for infrastructure managers This comes into force and is to be complied with from 04 April After the compliance dates or the date by which compliance is achieved if earlier, infrastructure managers are to maintain compliance with the requirements set out in this. Where it is considered not reasonably practicable to comply with the requirements, authorisation not to comply should be sought in accordance with the s Code. 5.2 Application railway undertakings There are no requirements applicable to railway undertakings. 5.3 Health and safety responsibilities Users of documents published by Rail Safety and Standards Board (RSSB) are reminded of the need to consider their own responsibilities to ensure health and safety at work and their own duties under health and safety legislation. RSSB does not warrant that compliance with all or any documents published by RSSB is sufficient in itself to ensure safe systems of work or operation or to satisfy such responsibilities or duties. RAIL SAFETY AND STANDARDS BOARD Page 9 of 18

10 Appendix A Guidance for Part 2 of this standard The content of this appendix is not mandatory and is provided for guidance only. In addition to the guidance below reference can also be made to BS EN :1997. Clause 6.2 of BS EN :1997 gives some information relating to protective provisions for electrical equipment in ac and dc electrified areas, and illustrates some techniques which may be employed in addressing voltage and current transfer. It should be noted that BS EN :1997 is being updated and the corresponding provisions are set out in clause 7 of pren :2008. Consideration of these proposed provisions may also be helpful pending publication of the final version. When considering the guidance below it is necessary to consider the distribution network operator (DNO) supply earthing arrangements (existing or proposed), and if direct connection of the DNO and railway systems is to be undertaken or avoided. The transfer of potentials and currents between the DNO and railway systems is influenced by the arrangements employed by both parties. The arrangements employed on the railway should not result in a risk to the DNO installation. Early dialogue with the DNO is strongly recommended. A.1 Earthing requirements for LV electrical installations in ac electrified lines areas (2.2 of this standard) A.1.1 General A The overhead contact wire is energised at a nominal voltage of 25 kv. Electric trains draw power from the overhead wire with current returning to the feeder station via the running rails, return conductors or aerial earth wire depending upon the specific location design. Each overhead line equipment (OLE) support mast is bonded to the traction return circuit and provides an earth path through the mast foundation. A A A Where there is a risk of an exposed conductive part (for example trackside LV equipment enclosure, station platform awning, etc) becoming electrically connected to live OLE (by flashover, breakage of conductors, etc) exposed conductive parts and services are bonded to the traction return circuit to ensure rapid disconnection of the traction power supply in the event of a fault on the traction supply system. Additionally, equipotential bonding is required where it is possible simultaneously to touch exposed conductive parts and the running rails, trains or other conductive objects connected to the traction return circuit. This is necessary to limit touch voltage to values not exceeding those specified in BS EN The final choice of whether to bond directly or indirectly depends on several factors, including the cost of providing a reinforced circuit protective conductor for indirect bond compared to the cost of direct bonding and the practicality of segregating the systems at the LV electrical equipment or elsewhere. A.1.2 LV electrical equipment within the LV electrical installations directly bonded to the traction return circuit A Parallel paths for traction current can be avoided by not connecting the LV electrical installation supply circuit protective conductor, cable armouring and other earthed metallic conductors to the LV electrical equipment (see Figure A.1 for a typical arrangement) when the LV electrical equipment is either directly bonded to the traction return circuit, or is in contact with a conductive structure having a direct bond to the traction return circuit. Page 10 of 18 RAIL SAFETY AND STANDARDS BOARD

11 Conductive structure connected to traction return circuit LV electrical installation LV electrical equipment LV supply in contact with traction return circuit L N L N L N E E (cpc) Armour / metallic screen E (cpc) return circuit directly bonded to traction return circuit A and A cpc and where applicable cable armour / metallic screen etc not to be connected A possible bond by agreement with DNO Supply LV supply main earth return circuit return circuit distributed earthing return current bus-bar supply main earth (Feeder Stations only) Figure A.1 LV electrical equipment within the LV electrical installations directly bonded to the ac traction return circuit A Separation is achieved by gapping the supply cable armouring, by either: a) The use of an insulated gland Or b) Cutting back the cable armouring to a maximum distance of 300 mm from the equipment entry point RAIL SAFETY AND STANDARDS BOARD Page 11 of 18

12 A A In both cases apply heavy-duty heat-shrink sleeving, or equivalent, to cover the gland or the cut back section and 150 mm of the cable outer sheath, and terminate any other earthed metallic conductor and additional circuit protective conductor before it reaches the equipment. A suitable permanent warning label should be installed (securely attached to the cable at the gapping location) which identifies that the cable has been gapped and should not be bridged. The preferred warning label wording (black text on yellow background) is as follows: Caution cable gapped (see Railway Group Standard ). Do not bridge this gap A A A bond may be required between the traction return circuit and the LV electrical installation supply main earth to provide earth fault protection for the equipment and / or minimise touch voltages. This bond between the traction return circuit and the LV electrical installation should not be fitted at feeder stations because of the high level of traction currents which could flow via the bond into the LV supply network. The type and rating of this bond should be agreed with the infrastructure manager responsible for the traction system and the DNO formerly known as the regional electricity company. The LV protection arrangement for gapped circuits needs to be carefully considered because of the discontinuity of the circuit protective conductor. The characteristics of the traction system return circuit and its earthing arrangements can allow conduction of the LV fault current via the traction return system. In some cases use of this path is adequate for protection even if a bond in accordance with A is not fitted (subject to the requisite earth loop impedance being achieved). Consideration should be given to provision of appropriate RCD protection. A.1.3 LV electrical equipment within the LV electrical installations indirectly bonded to the traction return circuit A When the LV electrical equipment requires to be bonded to ensure safety during traction fault conditions (for example flashover or conductor breakage) or to control touch potentials, but is not in contact with any metalwork which itself is connected to the traction return circuit, an alternative to direct bonding of the equipment is required (see Figure 2 for a typical arrangement). Page 12 of 18 RAIL SAFETY AND STANDARDS BOARD

13 Conductive structure connected to traction return circuit LV electrical installation LV electrical equipment A armour / metallic screen connected subject to consideration of traction fault current conduction LV supply L N E Armour / metallic screen L N E (cpc) and cpc connected and where applicable rated for traction fault current L N E (cpc) return circuit electrical equipment indirectly bonded to traction return circuit and A bond between LV main earth and traction return return circuit Supply LV supply main earth return circuit return circuit distributed earthing return current bus-bar supply main earth (Feeder Stations only) Figure A.2 LV electrical equipment within the LV electrical installations indirectly bonded to the ac traction return circuit A Where the LV electrical equipment is at risk from traction system faults (for example flashover) the LV circuit protective conductor provides a fault path which is completed by a bond between the LV electrical installation supply main earth and the traction return circuit. When assessing the risk to LV electrical equipment resulting from OLE conductor breakage or current collector failure, the LV RAIL SAFETY AND STANDARDS BOARD Page 13 of 18

14 equipment s location in relation to the overhead line zone and pantograph zone as set out in BS EN should be considered. Where the LV equipment is not directly at risk from traction system faults, the bond is used to control touch potentials between the LV equipment and exposed conductive parts connected to the traction return system. The type and rating of this bond should be agreed with the infrastructure manager responsible for the traction system. Care should also be taken when considering cable termination arrangements on LV equipment at risk from traction system faults where armour / metallic screens are present. A A suitable permanent warning label should be securely attached to the bond identifying that the traction return circuit is connected to the LV supply main earth and that care needs to be taken when disconnection is required. A.1.4 Non-electrified sidings in an electrified area A Non-electrified sidings in an electrified area should be treated as electrified lines unless fitted with insulated rail joints. A.2 Earthing requirements for LV electrical installations in dc electrified lines areas (see 2.3 of this standard) A.2.1 General A The conductor rail is energised at a nominal voltage of 750 V dc. Electric trains draw power from the conductor rail with current returning to the substation via the running rails. The running rails are not deliberately earthed but allowed to float electrically about true earth. A As a result of the high-traction current levels and corresponding volt-drop, the running rails may rise to some tens of volts above true earth. Also, because the running rails are not electrically insulated from the ground, the track ballast, surrounding area and the conductive framework of buildings nearby takes up a dc voltage between true earth and the running rail potential. A.2.2 Protection against dc leakage supplies from socket outlets A When an earthed electrical appliance makes contact with a running rail or conductive object at running rail potential dc current may flow dependent upon the actual voltage and circuit parameters. In the case of an earthed electrical appliance, this current flows in the earth lead. An arc may be produced when contact between the electrical appliance and the running rail, or conductive object at running rail potential occurs. Overheating is possible in the earth lead and possibly the circuit protective conductor of the electrical distribution system. Also, uncontrolled dc can cause corrosion in adjacent metalwork. A Special precautions are required where any socket outlet, within 30 m of a running rail at traction potential, may provide power to an electrical appliance to be used on or about an object in contact with the running rail. Because earthed portable appliances can come into contact with the running rail or conductive object in connection at running rail potential, precautions are required to ensure the current flow is prevented, or reduced so as not to impair the effectiveness of the appliance conductors or supply conductors. Techniques to achieve this depends upon the specifics of the application, but might include one or more of the following: a) Ensure the earthing path has a sufficiently high value (for example provide additional resistance but without prejudicing disconnection times) to ensure the dc current flow is not significant Page 14 of 18 RAIL SAFETY AND STANDARDS BOARD

15 b) Block dc current flow by inserting a suitable device such as a capacitor (with a discharge resistor in parallel) within the earth path of the supply but ensuring that the protection for ac faults remains effective c) Use of RCDs and multi-pole protection to detect and disconnect the appliance (phase and neutral circuits) if inadmissible currents flow. A A A Special precautions are required as the neutral conductor is earthed at the supply point, and neutral to earth faults can remain undetected and if unchecked, cause unacceptably high stray traction current to flow through the neutral conductor system. Techniques to achieve this will be dependent upon the specifics of the application, but might include the use of RCDs and multi-pole protection to detect and disconnect the appliance (phase and neutral circuits) if inadmissible currents flow. For a supply with a nominal voltage of 400 V 3 phase there should be no neutral connection at the socket outlet. Any neutral necessary for control circuits should be derived from an isolating transformer mounted on the appliance. For a supply with a nominal voltage of 110 V or 230 V, the supply should be derived from an isolating transformer, centre tapped to earth. A.3 Earth monitoring or earth proving device A.3.1 An earth monitoring system provides a pilot conductor to the device, usually in the form of a cable braid, which forms a loop with the circuit protective conductor. The continuity of the earth path is proven by passing a small electrical current through the loop. If the earth path is not continuous, or if it has a resistance higher or lower than set values, the power supply is not energised. A.3.2 A.3.3 Currents flowing through the body under fault conditions produces a momentary electric shock. Where appliances are used in hazardous situations, for example, where there is an increased risk of falling as a result of an electric shock or where there is an increased risk due to using water, then the use of an earth monitoring device, in addition to an RCD, should be considered. Guidance on methods of protection for areas where high pressure water jets are used is set out in HSE Guidance Note PM29 Electrical hazards from steam / water pressure cleaners etc. A.4 LV electrical equipment attached to a running rail A.4.1 All electrical equipment powered from a LV power supply and attached to a running rail should be energised from an earth-free, isolated power supply, operated at a nominal voltage not exceeding 110 V. RAIL SAFETY AND STANDARDS BOARD Page 15 of 18

16 Appendix B Guidance for Part 3 of this standard The content of this appendix is not mandatory and is provided for guidance only. B.1 Fuses B.1.1 B.1.2 Cartridge fuses should be used for short circuit and / or overcurrent protection. Rewirable fuses should not be used. B.2 Miniature circuit breakers B.2.1 Miniature circuit breakers (MCB), complying with BS EN series, may be used to protect final circuits. The selection of the correct type and rating of MCB to match the circuit duty depends on the methods used by manufacturers to determine device ratings. B.2.2 B.2.3 MCBs have a relatively low maximum fault current rating, and back-up fuse protection may be necessary. Miniature circuit breakers should not be installed where they may be used as a switch, for example as a light switch. The careful selection of location of MCBs during the installation design process can inhibit the use of the MCBs as switches. Page 16 of 18 RAIL SAFETY AND STANDARDS BOARD

17 Definitions Circuit protective conductor (cpc) (BS 7671) A circuit protective conductor is a protective conductor connecting exposed-conductiveparts of equipment to the main earth terminal. Electrical installation (BS 7671) An assembly of associated electrical equipment supplied from a common origin to fulfil a specific purpose and having certain co-ordinated characteristics. High voltage High voltage (HV) is normally exceeding low voltage. Low voltage Normally not exceeding 1000 V ac or 1500 V dc between conductors, or 600 V ac or 900 V dc between conductors and earth. Nominal voltage (BS EN ) Voltage by which an installation or part of an installation is designated. Temporary installation An installation that is not intended to become a fixed installation, regardless of the length of time. Touch voltage (BS EN ) Voltage between parts when touched simultaneously 1. equipment The term traction equipment is used to mean the electrical equipment and conductors necessary to power the trains on an electrified railway. It includes the incoming supply feeders, switchgear and transformers which control and provide the electrical current at the traction system s line voltage, the distribution network and overhead contact wire or conductor rail system including the traction return circuit. return circuit The path by which the traction current returns from the traction unit to the feeder station, or substation, incorporating the traction return rails, bonding connections, return conductors and booster transformers as appropriate. 1 The definition of touch voltage used in BS EN :1997 differs from those proposed in pren :2008. RAIL SAFETY AND STANDARDS BOARD Page 17 of 18

18 References The Catalogue of s and the s CD-ROM give the current issue number and status of documents published by RSSB. This information is also available from Documents referenced in the text RGSC 01 The s Code Other references BS 7671 BS EN BS EN PM29 Requirements for electrical installations. IEE Wiring Regulations. Seventeenth edition Railway applications. Fixed installations. Protective provisions relating to electrical safety and earthing Electrical accessories. Circuit-breakers for overcurrent protection for household and similar installations. HSE Guidance Note Electrical hazards from steam / water pressure cleaners etc Other relevant documents Other references SI 1999/3242 Management of Health and Safety at Work Regulations: 1999 Page 18 of 18 RAIL SAFETY AND STANDARDS BOARD