Guidance for Low Voltage Electrical Synopsis This document provides information in support of GI/RT7007 Low Voltage Electrical. Signatures removed from electronic version Submitted by Anne E Blakeney Standards Project Manager Authorised by Brian Alston Controller, Railway Group Standards This document is the property of Railway Safety. It shall not be reproduced in whole or in part without the written permission of the Controller, Railway Group Standards, Railway Safety. Published by: Railway Safety Evergreen House 160 Euston Road London NW1 2DX Copyright 2002 Railway Safety
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Page 1 of 15 Contents Section Description Page Part A A1 Issue record 2 A2 Implementation of this document 2 A3 Responsibilities 2 A4 Health and safety responsibilities 2 A5 Technical content 2 A6 Supply 3 Part B B1 Purpose 5 B2 Application of this document 5 B3 Definitions 5 B4 General requirments 6 B5 Protection and switiching 6 B6 General earthing requirments 7 B7 Earthing requirements for LV electrical installations in AC electrified lines areas 8 B8 Earthing requirements for LV electrical installations in DC electrified lines areas 10 B9 Temporary low voltage electrical installations 12 Appendices A B References LV electrical equipment within the LV electrical installations directly bonded to the AC traction return circuit 13 LV electrical equipment within the LV electrical installations indirectly bonded to the AC traction return circuit 14 15 RAILWAY SAFETY 1
Page 2 of 15 Document Withdrawn as of April 2009 A1 Issue record Part A Issue Date Comments One June 2002 Original Document This document will be updated when necessary by distribution of a complete replacement. A2 Implementation of this document The publication date of this document is 1 June 2002. This document supersedes the following s, either in whole or in part as indicated: Railway Group Guidance Note Issue No. Title GN sections superseded by this document Date(s) as of which sections are superseded GM/RC2503 1 Code of Practice for Low Voltage Electrical on Railway Network Premises All 3 August 2002 (document withdrawn as of this date) A3 Responsibilities s are non-mandatory documents providing helpful information relating to the control of hazards and often set out a suggested approach, which may be appropriate for Railway Group* members to follow. * The Railway Group comprises Railtrack PLC, Railway Safety, and the train and station operators who hold Railway Safety Cases for operation on or related to infrastructure controlled by Railtrack PLC. Railtrack PLC is known as Railtrack. A4 Health and safety responsibilities A5 Technical content In issuing this document, Railway Safety makes no warranties, express or implied, that compliance with all or any document published by Railway Safety is sufficient on its own to ensure safe systems of work or operation. Each user is reminded of its own responsibilities to ensure health and safety at work and its individual duties under health and safety legislation. The technical content of this document has been approved by: P Williment, Principal Electrification Engineer, Railway Safety H Peers, Principal Plant Engineer, Railway Safety J Allan, Principal Signalling and Telecoms Engineer, Railway Safety Enquires to be directed to Railway Safety Tel: 020 7904 7518 2 RAILWAY SAFETY
Page 3 of 15 A6 Supply Controlled and uncontrolled copies of this document may be obtained from the Industry Safety Liaison Dept, Railway Safety, Evergreen House, 160 Euston Road, London NW1 2DX. RAILWAY SAFETY 3
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Page 5 of 15 B1 Purpose B2 Application of this document Part B This document provides information in support of GI/RT7007 Low Voltage Electrical. B2.1 To whom the guidance applies This document contains guidance that is applicable to the duty holders of the following categories of Railway Safety Case: a) infrastructure controller b) station operator. B3 Definitions Circuit protective conductor (BS 7671) Circuit protective conductor (cpc) is a protective conductor connecting exposedconductive-parts 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 50122-1) 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 50122-1) Voltage under fault conditions between parts when touched simultaneously. Traction 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 catenary or conductor rail system including the traction return circuit. Traction return circuit (GL/RT1254) 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. RAILWAY SAFETY 5
Page 6 of 15 Document Withdrawn as of April 2009 B4 General requirements B4.1 B4.1 General requirements for LV electrical installations The general requirements for LV electrical installations given in BS 7671, as applicable to the rail industry, shall be followed. B4.2 B4.2 Cables to LV electrical equipment on or near to the running rail Except as follows, cables that cross the track and that are energised at a nominal voltage of more than 110 V shall not be routed on the surface. It is permissible to route cables energised at a nominal voltage of 110 V or less, or any cables connected to equipment attached to a running rail, on the surface across the track formation rather than in under track crossings. B4.2.1 The preferred methods of routing cables on the surface across the track formation are by the use of notched sleepers/bearers or ducted or hollow sleepers/bearers. B5 Protection and switching B5.1 B5.1 Residual current devices providing additional protection against direct contact in DC electrified lines areas When selecting a residual current device (RCD) to provide additional protection against direct contact in DC electrified lines areas, consideration shall be given to the effects of the level of DC leakage that may affect the operation of the RCD. The infrastructure controller shall produce and maintain a list of approved suppliers of RCDs that are immune up to declared levels of DC leakage and shall make this list available to other Railway Group members on request. B5.1.1 The level of DC leakage will be dependent upon the location and the traction return bonding arrangements. B5.2 B5.2 Fuses Rewirable fuses shall not be used for short circuit or overcurrent protection. B5.2.1 Suitable cartridge fuses are those compliant with BS 88 or BS 1361 as appropriate. B5.3 B5.3 Interlocking Interlocking shall be provided where it is necessary to prevent paralleling of independent LV electrical power sources. B5.4 B5.4 Miniature circuit breakers Miniature circuit breakers shall not be installed where they may be used as a switch, for example as a light switch. B5.4.1 Miniature circuit breakers (MCB), complying with BS EN 60898, may be used to protect final circuits. The selection of the correct type and rating of MCB to 6 RAILWAY SAFETY
Page 7 of 15 match the circuit duty will depend on the methods used by manufacturers to determine device ratings. B5.4.2 MCBs have a relatively low maximum fault current rating and back-up fuse protection may be necessary. B5.4.3 The careful selection of location of MCBs during the installation design process can inhibit the use of the MCBs as switches. B6 General earthing requirements B6.1 B6.1 Segregated HV and LV earth systems Where HV and LV earth systems are segregated, Railway Group members shall co-operate to design, install and maintain the LV electrical installation such that the values of touch voltages specified in BS EN 50122-1 are not exceeded. B6.2 B6.2 Circuit protective conductor Metallic conduit, trunking and cable trays shall not be used as a cpc where high currents associated with other electrical systems would affect the integrity of the cpc. B6.3 B6.3 Earth monitoring or earth proving device Where appliances are used in hazardous situations the use of an earth monitoring device, in addition to an RCD, shall be considered as part of the risk assessment required by the Management of Health and Safety Regulations. B6.3.1 Currents flowing through the body under fault conditions will produce 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. B6.3.2 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 cpc. 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 will not be energised. B6.3.3 Guidance on methods of protection for areas where high pressure water jets are used is given in HSE Guidance Note PM29 Electrical hazards from steam/water pressure cleaners etc. RAILWAY SAFETY 7
Page 8 of 15 Document Withdrawn as of April 2009 B6.4 B6.4 Touch potentials Where it is possible for a person simultaneously to touch exposed conductive parts that are connected to the equipotential bonding system of the LV electrical installation and the exposed conductive parts of the equipotential bonding system of a rail mounted vehicle, touch voltage values shall not exceed those specified in BS EN 50122-1. B6.4.1 Even though rolling stock is excluded from the scope of the Railway Group Standard this important interface needs to be considered. B6.5 B6.5 LV electrical equipment on or near to the running rail Except where agreed by the infrastructure controller, all electrical equipment attached to a running rail shall be energised from an earth-free, isolated power supply, operated at a nominal voltage less than or equal to 110 V. B6.5.1 Many existing train detection systems are not designed to withstand nominal voltages greater than 110 V. B7 Earthing requirements for LV electrical installations in AC electrified lines areas B7.1 B7.1 Earthing and bonding The requirements for AC electrified lines traction bonding are contained in GL/RT1254. B7.1.1 The overhead contact wire is energised at a nominal voltage of 25000 V. 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. B7.1.2 Where there is a risk of exposed metalwork becoming electrically connected to live OLE (by flashover, breakage of conductors, etc) conductive structures and services are bonded to the traction return circuit to ensure rapid disconnection of the traction power supply in the event of a traction fault. B7.1.3 Additionally, equipotential bonding is required where it is possible simultaneously to touch exposed metalwork 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 50122-1. B7.1.4 The final choice of whether to bond directly or indirectly will depend on the cost of providing a reinforced cpc compared to direct bonding and the practicality of segregating the earth systems at the LV electrical equipment or elsewhere. 8 RAILWAY SAFETY
Page 9 of 15 B7.2 B7.2 LV electrical equipment within the LV electrical installations directly bonded to the traction return circuit The LV electrical equipment cpc shall not be connected to the LV electrical equipment when the LV electrical equipment is either: a) directly bonded to the traction return circuit or b) is in contact with a conductive structure having a direct bond to the traction return circuit. B7.2.1 Parallel paths for traction current will be avoided by not connecting the LV electrical installation supply cpc to the LV electrical equipment (see Appendix A 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. B7.2.2 Separation is achieved by gapping the supply cable armouring by: 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 and c) applying heavy duty heat shrink sleeving to cover the gland or the cut back section and 150 mm of the cable outer sheath and d) terminating any additional cpc before it reaches the equipment. B7.2.3 A suitable permanent warning label (securely attached to the cable) identifies that the incoming service cable earth has been gapped and should not be bridged. B7.2.4 An equipotential 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 minimise touch voltages. This equipotential bond between the traction return circuit and the LV electrical installation should not be fitted at feeder stations. The type and rating of this bond should be agreed between the infrastructure controller and the regional electricity company (REC). B7.2.5 Gapped circuits should have RCD protection. RAILWAY SAFETY 9
Page 10 of 15 Document Withdrawn as of April 2009 B7.3 B7.3 LV electrical equipment within the LV electrical installations indirectly bonded to the traction return circuit In each of the following circumstances the LV electrical equipment cpc shall be connected to the LV electrical equipment and be rated to carry the prospective traction fault current when the LV electrical equipment requires to be bonded and either: a) the LV electrical equipment is not in contact with any metalwork which itself is connected to the traction return circuit or b) a direct bond as in section B7.2 is not selected. Where indirect bonding is used the fault path shall be completed by an equipotential bond between the LV electrical installation supply main earth and the traction return circuit. B7.3.1 When the LV electrical equipment requires to be bonded to ensure safety during traction fault conditions, 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 Appendix B for a typical arrangement). B7.3.2 The LV electrical equipment cpc provides a fault path which is completed by an equipotential bond between the LV electrical installation supply main earth and the traction return circuit. B7.3.3 A suitable permanent warning label should be securely attached to the equipotential 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. B7.4 B7.4 Telecommunications equipment within the LV electrical installations that is not allowed to be bonded to the traction return circuit Where the LV electrical installation feeds active telecommunications equipment which requires a noise free earth to operate reliably, the LV electrical installation cpc shall not be bonded to the traction return circuit, either directly or indirectly. Where the structure of the building that houses the telecommunications equipment room is metallic so that section B7.1 would apply, the telecommunications equipment room must be segregated from the remainder of the building and provided with an isolated electrical supply, which shall be independently earthed and bonded to equipment within the segregated room. 8 Earthing requirements for LV electrical installations in DC electrified lines areas B8.1 B8.1 General The requirements for DC electrified lines traction bonding are contained in GL/RT1254. 10 RAILWAY SAFETY
Page 11 of 15 B8.1.1 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. B8.1.2 As a result of the high traction current levels and corresponding volt-drop, the running rails may rise to 30 V 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 will take up a DC voltage between true earth and the running rail potential. B8.1.3 DC will flow when an earthed electrical appliance makes contact with a running rail or conductive object at running rail potential. In the case of an electrical appliance, this current will flow 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 cpc of the electrical distribution system. Also, uncontrolled DC can cause corrosion in adjacent metalwork. B8.2 B8.2 Non-electrified sidings in an electrified area Non-electrified sidings in an electrified area shall be treated as electrified lines unless fitted with insulated block joints. B8.3 B8.3 Protection against DC leakage The LV electrical installation shall prevent the adverse effects of DC leakage in the earth wire of any socket outlet, which may provide power to an electrical appliance that will be used on or near an object in contact with the running rail or other exposed conductive parts of the traction return circuit. B8.3.1 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. B8.4 B8.4 Protection against DC leakage entering the neutral conductor The LV electrical installation shall also prevent adverse effects from DC leakage which may enter the neutral of any other equipment or electrical distribution system because of earth faults. B8.4.1 Special precautions are required as the neutral conductor is earthed at the supply point and neutral to earth faults can remain undetected. B8.4.2 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. B8.4.3 For a supply with a nominal voltage of 110 V or 230 V, the supply should be derived from a BS 3535 isolating transformer, centre tapped to earth. RAILWAY SAFETY 11
Page 12 of 15 Document Withdrawn as of April 2009 B8.5 B8.5 Earth monitoring or earth proving devices The detection system of earth monitoring or earth proving devices shall be protected against the effects of DC leakage. B9 Temporary low voltage electrical installations B9.1 B9.1 Temporary LV electrical installations All temporary LV electrical installations, including construction and non construction sites, shall comply with clause 604 of BS 7671 and with HSG 141. B9.2 B9.2 Temporary supply Where an LV electrical installation is to provide a temporary supply to a rail mounted vehicle the Railway Group members shall co-operate to ensure compliance with this standard. 12 RAILWAY SAFETY
Page 13 of 15 Appendix A LV electrical equipment within the LV electrical installations directly bonded to the AC traction return circuit LV electrical installation LV electrical equipment E (cpc) LV supply LV supply main earth Section B7.2 b) - in contact with traction return circuit N L Metallic overhead line structure Section B7.2 a) - directly bonded to traction return circuit Section B7.2.4 - possible bond by agreement with REC Traction supply Traction supply main earth Traction return circuit Note: E (cpc) is not rated for traction fault current RAILWAY SAFETY 13
Page 14 of 15 Document Withdrawn as of April 2009 Appendix B LV electrical equipment within the LV electrical installations indirectly bonded to the AC traction return circuit LV electrical installation LV electrical equipment E (cpc) LV supply LV supply main earth Section B7.3 - not directly bonded to, nor in contact with, traction return circuit N L Metallic overhead line structure Traction supply Traction supply main earth Traction return circuit Note: E (cpc) is rated for traction fault current 14 RAILWAY SAFETY
Page 15 of 15 GL/RT1254 GI/RT7007 References Railway Group Standards and other Railway Group Documents Electrified Lines Traction Bonding Low Voltage Electrical The Catalogue of Railway Group Standards and the Railway Group Standards CD-ROM give the current issue number and status of documents published by Railway Safety. BS 88 BS 1361 BS 3535 BS 6701 BS 7671 BS EN 50122-1 BS EN 60309 BS EN 60898 Other References Management of Health and Safety Regulations Cartridge fuses for voltages up to and including 1000V A.C. and 1500V D.C. Specification for cartridge fuses for A.C. circuits in domestic and similar premises Isolating transformers and safety isolating transformers Code of practice for installation of apparatus intended for connection to certain telecommunication systems Requirements for Electrical Railway applications Fixed installations Part 1. Protective provisions relating to electrical safety and earthing Plugs and sockets for industrial purposes Specification for circuit breakers for overcurrent protection for household and similar installations PM29 HSG 141 HSE Guidance Notes: Electrical Hazards from Steam/Water Pressure Cleaners, etc Electrical Safety on Construction Sites RAILWAY SAFETY 15