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TN 038: 2014 For queries regarding this document [email protected] www.asa.transport.nsw.gov.au Technical Note TN 038: 2014 Issued date Effective date 01 May 2014 01 May 2014 Subject: Withdrawal of EP 12 30 00 01 SP Electrolysis from Stray DC Current This technical note is issued by the Asset Standards Authority as a notification to remove from use RailCorp document EP 12 30 00 01 SP Electrolysis from Stray DC Current, Version 3.0. EP 12 30 00 01 SP is a legacy document and should be used for reference purposes only. ASA guide T HR EL 12002 GU Electrolysis from Stray DC Current, Version 1.0 supersedes this document. Authorisation Technical content prepared by Checked and approved by Interdisciplinary coordination checked by Authorised for release Name Gevik Avetian Neal Hook David Spiteri Graham Bradshaw Position Principal Engineer Earthing, Bonding and Electrolysis Lead Electrical Engineer Chief Engineer Rail Principal Manager Network Standards & Services Signature 2014 Technical Note - EP 12 30 00 01 SP Electrolysis from Stray DC Current © State of NSW through Transport for NSW Asset Standards Authority Page 1 of 1 EP 12 30 00 01 SP ELECTROLYSIS FROM STRAY DC CURRENT Version 3.0 Issued May 2010 Owner: Chief Engineer Electrical Approved by: Wilfred Leung Chief Engineer Electrical Authorised by: Wilfred Leung Chief Engineer Electrical Disclaimer This document was prepared for use on the RailCorp Network only. RailCorp makes no warranties, express or implied, that compliance with the contents of this document shall be sufficient to ensure safe systems or work or operation. It is the document user’s sole responsibility to ensure that the copy of the document it is viewing is the current version of the document as in use by RailCorp. RailCorp accepts no liability whatsoever in relation to the use of this document by any party, and RailCorp excludes any liability which arises in any manner by the use of this document. Copyright The information in this document is protected by Copyright and no part of this document may be reproduced, altered, stored or transmitted by any person without the prior consent of RailCorp. Engineering Standard Superseded by T HR EL 12002 GU Engineering Standard Electrical Superseded by T HR EL 12002 GU RailCorp Engineering Standard — Electrical Electrolysis From Stray DC Current EP 12 30 00 01 SP Document control Version 3.0 Date January 2002 May 2010 Summary of change Last Technical Review Application of TMA 400 format Contents 1 Introduction .............................................................................................................................3 2 Scope and Application ...........................................................................................................3 3 Legislation ...............................................................................................................................3 3.1 Requirement of the Act ..............................................................................................3 3.2 Administration of the Regulations..............................................................................4 4 Description of Electrolysis.....................................................................................................4 4.1 General ......................................................................................................................4 4.2 Stray Current .............................................................................................................4 5 Special Situations ...................................................................................................................5 5.1 Continuous Structures ...............................................................................................5 5.2 Overhead Wiring Structures ......................................................................................6 5.3 Isolation from MEN Systems .....................................................................................6 5.4 Concrete Bridges .......................................................................................................6 5.5 Earthed Rail Locations...............................................................................................6 6 Minimisation Techniques .......................................................................................................6 6.1 Resistance to Earth ...................................................................................................6 6.2 Minimise Voltage Drop...............................................................................................7 6.3 Isolate Other Services ...............................................................................................7 6.4 Drainage Bonds and Cathodic Protection .................................................................8 6.5 For Others..................................................................................................................8 © RailCorp Issued May 2010 UNCONTROLLED WHEN PRINTED Page 2 of 9 Version 3.0 Superseded by T HR EL 12002 GU RailCorp Engineering Standard — Electrical Electrolysis From Stray DC Current 1 EP 12 30 00 01 SP Introduction This document provides information of a general nature on electrolysis and the cause, effects and minimisation techniques of stray dc currents. 2 Scope and Application The dc electrified traction system consists of an overhead wire system which is supplied with direct current (dc) at 1500 V from traction substations spaced 5 to 15 km along the tracks. The dc current required to operate the train traction motors is received by the train pantograph from the contact wire and the current then returns to the traction substations via the wheels of the train and the unearthed rail track system. The overhead wiring is positive with respect to the rails. Ideally, all current should return through the rails, but since they are in close contact with the ground through the sleepers and ballast, some current will 'leak' from the rails and return to the substation through the ground. This is called 'stray current' or 'leakage current'. The return traction current flowing in the rails causes a longitudinal voltage drop along the length of the rails. Although the rails are nominally isolated from the main mass of earth there is inevitably a distributed leakage resistance causing a varying potential difference with respect to earth. This potential difference is negative near the traction substations and positive between substations. The resulting potential difference is generally 10-70 V, which is not dangerous. It is neither practical nor desirable to completely insulate the rails from ground. Some 'small' leakage current is desirable to ensure that the voltage between rail and earth does not become dangerous. Another limiting factor in the rail to earth resistance is the signalling system which requires the rail to earth leakage resistance to be a minimum value of 2 Ω rail to rail per km. Thus the minimum allowable value is 1 Ω rail to earth per kilometre of rail. A typical track would usually have a value of approximately 8 Ω for one kilometre of rail and a very well ballasted track would be over 50 Ω to earth for one kilometre of rail. The problem of minimising electrolysis is closely related to the problem of earthing and/or bonding of metallic structures to prevent electric shock to people. The solutions to both problems have to be a compromise since the 'best' solution for one situation results in major problems for the other situation. 3 Legislation 3.1 Requirement of the Act There are no regulations contained in the NSW Electricity Safety Act, 1945 that deal directly with the causes and mitigation of electrolysis. However, the Electricity (Cathodic Protection) Regulation 1993 allows for the control and operation of drainage bonds and cathodic protection systems. Except in some special cases, refer to section 5.5, there is no deliberate earth on the RailCorp system. © RailCorp Issued May 2010 UNCONTROLLED WHEN PRINTED Page 3 of 9 Version 3.0 Superseded by T HR EL 12002 GU RailCorp Engineering Standard — Electrical Electrolysis From Stray DC Current 3.2 EP 12 30 00 01 SP Administration of the Regulations The NSW Ministry of Energy and Utilities administers the Electricity (Cathodic Protection) Regulation under the Electricity Safety Act and the Pipelines Act. In this work, it is largely aided by the NSW Electrolysis Committee. This committee’s secretariat operates within the Ministry of Energy and Utilities. The member organisations of the committee are currently: • • • • • • • • • • • • Agility Management Pty Ltd Duke Energy International Energy Australia Gorodok Pty Ltd Hunter Water Corporation Integral Energy NSW Ministry of Energy and Utilities Pacific Power RailCorp Sydney Water Telstra Corporation The Australian Institute of Petroleum Three Sub-committees meet throughout the year to consider problems, they are: • Sydney Electrolysis Technical Committee • Newcastle Electrolysis Technical Committee • Illawarra Electrolysis Technical Committee. The technical work is done by the technical committees which meet regularly to examine applications for new bonds and generally supervise electrolysis and cathodic protection (CP) work; it has the same bodies represented as on the Main Committee. In the operation of these Committees emphasis is on co-operation to correct any problems found. The Committee also has a specially equipped vehicle for field work, e.g. determining interference from proposed and existing bonds and investigating problems. 4 Description of Electrolysis 4.1 General Electrolysis is an electro-chemical reaction involving an electrolyte and metals which are carrying a DC current. It results in the corrosion of the metal which is carrying the current, at the point where the current transfers from the metal and enters the electrolyte. For steel the corrosion rate is 9 kg per ampere year. In the case of stray traction currents, the electrolyte is moist earth, while the metals are the rails and buried metallic services such as pipes and the sheathing on power and communication cables. The buried services are usually referred to in electrolysis literature as 'structures' but should not be confused with overhead wiring structures. The buried ‘structure' does not necessarily have to be underground - it just has to connect to ground at two points. 4.2 Stray Current Stray currents leave the rails 'far' from the substation, if there is a relatively low resistance to earth. The currents then use the path of lowest resistance to return to the substation. This usually involves 'entering' a buried structure and then passing from that structure to the ground at some point 'closer' to the substation. It then passes back to the rails at © RailCorp Issued May 2010 UNCONTROLLED WHEN PRINTED Page 4 of 9 Version 3.0 Superseded by T HR EL 12002 GU RailCorp Engineering Standard — Electrical Electrolysis From Stray DC Current EP 12 30 00 01 SP another point of relatively low resistance to earth and so completes the circuit to the substation negative. At the point where the current leaves the metal and enters the earth, corrosion of metal occurs. Note that electrolysis only occurs in the ground. Regenerative Braking fitted to newer trains causes the trains to act as 'mobile substations' causing the rail potentials to be more variable than the simple case described above. In the simple model, see Figure 1 below, substation earth was always positive to rail, but this is no longer always true. Whether or not a buried structure is likely to be damaged by stray traction currents is determined by the correlation of the structure to rail and the structure to soil potentials. If it is positive corrosion is likely, if it is negative then the structure receives protection from corrosion because of the stray currents. The most common railway examples of stray current paths are discussed in Section 5 below. Substation A 1000A Substation B 1000A OHW 1000A 1000A 2000A 1000A - Istray 1000A - Istray Rail Istray Stray Traction Current (Istray) Istray Istray Istray Istray Istray + Rail to Earth Volts - Figure 1 - Current Distribution and Rail to Earth Voltage Under Uniform Conditions 5 Special Situations 5.1 Continuous Structures Continuous structures such as metal lineside fencing and metal signalling troughing provide good paths for stray currents because they are close to the tracks for long distances and are connected to earth at many points (to 'pick up' and 'drop off' current). Another example is overhead earthwires which are erected over high voltage transmission lines to protect against lightning. The wires are earthed at each pole and also connected to the substation earth mat which provides a very good earth. © RailCorp Issued May 2010 UNCONTROLLED WHEN PRINTED Page 5 of 9 Version 3.0 Superseded by T HR EL 12002 GU RailCorp Engineering Standard — Electrical Electrolysis From Stray DC Current EP 12 30 00 01 SP For further information refer to Specification EP 12 10 00 21 SP - “Low Voltage Installations Earthing”. 5.2 Overhead Wiring Structures The OHW structure footing has a resistance to earth. A survey of structures has shown that the values of resistance to earth vary from 1.5 to 280 ohms. Where a OHW structure is spark-gapped to rail (refer to Specification EP 12 20 00 01 SP - “Bonding of Overhead Wiring Structures to Rail”) and the spark gap operates (becomes short-circuited) a good path for stray currents is created. For further information refer to Specification EP 12 20 00 01 SP - “Bonding of Overhead Wiring Structures to Rail”. 5.3 Isolation from MEN Systems The local Electricity Distributors low voltage supply commonly use a multiple earthed neutral (MEN) system of earthing. The neutral conductor is reticulated throughout the areas through which the RailCorp network operates and provides a good 'pick up' and 'drop off' facility. The earth electrodes will be corroded at the drop off point. An MEN supply is only permissible if the installation and its earthing electrode are at reasonable distance from the electrified rail or any metal which may be connected to it. For further information refer to Specification EP 12 10 00 21 SP - “Low Voltage Installations Earthing”. 5.4 Concrete Bridges The use of reinforced or prestressed concrete bridges raises special concerns when used for DC railways. If the reinforcing bars or stressing wires are not insulated from rail then these will carry traction current. Even if they are insulated, concrete is not a good insulator and there could be some stray current in the bars/wires. The length of these structures increases the possibility of leakage. Since the steelwork is necessary for the strength of the bridge, it is vital that corrosion does not occur. 5.5 Earthed Rail Locations As mentioned previously, the RailCorp traction system is not deliberately connected to earth, although one earth is 'allowed'. Some situations require that the rails be earthed for safety reasons. Examples are in coal and wheat loaders, where potential sparks due to a voltage between rail and earth would be catastrophic. For further information refer to Specification EP 12 10 00 13 SP - “1500 V Traction System Earthing”. 6 Minimisation Techniques The following minimisation techniques are recommended for any person engaging in work within the ‘railway corridor’ and near ‘1500 V track’. All mandatory requirements are covered in relevant documents. 6.1 Resistance to Earth Ensure the rails and associated negative connections have relatively high resistance to earth, in particular: © RailCorp Issued May 2010 UNCONTROLLED WHEN PRINTED Page 6 of 9 Version 3.0 Superseded by T HR EL 12002 GU RailCorp Engineering Standard — Electrical Electrolysis From Stray DC Current EP 12 30 00 01 SP • Keep rails clear of dirt and mud, particularly in sidings, yards, level crossings and through stations. • Steel sleepers are not used in the electrified area unless track circuited. (The track circuiting detects any low resistance to earth). • Non-electrified lines and sidings are separated from electrified lines by insulated rail joints. These are installed such that stabled trains do not short them out. • Spark gaps have not blown and the insulation of structure bonding cables is not damaged. • In tunnels, on bridges and under air-space developments, there is no contact between rails and reinforcing or other steelwork. • At Substations and Sectioning Huts, negative connections are insulated from earth and trackside negative rail busbars are not covered in mud or ballast. Rail Earth Contactors are not closed for longer than necessary. • In Car Sheds, traction return rails are not connected to building framework and are well insulated from earth by the use of insulating pads under the rails or epoxy coating of rails. • Overhead wiring structures which are bonded to rail via a spark gap do not contact earthed services such as station awnings, fences, water pipes etc. 6.2 Minimise Voltage Drop Ensure the voltage drop along the rail is minimised. This means ensuring the electrical resistance of the return circuit is minimised. • • • • • Use as many running rails as possible for traction return. Install tie-in bonds to share the current between tracks. All rail bonds and impedance bonds are correctly installed. Minimise substation spacing. Substation voltages are equalised. Adjacent substations should be balanced as far as output voltages are concerned. Equal voltages will keep the return rail current and therefore the resulting volt drop in the rail at a minimum. Accordingly the voltage available to drive a stray current from the rail to, say, Telstra cable sheath and thence to the Substation would be minimised. 6.3 Isolate Other Services • Keep metallic services 'away' from the track so there is less chance of 'picking up' appreciable dc leakage current. • All low voltage supplies use Isolating transformers. local Electricity Distributor neutral and earthing systems should not enter Railway Corridor. • Water and Gas pipes servicing buildings on the Railway Corridor and near 1500 V track to have an isolating joint installed at the boundary. • Water and Gas pipes crossing or laid along the Railway Corridor and near 1500 V track to be insulated from earth. This also applies to other services such as power or communication cables with metallic sheaths. • Metallic lineside fencing to have insulating panels installed every 500 m. • Metallic signalling troughing along the track to have insulating sections every 500 m. Care must be taken to ensure that metallic lids do not 'bridge out' the insulated trough section. • Fencing at stations and electrical substations is not to be connected to the lineside fencing. • Concrete poles should not be used on the Railway Corridor and near 1500 V track. Other local Electricity Distributors 'advised' not to use concrete poles near 1500 V track, especially if overhead earth wire or neutral wire fitted. • Ensure all metallic structures such as footbridges, bus shelters etc. are isolated at boundary of Railway Corridor. This is usually achieved by installing two 'gaps' in © RailCorp Issued May 2010 UNCONTROLLED WHEN PRINTED Page 7 of 9 Version 3.0 Superseded by T HR EL 12002 GU RailCorp Engineering Standard — Electrical Electrolysis From Stray DC Current EP 12 30 00 01 SP the steelwork, about 2m apart. Special care is needed if there is lighting installed, to ensure the local Electricity Distributor’s earth is not connected to the steelwork which forms part of any overhead wiring structure, station or bridge. • Within Railway Corridor ensure there are no long lengths of metallic water/gas/air pipes. This is particularly applicable to car sheds. • RailCorp high voltage cable screens should not be continuous between substations. Numerous methods have been tried to minimise corrosion for bridges. Some are: • Insulating membranes have been installed in the concrete between the rails and reinforcing but this is prone to damage during construction and cannot be repaired after the bridge is built. • All steel has been bonded together and the current flow monitored to ensure there is minimal leakage. If leakage is excessive, Drainage Bonds (see later) can be installed. • Some major prestressed concrete bridges such as Rushcutters Bay and Woolloomooloo viaducts were successfully insulated by an insulating layer of epoxy grout between the roadbed concrete and the structural concrete. 6.4 Drainage Bonds and Cathodic Protection Cathodic protection (CP) relies on making the metal to be preserved negative to the soil thus avoiding corrosion. However with relatively large voltages encountered with stray currents, it is not fully effective and railway drainage bonds may be useful, see Figure 2. A drainage bond relies on a deliberate metallic connection of the structure to be provided to the rail in order that the stray currents return to the rail without first going into the soil. The bond has some resistance to limit the current returning to the rail from a particular underground structure, (the drained structure becomes negative to soil and therefore attracts current from adjacent structures); excessive drainage from one structure may cause hazard to other structures. For a steel structure it is preferable to keep the potential between 0.85 V and 2 V negative to soil; damage such as hydrogen embrittlement or disbonding of insulation may take place at higher negative voltages. The standard method used for determining the potential affecting the structure is by Cu/CuSO4 half-cell to make repeatable contact with soil. The corrosion effect on the underground structure is very largely dependent on whether it is bare or insulated, (coated). Although insulation of the metal if perfectly applied and maintained gives full protection against both types of corrosion, defects in insulation are unavoidable. Because of them current would now be concentrated in the small defect area where corrosion can take place at a relatively high rate. Therefore this insulation is usually supplemented with the CP system. However full protection against stray current potentials cannot be obtained as CP voltages are usually lower than the traction potentials involved. 6.5 For Others • • • • © RailCorp Issued May 2010 Use isolating joints to divide the buried structure into short lengths. Select routes away from the route of the DC traction system. Use insulating coatings. Use Drainage Bonds and Cathodic Protection. UNCONTROLLED WHEN PRINTED Page 8 of 9 Version 3.0 Superseded by T HR EL 12002 GU RailCorp Engineering Standard — Electrical Electrolysis From Stray DC Current EP 12 30 00 01 SP Figure 2 - Typical Electrolysis Drainage Bond Panel Fuse = 25 Amp Lamps = 32v, 250W Resistor = 0.30 Ω tapped @ = 0.22 & 0.26 Ω connected @ = 0.26 Ω Diode = BYX52R Conductance bond = 1.5 S @ 4.0 V © RailCorp Issued May 2010 (I=6 A) UNCONTROLLED WHEN PRINTED Page 9 of 9 Version 3.0