electrical earthing & bonding in the melbourne … · printout may not be up-to-date; refer to...

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SPECIFICATION

ELECTRICAL

L1-CHE-SPE-033

ELECTRICAL EARTHING &

BONDING

IN THE

MELBOURNE UNDERGROUND

RAIL LOOP Version: 1

Issued: 12 May 2016

Owner: Engineering

Authored by:

Andrew Russack

Head of Engineering - Electrical

ELECTRICAL SPECIFICATION ELECTRICAL

EARTHING & BONDING IN THE MURL

L1-CHE-SPE-033 Version: 1 Effective from: 12th May 2016

Approving Manager: Head of Engineering - Electrical Approval Date: 12/05/2016 Next Review Date: 12/05/2019

PRINTOUT MAY NOT BE UP-TO-DATE; REFER TO METRO INTRANET FOR THE LATEST VERSION of 13

Approval

Amendment Record

Approval Date Version Description

12/05/2016 1 Initial Issue






Table of Contents

1 Purpose .............................................................................................................................. 4

2 Scope ................................................................................................................................. 4

3 Acronyms and Abbreviations ........................................................................................... 4

4 Definitions .......................................................................................................................... 4

5 Reference Documents ....................................................................................................... 5

6 Responsibilities ................................................................................................................. 5

7 Requirements..................................................................................................................... 6

7.1 MURL Earthing & Bonding Principles..................................................................................... 6

7.2 Earthing System - Tunnel and Station Structures .................................................................. 6

7.3 Track Insulation ...................................................................................................................... 7

7.4 Connections to the Running Rails .......................................................................................... 8

7.5 Cross-bonding of Tracks in the Tunnels................................................................................. 9

7.6 Melbourne Central Tie Station .............................................................................................. 10

7.7 Insulation of Underground Station Platforms near the Trains .............................................. 10

7.8 Insulation of Melbourne Central and Flagstaff Station Roof Structures from Tram Tracks . 10

8 Appendices ...................................................................................................................... 11

MURL Schematic Earthing and Bonding Diagram .............................................................................. 11

Cross-bonding of Running Tracks at Underground Stations ............................................................... 11






1 Purpose

To set out the electrical earthing and bonding requirements within the underground portion of the Melbourne Underground Rail Loop.

This document is based on and supersedes the document entitled “Protection of Structures and Personnel from Leakage Currents from Train and Tram DC Traction Systems”, developed as part of the MURL project. (Refer Section 5. ‘Referenced Documents’ for further details)

2 Scope

This specification sets out the electrical earthing and bonding requirements within the Melbourne Underground Rail Loop. It applies only within the underground sections, including the tunnel ramps and underground station areas.

This specification does not cover the earthing and bonding requirements for the above ground section of the MURL, also known as the Overpass or Through Suburban Lines between Flinders Street and Southern Cross Stations.

3 Acronyms and Abbreviations

MURL Melbourne Underground Rail Loop

OCL overhead contact line system

RODES Remote Overhead De-energisation and Earthing System

4 Definitions

Bonding (electrical)

The electrical connection of conductive materials to form a continuous electrical conductor. Bonding is generally applied to

1. maintain electrically conductive materials at the same potential; and/or

2. Manage the return path of fault currents; and/or

3. Manage the path of traction power return currents.

Earth The general mass of earth; or

The electrical connection between an electrically conductive piece of infrastructure to the general mass of earth.

Earthing The action of making an electrical connection between electrically conductive infrastructure and the general mass of earth.

MURL Melbourne Underground Rail Loop is defined as the tunnels, stations and the overpass completing the loop between Flinders and Southern Cross Stations carrying the Through Suburban Lines.






MURL Overpass Bridge and viaduct structures carrying the Through Suburban Lines between Flinders Street and Southern Cross stations. Also known as the ‘new viaduct’.

RODES System for the remote de-energisation of the OCL system in the MURL, primarily to expedite emergency recovery activities in a safe manner.

Shall Is used as the descriptive word to express a requirement that is mandatory to achieve conformance.

Tunnel Any section of the MURL that is below ground level, including the whole of the ramps leading into the tunnel proper from normal ground level.

Should Is used as the descriptive word to express a requirement that is recommended in order to achieve compliance. The term ‘should’ can also be used if a requirement is a design goal but not a mandatory requirement.

5 Reference Documents

Electricity Safety Act 1998

Electricity Safety (Installations) Regulations 2009

AS3000: 2007 Electrical Installations

EN50122-1:2011 Railway Applications – Fixed Installations – Electrical Safety, Earthing and the Return Circuit – Part 1: Protective provisions against electric shock

EN50122-2:2010 Railway Applications – Fixed Installations – Electrical Safety, Earthing and the Return Circuit – Part 1: Protective provisions against the effects of stray currents caused by d.c. traction systems

L1-CHE-STD-015 MEST 000002-06 Electrical Networks Principles and Performance

L1-CHE-STD-010 MEST 000002-03 MTM Railway Bridges - Electrical Protection and Bonding Standard

L1-CHE-STD-016 MEST 000002-05 Track Bonding For Signalling and Traction Return Current

Melbourne Transit Authority - Melbourne Underground Rail Loop Administration: “Protection of Structures and Personnel from Leakage Currents from Train and Tram DC Traction Systems”, John Connell-Mott, Hay and Andersen, Hatch, Jacobs. Ref: 0268J. October, 1985

6 Responsibilities

Head of Engineering – Electrical is responsible for the content of this document.

Any changes to the configuration or disconnection of the earthing and bonding system in the MURL shall not be made until approved by the Head of Engineering – Electrical.






7 Requirements

7.1 MURL Earthing & Bonding Principles

An electrified railway system shall consider the potential effects of step and touch voltages under all operating and fault conditions. The earthing and bonding is put in place to mitigate the effects of step and touch voltages and to ensure that fault conditions are detected and supply can be disconnected to minimise damage to plant and equipment.

An additional issue to be considered with an electrified railway supplied via direct current is the effect of driven electrolysis due to the ‘leakage’ of some of the return current from the running rails. There always exists a finite impedance (resistance is the major component in the steady state) between the running rails and the general mass of earth. This resistance allows current to return to the substation negative bus via the earth. This fact is not in itself a problem.

However, where electric current leaves a metallic structure in the presence of an electrolyte, electrolytic corrosion (electrolysis) will occur. For iron/steel the effect will be the loss of material through oxidation at a rate of 9.2kg of material per annum for each ampere of electric current flowing. With the 1 500VDC traction power system supplying many thousands of amperes, a significant amount of material can be lost in a very short time.

Most at risk are the running rails themselves in sections of the track distant from traction power substations, particularly if the ballast is in poor condition and moist, and buried metallic infrastructure, particularly if running parallel with the rail corridor.

The MURL tunnel system predominantly sits in a very electrically conductive mix of soils and rock strata. If stray current were allowed to enter the tunnel steel structures and reinforcing and exit into the surrounding body of earth, a significant amount of damage could occur to the tunnel structure in a very short time.

The principle adopted in the underground sections of the MURL is to connect together all concrete reinforcing, exposed steel and other exposed conductive materials except the running rails and the OCL system. The running rails are insulated from the tunnel floor.

7.2 Earthing System - Tunnel and Station Structures

All tunnel wall reinforcement, station structural steel and soldier piling is connected together electrically by welding where necessary. The quantity of steel involved in the three underground stations and its area of contact with 'mother' earth, which has zero resistance, is enormous. This matrix of steel therefore provides an excellent 'earth' for electrical purposes.

In parallel with the tunnel reinforcement and bonded to it electrically at approximately 60 metre intervals, two bare stranded copper cables, each having a cross-sectional area of 150 square millimetres, run the full length of each tunnel and assist in connecting the structural steel reinforcement of the three underground stations together electrically. These are mounted clear of the tunnel walls to prevent corrosion and in addition to the foregoing are used for the connection into the earthing system of all exposed metal and other items to be protected. The earth cables are cross-bonded at regular intervals and at the stations, to those in the other tunnels.

The items connected into the earthing system via these cables (earth bars) are;

1. All exposed metal in the tunnels with the exception of the running rails, train stops and OCL system and items of equipment connected to these;






2. Box-tunnel reinforcement.

The box tunnel sections are each fitted with exposed steel plates near each end which are connected electrically by welding to the steel reinforcement therein for stray current measurement purposes. These 'monitoring points' are positioned so that adjacent box section plates and hence their internal steel reinforcing, can be easily connected together by short rods to provide electrical continuity of the reinforcement throughout all Box sections. In practice, this was achieved by the parallel earthing cables, connecting one monitoring point of each Box section direct to one of these cables.

3. All 1500 volt DC traction power Insulator pins.

4. All steady arm brackets and supports and anchor brackets.

5. All transformer and generator 'star' points and alternator neutrals.

6. All earths required for the 1500V DC, 400/230 volt AC and other power supplies.

7. All metallic air lines, water pipes and the metallic jackets or sheaths of all incoming/outgoing services to/from the stations and tunnels having exposed metallic jackets or sheaths. All such services however have been and shall continue to be provided with insulated joints in their jackets or sheaths at their points of entry or exit to the Tunnel system, whether at the stations, Tunnel portals, emergency exits, draught relief or ventilation shafts, etc.

As a prevention against corrosion and electrolytic action, no copper earth plates or rods shall be used.

Where copper/copper-alloy to steel/iron connections are proposed to be installed, consideration must be made for management of the interface between the two metals of different electronegativity.

All steel racks, brackets and the like shall be hot-dipped galvanized.

All metal-sheathed, bare copper cables and bars, cable racks and brackets shall be mounted so that there is an air space of at least 20mm between all such bare cables and bar, cable racks and brackets and the tunnel walls.

7.3 Track Insulation

The following items are bonded together electrically and in some cases also mechanically and shall be insulated and remain effectively insulated from earth, or earthed structures, at all times:

1. running rails;

2. track switches/points;

3. point machines;

4. track circuits, including pin-point detectors;

5. Impedance and cross-bonds.

The primary insulation consists of the insulation of the running rails from the double sleepers and rigid invert by large rubber pads 12mm thick between the steel Pandrol rail mounting plates and the sleepers, or rigid invert, as applicable. These pads are larger than the Pandrol mounting plates by 22mm approximately on all sides to minimise current leakage and although the Pandrol plate holding down bolts pass through these pads, they are insulated from the plates by special nylon bushes.






The double sleepers are further insulated from the invert by four or more rubber discs 350mm diameter by 15mm thick beneath each and a side pad at each end 300mm long, 150mm wide and 50mm thick. In addition, each double sleeper is insulated from its neighbouring sleepers by two spacer pads 300mm long x 150mm wide x 65-83mm thick (as required) along each side.

The RS-STEDEF sleepers are insulated from the rigid invert by two rubber boots, each of which encompasses the lower half of one of the two concrete blocks forming each sleeper.

Any train stops, pin-point detectors and other equipment connected to the rails shall also be satisfactorily insulated from the sleepers and/or the rigid invert. Particular attention shall also be paid to the elimination of metallic paths to such devices per medium of wire-reinforced air connections and electrical conduits, etc.

For the safety of buried structures it is important that the weekly inspections by patrolmen during routine track patrols and the yearly and two-yearly tests outlined in the schedule appended, be undertaken to ensure that the desired standard of rail insulation is maintained and above all, that the rails are not accidentally, or otherwise, 'earthed' by pipes, crow-bars, maintenance tools, drink cans, brake dust, ground water or other conducting liquids or materials.

The minimum resistance to earth of each pair of running rails in parallel with all signalling, communications and safety equipment connected to them, should be maintained above 5 000 ohms per lineal metre at all times. To facilitate testing, insulated joints are provided in the eight running rails at each end of the tunnels near the top of each ramp.

The 'bridges' across the insulated joints, which are essential during train operations for the traction power return currents to return to the substation negatives, shall only be removed collectively when the relevant overhead sections are de-energised. These ‘bridges’ must be replaced before the overhead sections are re-energised.

7.4 Connections to the Running Rails

These have been and shall continue to be, limited to items of signalling, communications, cross-bonding and train control systems, etc. that are effectively insulated from earth. Items such as train stops, which are contacted by the trains and pin-point detectors and point machines which are connected to one rail, shall he effectively insulated from earth and except for the Train Stops, be electrically bonded to a running rail.

The train stops have been repositioned on the curb. In this position they shall be insulated from earth and their cases left 'floating', i.e. neither connected to earth nor a running rail. This is considered to be a safe practice in this instance since they are operated from a 110 volt AC unearthed supply. This instruction also applies to the train stops mounted on the rigid invert on the ramps, through the box tunnel sections and adjacent to the STEDEF sleepers in the Burnley tunnel, south of Parliament Station.

Particular attention shall be paid to the avoidance of metallic paths via pneumatic (control air) connections and electrical conduits between the running rails and the Invert, or earthed structures. To ensure that the cases of these train stops are not inadvertently earthed by the use of conductor (wire)-reinforced pneumatic connections and electrical conduits, it is required that their exterior surfaces be painted in a distinctive colour and a plate be attached stating that the case of this apparatus is insulated from both the running rails and Earth and that its resistance to either shall be maintained above 10 000 ohms at all times.

Any diodes or clamping devices installed must be remotely indicated to Electrol to ensure a failure can be identified and rectified quickly.






Spark gaps shall not be installed between the rail and any structure as the breakdown of a spark gap could remain undetected and permit heavy currents to flow from the rails into the steel structures and onto the general mass of earth, causing accelerated electrolytic corrosion.

RODES control panels and short circuit switch cubicles are bonded to traction return.

7.5 Cross-bonding of Tracks in the Tunnels

The tracks in the four MURL tunnels are crossbonded - parallel connected electrically - at Flagstaff, Melbourne Central and Parliament Stations to reduce the resistance of the return current path from the loads (trains or faults) to the Substation negatives via the rails and hence:

Provide an increased margin of safety in respect to the tripping of the traction power circuit breakers by over-current during overhead-to-rail (or train) faults.

Reduce the current in each rail by distributing it over several rails.

Provide additional and alternative paths via the rails for return currents.

Reduce the rail-to-earth potentials for increased safety to passengers and staff during heavy overhead- to-rail for train) faults and minimising at all times, leakage currents returning to the Substation negatives via the earth and corroding steel pipes and structures.

The cross-bonding of the four tunnel tracks at each of the three underground stations is made to each of the four pairs of rails via centre-point tappings to the inductive loops of parallel-tuned circuits each of which is tuned to the audio frequency of the track circuit employed at the point of connection to the track concerned. This is essential to prevent interference between the vital safety signalling tracks circuits employed on adjacent tunnel tracks.

It is also essential, for the same reason that the tuned-circuit tuning capacitor of each cross-bond connection be directly connected between the two running rails of each track a short distance (about 200 to 300mm) ahead of the connections of its associated inductor loop so that the capacitor will short-circuit the track circuit frequency, cause the Track Circuit to operate in a 'fail safe' manner and prevent spillover detrimentally into another track, should a cable or connection on the associated inductor he accidentally or otherwise broken away from a rail.

At Melbourne Central Station, the cross-bond is also used to connect to the rails, the negative of a 1500 volt DC supply for the operation, holding and tripping of the Circuit Breakers in the Tie Station there. As the disconnection of this lead from the rails while the Traction Power is 'ON' will raise its potential to the highly dangerous level of 1500 volts, under no circumstances should it be disconnected from the cross-bonding cable, or the Cross-bonding cable be disconnected from all four rail tracks, while the traction power is 'ON'.






7.6 Melbourne Central Tie Station

The 1500 volt traction power negative of the Melbourne Central Tie Station is connected to the four sets of running tracks via the track cross-bonding cables. The following items are connected to the 1500 volt DC negative and consequently, while the negative is connected to the rail, are at rail potential in respect to earth.

All DC Circuit Breaker cubicle frames

The bus-tie screens

The 1 500V DC fuse bases

On the other hand, the following items are connected to earth and consequently are at earth potential -

All line isolator bases

All line cable screens

Cable trays

Positive terminal of the 50 volt DC battery

SCADA cabinet

Any connection between items from both groups therefore 'earths' the running rails and permits heavy traction power currents to flow back to the substation negatives via the earth.

Diodes are installed between the rail and earth (steel structure) at Melbourne Central Isolator Room to ensure any leakage current returns directly to the running rails and not via the general mass of earth.

7.7 Insulation of Underground Station Platforms near the Trains

As the metal trains will be at the same potential as the running rails, which under normal operating conditions may typically reach 60 volts above earth and higher during fault conditions, a strip of each platform at each underground station between 450 millimetre (minimum) to one metre wide from the platform edge, depending upon paving tile sizes and their arrangement, has been insulated with an epoxy or equivalent type resin to prevent passengers, who may be boarding or leaving a train at the instant a major electrical fault occurs in the vicinity on the DC traction power system, from receiving a mild but discernible electric shock.

7.8 Insulation of Melbourne Central and Flagstaff Station Roof Structures from Tram Tracks

As the tram tracks in La Trobe Street are virtually laid on the roof structures of Melbourne Central and Flagstaff Stations, stray currents from the tram rails into these structures are being controlled in each case by the installation of a high resistance Fortecon membrane between the tram tracks and the roof structure. This membrane, which is in addition to two layers of Hyload 50 pitch-polymer sheeting for water-proofing/insulation purposes, covers the entire roof structure and extends down the wall structures for about 150 millimetres.

The control of the stray currents from the tram tracks is also assisted by the use of an elementary form of overhead multiple balanced negative feeders. Refer Yarra Trams for further information.






8 Appendices

MURL Schematic Earthing and Bonding Diagram

Cross-bonding of Running Tracks at Underground Stations






Appendix 1: MURL Schematic Earthing and Bonding Diagram

[under development]






Appendix 2: Cross-bonding of Running Tracks at Underground Stations