Download EP 12 30 00 01 SP - Asset Standards Authority

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Superseded by T HR EL 12002 GU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Superseded by T HR EL 12002 GU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