Bonding of temporary metal fences at a Railway station platform

Earthing and bonding associated with railways, especially 25 kV traction supplied ones is an important subject for a number of reasons.  Note that large amounts of traction energy are involved (a larger train can draw up to 300A from a nominal 25kV supply, which is a rather large 7.5 MVA), and more than one train can be drawing current simultaneously, so we are talking about a large distribution network.


The railway is a particularly challenging environment.  It isn't rocket science, but it is quite particular and has some unique requirements.  NR have a vast raft of standards that control who can do what, and how to the network, this is necessitated by the scale of the undertaking and to demonstrate to the regulators that they are managing the numerous risks in an acceptable way, especially bearing in mind the number of people who are exposed to the general environment.


As Graham has stated, it is important that a competent person has designed and specified what you need to do.  If this is a major project, I'd expect that a Principal Designer will have been nominated who will oversee and coordinate the designs of the various aspects of any works, and that will include various interdisciplinary reviews to ensure that any new works won't affect existing systems.  I would expect an earthing and bonding plan to be produced which will show exactly what needs to be connected where and how, regardless of whether it is a large or small project, and that it will have been accepted by NR's asset managers.  This is very important for fences, bridges etc. and is often overlooked.


Assuming that you will have been provided with such a plan, and that your question is really about understanding just what functions the fence 'earthing' might need to fulfil, then please note the following overview of railway earthing and bonding, it is deliberately kept at a high level because there are quite a few intricacies and subtleties that aren't necessary here.  I will expand slightly on what others have already mentioned.


In a 25kV traction system, a number of supply transformers are located around the country, fed from the national grid, so typically a 132kV to 25kV transformer will be installed at a traction substation and its secondary will be connected between the OverHead Line (OHL) and the actual running rails.  The running rail is also earthed at the substation.  So the train draws power from the OHL via the pantograph, and the return current flows through the wheels of the train and the running rails back to the supply transformer.


In some ways, the running rail is a bit like a CNE conductor in a TN system, the rails aren't really insulated from earth, and regularly connect to conductive structures that sit in the ground, so most current flows through the rails* though some will flow back through the earth, or other parallel paths.  Some railway signalling uses track circuits, where the train wheels short the two rails together to signal to the signalling system that a train is present at that location.  In these cases, only one of the two rails carries return currents. 

*various electromagnetic noise immunisation techniques are used to reduce current through the rails, using additional conductors that are often installed high up on OHL equipment stanchions, this tends to cancel induced noise in lineside telecoms cables that otherwise could result in several hundred volts being induced in them, though some current still flows in the rails.


So with the above as a context, here are some earthing and bonding functions associated with our ac electrified railway:

Induced Voltages

In the same way that a lineside telecoms conductor could theoretically have a large voltage induced in it, so can long conductive items such as fences.  Any conductive fence that is more than 3m long and in proximity to the railway should be bonded to control induced voltages.  Lineside palisade fences are often bonded.

Protection from Shock - Falling Conductors

Overhead conductors do fall from time to time, whether from storms, tree damage or faulty trains snagging the conductor or its supporting structure.  A 'collapse zone' is defined as being the zone into which a falling cable could reasonably expect to land/collapse.  For a short period of time until protection is applied, anything that the conductor touches in that zone would be energised to 25 kV unless we did something to limit it.  The standards require that conductive items that are in the collapse zone have a conductive path back to traction return (e.g. the running rails) using 158 mm2 aluminium bonds so that in the event that a wire collapses, a fault current will flow back to the transformer by the rails, and the bond sizes are set such that the maximum short-term fault voltage is kept to tolerable limits.


In station areas there are usually various conductive LV system components e.g. CCTV, Lighting Columns, shelters, ticket machines etc, as well as passive conductive items such as fences, railings, handrails etc as well as footbridges and so on, and they can all theoretically become in contact with a failing OHL.  Therefore, these parts are bonded to the station main earth busbar, and large bonds then connect the busbar(s) to the running rails and therefore are protected in the same way as is mentioned above.

Protection from Shock - Touch Potentials

A fence could be considered to be an extraneous conductive part, and anything within reach should be considered from a touch potential perspective.  This could include, for example, a new fence that was run to within 2.5m of an equipment case, or a fence that was reachable from the body of a train.

Return Currents

As was already mentioned, return currents also flow through the earth, and will essentially flow back to source by any means that they can.  If you have a long fence, bonded to the rails at several locations, then you would have a low resistance parallel path through the fence in which a notable amount of return current could flow.  This is why, typically, long lengths of lineside fences are gapped every 400m, with a single large bond connecting the middle of the fence run to traction return.  So in all instances, potential paths for return current should be considered at the design stage.

Rise of Earth Potential

In some locations, especially near substations, there can be 'hot spots' where there could be significant Rise of Earth Potentials, and with the risk of transferring such potentials in the events of supply faults.  So careful earthing design and gapping where appropriate needs consider such risks.


I hope that was of some use, it wasn't meant to be heavy, but I have personally seen people (including myself in earlier days) underappreciating some of the sometimes surprising subtleties that are only understood with time and immersion in the environment.  It is important that a competent person specifies the work, and that competent people familiar with the risks undertake it.  The railway is a very unforgiving place, whether from the risk of being struck by rail vehicles, or from the instructure, e.g. the civils engineering surveyor who was measuring a concrete bridge and whose tape rule slipped and touched the overhead line.

Earthing and bonding associated with railways, especially 25 kV traction supplied ones is an important subject for a number of reasons.  Note that large amounts of traction energy are involved (a larger train can draw up to 300A from a nominal 25kV supply, which is a rather large 7.5 MVA), and more than one train can be drawing current simultaneously, so we are talking about a large distribution network.


The railway is a particularly challenging environment.  It isn't rocket science, but it is quite particular and has some unique requirements.  NR have a vast raft of standards that control who can do what, and how to the network, this is necessitated by the scale of the undertaking and to demonstrate to the regulators that they are managing the numerous risks in an acceptable way, especially bearing in mind the number of people who are exposed to the general environment.


As Graham has stated, it is important that a competent person has designed and specified what you need to do.  If this is a major project, I'd expect that a Principal Designer will have been nominated who will oversee and coordinate the designs of the various aspects of any works, and that will include various interdisciplinary reviews to ensure that any new works won't affect existing systems.  I would expect an earthing and bonding plan to be produced which will show exactly what needs to be connected where and how, regardless of whether it is a large or small project, and that it will have been accepted by NR's asset managers.  This is very important for fences, bridges etc. and is often overlooked.


Assuming that you will have been provided with such a plan, and that your question is really about understanding just what functions the fence 'earthing' might need to fulfil, then please note the following overview of railway earthing and bonding, it is deliberately kept at a high level because there are quite a few intricacies and subtleties that aren't necessary here.  I will expand slightly on what others have already mentioned.


In a 25kV traction system, a number of supply transformers are located around the country, fed from the national grid, so typically a 132kV to 25kV transformer will be installed at a traction substation and its secondary will be connected between the OverHead Line (OHL) and the actual running rails.  The running rail is also earthed at the substation.  So the train draws power from the OHL via the pantograph, and the return current flows through the wheels of the train and the running rails back to the supply transformer.


In some ways, the running rail is a bit like a CNE conductor in a TN system, the rails aren't really insulated from earth, and regularly connect to conductive structures that sit in the ground, so most current flows through the rails* though some will flow back through the earth, or other parallel paths.  Some railway signalling uses track circuits, where the train wheels short the two rails together to signal to the signalling system that a train is present at that location.  In these cases, only one of the two rails carries return currents. 

*various electromagnetic noise immunisation techniques are used to reduce current through the rails, using additional conductors that are often installed high up on OHL equipment stanchions, this tends to cancel induced noise in lineside telecoms cables that otherwise could result in several hundred volts being induced in them, though some current still flows in the rails.


So with the above as a context, here are some earthing and bonding functions associated with our ac electrified railway:

Induced Voltages

In the same way that a lineside telecoms conductor could theoretically have a large voltage induced in it, so can long conductive items such as fences.  Any conductive fence that is more than 3m long and in proximity to the railway should be bonded to control induced voltages.  Lineside palisade fences are often bonded.

Protection from Shock - Falling Conductors

Overhead conductors do fall from time to time, whether from storms, tree damage or faulty trains snagging the conductor or its supporting structure.  A 'collapse zone' is defined as being the zone into which a falling cable could reasonably expect to land/collapse.  For a short period of time until protection is applied, anything that the conductor touches in that zone would be energised to 25 kV unless we did something to limit it.  The standards require that conductive items that are in the collapse zone have a conductive path back to traction return (e.g. the running rails) using 158 mm2 aluminium bonds so that in the event that a wire collapses, a fault current will flow back to the transformer by the rails, and the bond sizes are set such that the maximum short-term fault voltage is kept to tolerable limits.


In station areas there are usually various conductive LV system components e.g. CCTV, Lighting Columns, shelters, ticket machines etc, as well as passive conductive items such as fences, railings, handrails etc as well as footbridges and so on, and they can all theoretically become in contact with a failing OHL.  Therefore, these parts are bonded to the station main earth busbar, and large bonds then connect the busbar(s) to the running rails and therefore are protected in the same way as is mentioned above.

Protection from Shock - Touch Potentials

A fence could be considered to be an extraneous conductive part, and anything within reach should be considered from a touch potential perspective.  This could include, for example, a new fence that was run to within 2.5m of an equipment case, or a fence that was reachable from the body of a train.

Return Currents

As was already mentioned, return currents also flow through the earth, and will essentially flow back to source by any means that they can.  If you have a long fence, bonded to the rails at several locations, then you would have a low resistance parallel path through the fence in which a notable amount of return current could flow.  This is why, typically, long lengths of lineside fences are gapped every 400m, with a single large bond connecting the middle of the fence run to traction return.  So in all instances, potential paths for return current should be considered at the design stage.

Rise of Earth Potential

In some locations, especially near substations, there can be 'hot spots' where there could be significant Rise of Earth Potentials, and with the risk of transferring such potentials in the events of supply faults.  So careful earthing design and gapping where appropriate needs consider such risks.


I hope that was of some use, it wasn't meant to be heavy, but I have personally seen people (including myself in earlier days) underappreciating some of the sometimes surprising subtleties that are only understood with time and immersion in the environment.  It is important that a competent person specifies the work, and that competent people familiar with the risks undertake it.  The railway is a very unforgiving place, whether from the risk of being struck by rail vehicles, or from the instructure, e.g. the civils engineering surveyor who was measuring a concrete bridge and whose tape rule slipped and touched the overhead line.