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.

Excellent summary Brian.

To put a bit of context on this, with 25kV applied on the supply we may well find voltages of the order of 1000V drop along the catenary, 23kV across the locomotive and 1000V along the return (8% vd).

This means that earth at the locomotive could be 1000V above earth at the substation (oops). Clearly this needs careful handling even in normal situations.

Apart from safety considerations the Railway engineers need to be sure that the signalling will operate under running and 25kV fault conditions. Signalling is relatively low voltage but they need to make sure that no single fault on the signaling system can ever give a green light where it should be red. Not easy when you can have 1000V between different points at earth potential!

Regarding the return current, there is a return conductor along the catenary supports and every so often you may see a transformer on a pole. This is a 1:1 ratio transformer and ensures that whatever current is flowing in the catenary is also flowing in the earth return. This has the effect of "sucking" the return current back into the return cable rather than letting it meander across the countryside looking for the lowest resistance path.

Yes it is something of a curious system.

The feeder design actually allow a greater voltage drop in the catenary to allow economy of design and allows feeder stations to be located further apart, with consequent lower number of national grip supply points.  I have seen models with a worst case far-end catenary voltage of less than 20kV (some of the trains decide that enough is enough if the supply voltage drops to 19kV and pull their own plug).

Fortunately the system has a much lower impedance in the return paths, so we are able and obliged to design the system so as to keep rail (traction return) voltages below 25V at stations otherwise there would be intolerable touch potentials at the rails, or at any bonded metalwork, much of which is accessible to rail staff and the general public alike.

One can see that the network is far from simple, with multiple effective earth connections in the return path.  There are other subtleties which I won't bore anyone else with here.

On a final point, the booster transformer system is being phased out because certain unscrupulous people would steal the copper conductors (a hazardous occupation, with at least one culprit 'paying the price' instead of 'getting the price' later).  Longer term plans will replace this with high level aluminium conductors as part of improved system designs (AT).

Thanks for the update Brian.


When you say the voltage is kept to 25V at stations, is that 25V relative to local earth or 25V relative to earth at the substation?


When the far end catenary voltage is down to 20kV, how much of  the volt drop along the catenary and how much is along the return path?

Hi Harry,

It is important that accessible voltages are kept to safe levels wherever they can be accessed relative to local earth, e.g. the ground on which a person might stand, so this is 25V w.r.t. local earth.

Re system volt drop, most of the volt drop is in the Catenary or OHL contact wire, because it's impedance is much higher than that of the traction return circuit, which also has many parallel return paths which keeps its potential low with respect to remote (and local) earth.

Developments such as the autotransformer (AT) system effectively increase system capacity by transmitting 25kV to the OHL system, and also an antiphase supply via a parallel conductor making 50kV available to autotransformers that cleverly make more power locally available to the train whilst reducing current returning via the rails. Several of the main lines, e.g. the new Great Western electrification use the new AT scheme, which is compatible with both new and old rolling stock, and have the advantage of increasing the length of section that can be supplied from a single supply substation.

That had me looking stuff up.

To maybe save others the legowork, I have found a fairly simple if a bit maths heavy model of this antiphase lines and  AT system described  to force  a near centre zero system here  if anyone else is interested.

Thank you very much to all.


I'm definitely out of my comfort zone with respect to designing anything to do with railway station temporary bonding; I am only now just beginning to understand the hazards involved.


I'll definitely have to get someone who is competent on railways to do some designs for me. (My design experience is very limited to simple electrical circuits and I will need to submit designs in order to complete the works I have been asked to do - Network rail will also assess the designers qualifications for approval before they will accept the designs proposed) 


So I have to design temporary bonding for these fences that separate the works area from the public. The existing bonded temporary fences are bonded in what look like 25mm bonds to pipe clamps that seem to be much like domestic bonding clamps.


We're primarily a simple design and  installation crew of 4 and usually I take care of all of the PQQS, pricing, meetings, Testing, the QS role etc and the other guys assist me to do the actual install. I'd also need to design temporary lighting, small power etc, which I can do but if Network rail wont accept my qualifications then I may need a design person for that sort of stuff too.


Is there Anyone interested in doing this for me when needed with Qualifications likely to be accepted by Network Rail  for the area around Ealing Broadway in London?


Kind Regards


Tatty