Bonding of temporary metal fences at a Railway station platform

Well, yes, HV and how the return of some of the traction current is shared through the ground, particularly close to infrastructure such as buildings etc, which necessarily include earthing systems themselves.

mmmmm..........may I please start with traction current - what is that? ?


How does the current "return" - surely current in = current out and none of it should be going via an earth anywhere? Unless maybe its got to do with HV and induced currents of metalwork within a certain range of the HV lines?


My ignorance remains vast; I'm no earthing expert and have never spent any time what so ever on the finer details of earthing and bonding beyond - what needs earthing and bonding in a house or small commercial building.

In all big commercial buildings I just check that the main earthing and bonding is there but very often I don't have much of a clue what I'm looking at, let alone if its correct for the size of supply. I usually only need to worry about a floor on a multi-storey office block and not about the supply - that's always someone else's problem/responsibility. 


Do the guidance notes on earthing and bonding cover this specific area?

Induced current is one problem.


However, the overhead wires provide 22 kV "line" overhead, with the Neutral (earth) return through the wheels of the train. Whilst there is a wire for the 22 kV neutral to return to the transformer, some of the current is shared through the actual ground itself back to the earth of the 22 kV traction supply transformer.

Earthing on sprawling infrastructure like railways and very large buildings / campuses is very different to what you would consider for a single building.

Thank you Graham. 


Wow I did not know that the actual track was a current carrying conductor...... I assume that there must be multiple earthed connections all the way down it (Perhaps to rods? or is the metal to ground contact sufficient due to the area of coverage?) 


So in bonding this fencing and stairwells between the platform and the overhead concourse, I'm trying to achieve zero potential difference between those items and an HV earthed point?


Does that mean my bonding needs to go back to the HV side of the transformer somehow/somewhere, because I'm sure from the little I've seen, the existing fence bonding seems to go into local - assumed by me - LV distribution cabinets/boxes - but I havn't seen into them yet and havn't had a chance to do so yet. 


Anything to do with HV is totally outside of my experience, including the earthing side, but I'm half sure this is an LV earthing thing. Is that correct?


Kind Regards


Tatty

There is a Neutral return wire connected to the rails, but yes the large contact area means some of the traction current can travel through the ground.


Now, as to the design of the particular station, the LV may well already be connected to an appropriate earthing system - so that may well be correct.


Have you been told where to bond, or just asked to bond it?

In bonding fences, stairwells etc, it's to bond locally regardless of whether it's HV or LV.


You'd definitely be advised to check with the engineering team for the station or line, and get hold of the relevant Railtrack standards.

Thank You Graham

I think it's 25kV rather than 22kV and Network Rail rather than Railtrack  - but otherwise I'm in agreement with Graham. The bonding conductors can sometimes be much (much) larger than you'd expect from BS 7671 - presumably to cover the case where the return traction current is flowing through various bits of metalwork - which your temporary fencing might happen come into contact with.

  - Andy.

Sorry ... yes, I must have been half asleep this morning, definitely Network Rail, and definitely 25 kV