Earth bonding on hand rails.

[morgz94]

I live close to an unmanned unsignalled foot crossing over the Great Western MAin Line. I've noticed that there is some heavy guage bonding attached to the base of the safety railing either side of the crossing. I couldn't get a photo as I had left my phone behind. Could anyone tell me please what the purpose of this is? Thanks.

 

[Tio Terry]

Safety. All metalwork is bonded together to avoid different potentials across two different earths within touching distance.

 

[pdeaves]

As Tio Terry says. The railway there is electrified so it primarily protects against induced electrical currents.

 

[Joseph_Locke]

And because the hand rails are close to or within the fall zone of the live catenary, the bonds are rated to deal with a full fault current (IIRC 12kA) so your normal domestic equipotential earth wire isn't enough.

 

[edwin_m]

We used to get a warning that if you cut through something like a metal conduit alongside an AC electrified route a dangerous voltage could appear between the two halves, so you should electrically connect them by attaching a wire before making the cut.

This is, I guess, because some of the traction return current flows through the earth around the conduit, by a mixture of leakage and induction, so there will be a voltage gradient through the earth. As the conduit is a better conductor than the earth, the two halves it will be at the same potential as the earth some distance away in each direction so the difference will be large.

Electrically connecting the handrail on each side of the crossing will avoid a similar hazard if someone could touch both at once. This is as well as the various other hazards mentioned above.

 

[MarkyT]

And because the hand rails are close to or within the fall zone of the live catenary, the bonds are rated to deal with a full fault current (IIRC 12kA) so your normal domestic equipotential earth wire isn't enough.

That fault current figure surprised me at first but when I considered the often vast size of 25kV substation areas and the large number trains within one that could be drawing current simultaneously it makes sense. While 750V currents are so much higher per train, the substation areas on DC are tiny by comparison.

 

[morgz94]

Hi, many thanks, this interesting reading.

 

[satisnek]

I can remember standing at one of the platform ends at Glasgow Central in the late 1980s, by a metal railing where one platform face finished before the other. This railing was slightly loose and wobbly, and touching it I could feel a fairly strong tingle. Things were rather more lax back then!

It's the same principle used, apocryphally, by farmers who had high tension power lines running across their land and obtained free electricity without making a physical connection!