Bonding of temporary metal fences at a Railway station platform

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.

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.