Signal Voltages.

[calliope]

Hello, I'm new but I have looked through the whole forum since I have been lurking and I must say this is a fascinating site.

I've got a question which might seem silly but it's something I've always wondered.

Do the signals on the Underground run on the same voltage as the track? I would guess that the trains run on higher voltage and current and that they are two separate systems, is this right?

Look forward to posting again but until then I will carry on reading, and a big thank you to whoever set this site up.

[Colin]

Welcome The only silly question is the one that you don't ask ;D
The traction current is a total of 630v DC, 420v in the outside positive rail and 210v in the centre negative rail. The signal main is 110v AC, with the track circuits in the running rails being much lower than that, again (usually) in AC format. It is possible to get high voltages in the running rails by such things as a metal object feeding traction current (known as a negative earth), or, as used on Network Rail, running rail return of traction current. I'm sure someone will be along with a more technical answer, but that's the basic's.

There's also a bit about track voltages here:
districtdave.proboards39.com/index.cgi?board=offtopic&action=display&thread=1127255082&page=1

[Phil]

As usual, Colin's knowledge of the threads on the forum reaches parts that the others' (including staff's) can't reach

(It's when he ticks us off for not knowing, that's embarrassing :

[calliope]

Thank you very much for the answer and that has led me on to another question.

Why do underground trains have two electrical conductor rails when overground trains only have one? Why don't underground trains return the current to the track through the running rails like Network South East ones?

I hope this isn't a silly question, I have no connection with working on the underground but am interested in all types of transport and again, this is something I have always wondered. Thanks for any replies.

[Dmitri]

calliope said:

Why do underground trains have two electrical conductor rails when overground trains only have one? Why don't underground trains return the current to the track through the running rails like Network South East ones?

I hope this isn't a silly question

I'd say it is a beaten-to-death question .

www.trainweb.org/tubeprune/tractioncurr.htm#Why%204%20Rails?

[Deleted]

calliope said:

Thank you very much for the answer and that has led me on to another question.

Why do underground trains have two electrical conductor rails when overground trains only have one? Why don't underground trains return the current to the track through the running rails like Network South East ones?

The LU four-rail system uses two power rails that are indirectly connected to earth. The outer rail is charged with a nominal potential of +420V above earth, and the centre rail is charged with a nominal potential of -210V below earth. This is done by indirectly connecting the rails to earth via two large resistor banks: a 20kV bank for the positive rail, and a 10kV bank for the centre rail.

This system has advantages that the three-rail systems do not have:

- it can withstand an earth fault on either the positive side or the negative side; the indirect earth connection via the resistances means that the traction section with the fault merely settles with the faulted rail at 0V and the unfaulted rial at +630V. IOW, the section does not open its supply breakers and can still be used to run trains, as long as the unfaulted rail is not faulted.

- it does not use the running rails as a return path: this makes aetearlscourt and BAET happy as they do not have to spend time changing out impedance bonds and messing with track circuits designed to withstand DC traction current.

- it insulates the traction current from the lineside: this prevents the electrical current from lazily returning to earth via lineside structures, i.e. signal masts, cable trunking, the air main, or the cast iron tunnels. Especially the cast iron tunnels, as any lazy return via them would result in electrolytic corrosion that would make stanmore K unhappy.

<fx: stampede of LU staff racing to poke holes>

calliope said:

I hope this isn't a silly question, I have no connection with working on the underground but am interested in all types of transport and again, this is something I have always wondered. Thanks for any replies.

As COLIN said, the only silly question is the unasked one.

[calliope]

Thank you to both of you to your replies!

As I said, I have no connection with the Underground, merely an interest in all transport. In this respect, Dmitri might, given his location, be mildly interested in my own website, here

Many thanks to all and I hope you will continue to give me as much help in times to come.

[Deleted]

the signalling main is actually 600V A/C before it gets transformed down to 100V at signalling locations and is normally at 125Hz but some older sites and thoses next to network rail run at 33 1/3Hz the track voltages are between 5V and 30V A/C

[Colin]

rbruce1314 said:

As usual, Colin's knowledge of the threads on the forum reaches parts that the others' (including staff's) can't reach

(It's when he ticks us off for not knowing, that's embarrassing :

;D ;D ;D ;D ;D ;D ;D ;D ;D ;D ;D