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Post by nickf on Mar 30, 2010 10:08:42 GMT
I travelled on a South West train that was diverted from Wimbledon via East Putney to the Windsor Lines via Point Pleasant Junction and I wondered what is involved when a train designed for 3rd rail pickup travels over a stretch of track which is 4th rail. Does it just put up with reduced voltage, 420 volts instead of 750 volts? If so, how does its running rail return system complete the circuit? Are there implications for track circuiting?
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Phil
In memoriam
RIP 23-Oct-2018
Posts: 9,473
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Post by Phil on Mar 30, 2010 10:20:08 GMT
The same thing happens on the Richmond branch every day, not to mention the Bakerloo!  All that matters for 4th rail trains is that there is a full voltage between 3rd and 4th rails. In tunnels it's best to keep voltages to earth as low as comfortable so as you say it's +420 and -210. On NR sections the neggy rail is simply held at 0v and the posi at +750 since it's in the open and the earthing problems due to tunnel dampness etc. are absent. |
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Post by nickf on Mar 30, 2010 10:34:27 GMT
AAhhhh!! Right! Thanks. So the 4th rail train puts up with 3rd rail practice rather than vice versa. And the increased voltage is within the tolerances, or does some voltage regulation circuit kick in?
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Post by Dstock7080 on Mar 30, 2010 10:50:10 GMT
On NR sections the negative rail is permanently bonded onto the usual return running rails.
C Stock and D Stock have 750v rated fuses.
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Post by nickf on Mar 30, 2010 10:57:23 GMT
Gentlemen, thank you very much indeed.
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Post by tubeprune on Mar 30, 2010 15:05:08 GMT
Just to add, that all 630/750v equipment is built to stand much higher voltages.
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Post by nickf on Apr 2, 2010 9:25:58 GMT
Given that when an Underground train is travelling on NR rails (750 volts) it is getting almost 20% more than the 630 volts provided on its home rails, is there a noticeable difference in performance that has to be compensated for in driving technique?
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Post by happybunny on Apr 2, 2010 9:53:07 GMT
Given that when an Underground train is travelling on NR rails (750 volts) it is getting almost 20% more than the 630 volts provided on its home rails, is there a noticeable difference in performance that has to be compensated for in driving technique? No, nothing at all. |
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Post by nickf on Apr 2, 2010 13:55:33 GMT
Thanks for that.
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