Deleted
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Post by Deleted on Jun 14, 2008 9:48:37 GMT
How many other electric railways run in small-bore cast iron tunnel? Therein lies the root answer for the perpetuation of the 4th rail system. Moscow, St Petersburg, Prague have cast iron tunnels. As has been discussed before there are actually quite few metro & guided transit systems with 3rd/4th rails, but just not implemented the same way as on LU. |
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Tom
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Post by Tom on Jun 15, 2008 9:46:50 GMT
Does the size of the bore actually matter, is the 3rd rail on GN&C significantly further from the metalwork than on Underground lines? No - it is still laid out in the same style as LU. Apart from welding up rail joints, lowering the negative rail to sleeper level and bonding it to the running rails, and rerailing the Moorgate overruns to enable installation friction arrestors, the P/Way hasn't changed much since the line was inherited from LT over 30 years ago. It is worth noting however, that the lower levels of the GN&C tunnels were not cast iron, but lined in brick. They also used a 4-rail system from opening, but both rails were on the outside, so if an earth developed in a fully iron tunnel there there would have probably been a greater risk of it becoming a dead short. |
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Post by railtechnician on Jun 21, 2008 18:41:19 GMT
Surely by removing the 4th rail, you also remove the redundancy feature of a 4 rail system? Currently there is a 630v potential difference between the two current rails, as one is +420 and the other is -210. When one rail develops an earth fault, due to the way the system works, the other rails voltage is increased to 630v to allow trains to continue running rather than become stalled in a tunnel section. That's what I was always told anyway. Yes LT traction supplies are earth free, hence an earth fault on either pozi or neggy rails will be detected but will not affect the 630 volt potential difference between the two. Doing away with the neggy would result in loss of traction with a positive earth fault. As to the way it works the only reason that the pozi reads 420 volts to earth and the neggy 210 volts to earth is because every traction section has both traction rails connected to the continuous running rail via resistors to enable the detection of earth faults. All an earth fault does is shift the reference point but the potential difference between pozi and neggy is always 630 volts unless there is an earth fault on each traction rail and then you have 0 volts ! |
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Deleted
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Post by Deleted on Jun 21, 2008 18:43:49 GMT
and a big bang  |
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Post by railtechnician on Jun 22, 2008 9:53:55 GMT
Incidentally I have been told by a retired member of the Rolling Stock Engineer's department that the nominal 750v dc on the NR fed sections of the District line can rise to as much as 900 volts and that quite frequently it is at 800-850 volts which accounts for the sudden jolt often felt when moving from LU traction to NR traction. Apparently LU rolling stock is quite happy at 900 volts but then LT always was a stickler for 'belt and braces' so I wouldn't mind betting that anything built in its day would withstand 1000 volts.
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