Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Apr 13, 2016 21:16:51 GMT
The traction current is already 750v DC (led to believe its more nearer to 700v) at that point, the traction current boundary is just below Chiswick Park station. Hello, Sorry to dig up a very old topic, I have a few questions regarding the power supply at the connection between 3 rails supply (NLL) and 4 rails supply (District). I must confess having no experience of this problem: in France, when we used 3 rails system, we had only common tracks with OLE systems: 1500v DC (on the Montparnasse network)or 25kV AC (on the Saint Lazare network). I have observed on the common (used by both LUL and Overground trains) stretch between Richmond and Gunnersbury that from places to places, there were places with an approx one car long isolated section on the centre (4th) rail. Are these intended to "detect" a district train and to ensure switching to LUL system ahead of it? Or, is the supply purely "3 rails" type, with the third rail at the same potential than the running rails (apart from the track circuit system)? And, if so, how is the transition east of the junction dealt with ? |
|
|
|
Post by norbitonflyer on Apr 13, 2016 22:19:13 GMT
I'm sure there was a thread on this not long ago, but essentially as I recall, on the 4-rail system the potential is normally +440V on the positive and -220V on the negative, to give a potential difference between the rails of 660V
On a section used by 3-rail and 4-rail trains the "negative" (centre) rail is held at earth potential and the "positive" at the full 660V (or whatever), so the potential difference is still 660V. The train only sees a potential difference - it would work just as well if it were at +3000V and +3660V !
As long as the potentials of both rails change at the same place the difference across the shoes will always be 660V.
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Apr 13, 2016 22:26:57 GMT
LU's 4th rail system is currently 630v not 660v
|
|
|
|
Post by norbitonflyer on Apr 13, 2016 22:44:58 GMT
LU's 4th rail system is currently 630v not 660v +420V and -210V then. |
|
roythebus
Pleased to say the restoration of BEA coach MLL738 is as complete as it can be, now restoring MLL721
Posts: 1,275
|
Post by roythebus on Apr 23, 2016 7:17:52 GMT
The change-over is not switched at all. As at Putney Bridge and probably Queens Park where LUL meets NR, there is a long "neutral section" which separates the 2 power supplies. Whilst there are no long gaps in the current rails, there are only short isolation gaps to ensure the 2 supplies don't meet. The running rails are also isolated for the same reason.
The current rails are left in place in case a train stops on the neutral section. Long jumper cables are located nearby and can be clipped onto the current rails to provide power to get the stalled train back "on the juice".
|
|
|
|
Post by bassmike on Apr 23, 2016 11:47:19 GMT
LU's 4th rail system is currently 630v not 660v +420V and -210V then. Can be anything between 550 volts or less and over 800 volts depending on local conditions. |
|
Tom
Administrator
Signalfel?
Posts: 4,196 
|
Post by Tom on Apr 23, 2016 19:58:51 GMT
The change-over is not switched at all. As at Putney Bridge and probably Queens Park where LUL meets NR, there is a long "neutral section" which separates the 2 power supplies. Whilst there are no long gaps in the current rails, there are only short isolation gaps to ensure the 2 supplies don't meet. The running rails are also isolated for the same reason. The current rails are left in place in case a train stops on the neutral section. Long jumper cables are located nearby and can be clipped onto the current rails to provide power to get the stalled train back "on the juice". Queen's Park doesn't have a long neutral section, there's just the regular 15m gap. Putney Bridge was at one point, designed to be switchable - the drawings for the switching arrangement still exist. |
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on Apr 23, 2016 23:22:40 GMT
Same as there is drawings to have the sub gap more towards East Putney so we can actually get to our assets without the need of a NR possession every night.
|
|
|
|
Post by bassmike on Apr 24, 2016 12:28:44 GMT
I believe that Putney bridge to East Putney has(or had) a resistor arrangement to allow trains to cross the boundary without losing power completely. (mentioned by B.Prigmore in lurs magazine titled"A rough ride over Putney bridge")
|
|
|
|
Post by Dstock7080 on Apr 24, 2016 12:58:14 GMT
When I first started on the District there were changeover switches at both Putney Bridge and Gunnersbury, a long section of dead conductor rail with a short current rail gap at either end.
When a train passed over the first gap the entire length was now energised as the shoes bridged the gap.
When the whole train was now located fully on the long section the current was effectively lost and the MGs/MAs would run down on the whole train, until the first car crossed the second gap at the far end.
This was evidenced by the whole train being in darkness for a few seconds.
Now standard 48' rail gaps are used.
|
|
roythebus
Pleased to say the restoration of BEA coach MLL738 is as complete as it can be, now restoring MLL721
Posts: 1,275
|
Post by roythebus on Apr 25, 2016 15:51:54 GMT
I suppose that makes sense. In my days on the DR we had "unit stock" COP/R and "car"stock, Q. Unit stock has a 650 volt jumper through each unit only.
Car stock was individual cars and had a through jumper for the 650v, hence the need for an 8-car gap. Presumably there was a similar arrangement at Queens Park many years ago but that probably went along with the standard stock.
There was one well-known Parsons Green motorman who got gapped on Putney Bridge one evening. when asked by a passenger what had happened his witty reply was "need a shilling for the meter guv'." At the time Mr.Maxwell, the operating manager lived on the Wimbledon branch. Guess who asked the question??
|
|
