4 rail and 3 rail Power supplies

[Deleted]

Can anyone help me on this point about the transition from LU, 4 rail system to what becomes effectively 3 rail in the shared tracks with 'the big stuff'. I understand about the LU system being a 4 rail system with the outer conductor being +440V and the centre return at -220V; giving 660V available for traction (give or take a volt or two) with the running rails at around zero volts. When running on shared tracks, I understand that the centre rail is still used but is bonded to the running rails but with the conductor rail now at between 650V and 750V; so there is now 650V and 0V between the conductor rails, but there is still 650 V or so available for traction . When an LU train makes the transition, what provisions are made to prevent a short circuit by bridging of the 2 systems. I presume that this is achieved by providing a dead section. likewise if, on LU, there are no power cables running through the train, any track dead sections would need to be at least 1 car length. Since the electrical equipment is now exposed to 650/750V to earth, (as opposed to 440V), do the electrics have to be to a higher insulation standard, or is this within their specification.

However I would welcome information from someone more knowledgeable than myself.  Thanks in advance.
Paul Kidger
 

[Deleted]

There is a gap between NR and LU traction of approx 1 train carriage in length

The train doesn’t care if it’s 630v or 750v as the stock that is cleared to run over these parts are actually rated upto 1000v but of course 750v plus a small margin for spikes.

[t697]

Dec 28, 2017 17:35:02 GMT @aetearlscourt said:

There is a gap between NR and LU traction of approx 1 train carriage in length

The train doesn’t care if it’s 630v or 750v as the stock that is cleared to run over these parts are actually rated upto 1000v but of course 750v plus a small margin for spikes.

And LUL trains have no connection on the train between the collector shoes on one car and the collector shoes of any other car, so the approx. 1 car gap length is enough.

[Deleted]

They did have a issue with a S8 when first introduced at Harrow on the Hill when it did energise the next traction section when it was off. Im sure they would of found the issue and done a redesign, could anyone confirm this ?

But for many years a through bus bar on the train for the traction supplies has not exsisted due to the above reasoning

[philthetube]

All rail gaps were make to be long enough to prevent power transfer around 20 years ago, I did not hear about an incedent at Harrow, and to be honest it would surprise me, but that is not to say it did not happen.

[Deleted]

It was when a driver was given a wrong signal but the following section was dead and the power control room got an alive indication when the train made its way onto the next section.

[Dstock7080]

Prior to installing the 48’ (14.6m) single rail gaps at Gunnersbury and Putney, they previously had automatic changeover switches.
Two short rail gaps at the ends of a trains-length normally dead section of track. When the train entered from say the LT side the section was live at 630v, when the whole train was detected between the rail gaps the changeover switch operated and the whole section was then live at 660/750v. The opposite occurred in the other direction.
This operation was visible as the whole train would temporarily go into darkness before the changeover switch operated.

[bassmike]

Anyone who has early "Underground News" magazines can probably find an article called "A rough ride over Putney Bridge" by Brian Prigmore which explains about the changeover section. Cant remember which issue but will try to look back when able.

[Deleted]

Dec 28, 2017 17:35:02 GMT @aetearlscourt said:

There is a gap between NR and LU traction of approx 1 train carriage in length

The train doesn’t care if it’s 630v or 750v as the stock that is cleared to run over these parts are actually rated upto 1000v but of course 750v plus a small margin for spikes.

Thanks for that information and thanks to the other replies. I thought that the equipment would be designed electrically for 750V plus, with about 1000V as the rated voltage and  probably not too far away from BR diesel and electric stock.  

[Dstock7080]

Dec 29, 2017 13:21:34 GMT bassmike said:

Anyone who has early "Underground News" magazines can probably find an article called "A rough ride over Putney Bridge" by Brian Prigmore which explains about the changeover section. Cant remember which issue but will try to look back when able.

after digging, i found it: October 1967, page 155: We Surged Our Way Across Putney Bridge

Other archive copies of “Underground News” including Indexes are now available on the LURS website at:
www.lurs.org.uk/historicalarchive.htm