On Apr 12, 2:33*pm, wrote:
I read about this in wikipedia but is it actually going to happen? If so what
will happen on sections where tube and sub surface stock run on the same track
*- ie raynors lane to uxbridge and acton town to ealing common. Will these
sections be kept at 630V or will the piccadilly stock be able to handle 750V
anyway?

B2003

The issue on LUL is that at a nominal voltage of 750 V, the maximum
voltage that could be seen at the train could be 900 V (under light
loading conditions) and equipment in some of the older stock could be
vulnerable to damage at such a voltage (eg by flashover). This
particularly applies to starting resistances. It is for this reason
that the Southern's line voltage is kept to 660 V in the inner
suburban area (to about 15 miles from London, plus branches like
Tattenham Corner and Epsom Downs), owing to the interfaces at Waterloo
(W&C), East Putney, Wimbledon, Gunnersbury and Richmond, and the
complexity of the inner suburban network.

(Although not used by Southern trains, the Richmond - Gunnersbury
section is part of the SR electrification system and is separated from
the NLL system at Gunnersbury Junction by a gap in the con-rail. A
similar gap also exists on the District Line.)

The line voltage is graded upwards outside the suburban area; on the
Brighton Line, for example, the last 660 V substation is Coulsdon
North (IIRC).

The NLL and Euston - Watford DC electrification remains at 650 V owing
to the inteface with the Bakerloo Line at Queen's Park. Beyond there,
instead of the standard LU +400 and -230 V arrangement, the outside
conductor rail is at 650 V and the centre one is bonded to the running
rails; these cross-bonds can be seen at regular intervals. Beyond
Harrow and Wealdstone, the redundant centre rail is retained to reduce
the resistance of the traction return circuit, as the DC lines only
use single-rail track circuits, unlike the Southern which uses double
rail.

I understand that LUL has a long-term strategy to upgrade the line
voltage to 750 V, and all new stock is capable of this, but until the
last of the older stock is withdrawn (I believe that the D stock
cannot operate at 750 V but subsequent builds can), this will not be
possible across the complete network.

HTH.