On Sat, 29 Nov 2008 12:52:12 -0500, Christopher A. Lee
wrote:

On Sat, 29 Nov 2008 09:26:38 -0800 (PST), MIG
wrote:

On Nov 29, 2:52*pm, Christopher A. Lee wrote:
On Sat, 29 Nov 2008 14:06:54 -0000, wrote:
"Charles Ellson" wrote in message
.. .
On Mon, 24 Nov 2008 20:32:20 -0000, wrote:

Is a fourth rail really necessary out that way, however?

It is if you want the same train to work both there and on the
Underground.

Was the 38 stock on Island Line modified to work only on third rail? What
did that require, if that was the case?

Modification to the same general standard as other SR 3rd-rail stock,
returning current via the wheels and chassis instead of via a central
pickup to a conductor rail. Older LU stock might also have required
upgrading of cables if the insulation was not suitable for 660v (or
750v?) use; under normal conditions an LU train running on LU 4-rail
track doesn't have any parts at more than 440v to earth but when
running on sections of "hybrid" 3or4-rail the highest normal voltage
to earth is 440V to earth. Under fault conditions (centre conductor
rail earthed) on LU the 3rd rail voltage can rise to the full supply
voltage and current stock has cable insulation already rated to cope.

I always understood
that they were really needed only for the tub sections of the tube, to
help
power flow into the motors.

No the LU 4-rail power supply is intended to keep the traction current
within the two conductor rails and not find its way back via other
bits of metal with consequent damage.

In case of flooding, perhaps?

No. Current leaking to ground and causing electrolytic corrosion.

Remember, there were already pipes etc under London before the tubes
were built, and the tubes were lined with cast iron segments..

Tram and streetcar track had the running rails at minus 10 volts so
that stray current leaked from ground to the rails instead of vice
versa.

LU's centre rail is I believe at minus 200 volts.

That's interesting, because I was under the impression that there was
no danger from the middle track, but 200 V could presumably give a
whack if there was any current.

There isn't much current if there's nothing in section. You used to
see track workers hopping on and off the centre rail and walking along
it.