On Sat, 29 Nov 2008, Steve Fitzgerald wrote:

In message , Christopher A. Lee
writes
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.

Traction current is fed at +420v (positive) on the outside rails and -210v on
the centre rail (negative) the sum of these giving a traction feed of 630v
dc.

They are 'loosely' tied to earth through resistances in the sub stations
that feed the supply. Despite being an electrician in a previous life
and now a driver, I struggle to get my head round 'loosely tied to
earth' and what it means in real life.

I recollect an incident a few years ago when the traction current had
been discharged (turned off) due to person under a train. For
complicated reasons, the centre rail had become live at +420v.

This must be some new meaning of the term 'turned off' of which i was not
previously aware! As you say, complicated reasons - but this sounds like
the kind of thing that really, really shouldn't happen. Crumbs.

tom

--
This is your life and it's ending one minute at a time.

On Sat, 29 Nov 2008 18:33:29 -0000, wrote:

"MIG" wrote in message
...

: 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.

Doesn't the live rai change back and forth between third and fourth
position? I remember hearing that the live rail is always in the third
position when in a station, because there is less of a risk of somebody
getting fried, should they fall onto the tracks. But the positive can be in
fourth position just before and after the station.

No. The outside rail goes outside the opposite running rail from the
platform in stations.

In message , Tom
Anderson writes

I recollect an incident a few years ago when the traction current had
been discharged (turned off) due to person under a train. For
complicated reasons, the centre rail had become live at +420v.

This must be some new meaning of the term 'turned off' of which i was
not previously aware! As you say, complicated reasons - but this sounds
like the kind of thing that really, really shouldn't happen. Crumbs.

It had been correctly discharged but been re-fed in error from
elsewhere. You will note that what should normally have been -210v had
now become +420v.
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)

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.

On Sat, 29 Nov 2008 18:53:49 +0000, Tom Anderson
wrote:

On Sat, 29 Nov 2008, Steve Fitzgerald wrote:

In message , Christopher A. Lee
writes
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.

Traction current is fed at +420v (positive) on the outside rails and -210v on
the centre rail (negative) the sum of these giving a traction feed of 630v
dc.

They are 'loosely' tied to earth through resistances in the sub stations
that feed the supply. Despite being an electrician in a previous life
and now a driver, I struggle to get my head round 'loosely tied to
earth' and what it means in real life.

Doing it by memory from an old issue of "Underground" the resistor
network is thus :-

POS RAIL - ~400ohms - EARTH - ~200ohms - NEG RAIL

Each resistor has a relay across it. If the power supply fails both
relays drop out causing an alarm; if either conductor rail is earthed
then one of the relays will drop out and bring up an alarm to indicate
an earth fault (if both rails are earthed then you have a short
circuit anyway). Transient earth faults can be caused by windblown
metallic debris so IIRC the alarms will be reset up to three times in
succession before someone is sent out to investigate.
The "loose tie" to earth is an electrical consequence of the detection
system and a design method of limiting the maximum conductor rail
voltage under non-fault conditions. Purely "floating" (nothing
connected to earth) systems can lead to such things as fault voltages
which are the sum of two or more supplies if the wrong wires come into
contact.

I recollect an incident a few years ago when the traction current had
been discharged (turned off) due to person under a train. For
complicated reasons, the centre rail had become live at +420v.

This must be some new meaning of the term 'turned off' of which i was not
previously aware! As you say, complicated reasons - but this sounds like
the kind of thing that really, really shouldn't happen. Crumbs.

One train (or two?) bridging two sections ? It doesn't need to be a
"good" connection to make a conductor rail lethally live. If it isn't
the same incident ISTR there was also at least one incident involving
incorrect closure of the switches (or failure to re-open one/them)
between two adjacent sections.

In article ,
(Charles Ellson) wrote:

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.

On hybrid sections (e.g. East Putney to Wimbledon) the 4th rail is at
+630v.

--
Colin Rosenstiel

On Sat, 29 Nov 2008 14:06:54 -0000 someone who may be
wrote this:-

Was the 38 stock on Island Line modified to work only on third rail?

Undoubtedly.

What did that require, if that was the case?

Leaving aside the incidentals, in a four rail system the traction
electricity flows from one conductor shoe via a cable to the motor
and then via another cable to the other conductor shoe. The traction
current is thus confined to its own separate circuit and will not
interfere too much with other circuits.

In a three rail system the traction electricity flows from the
conductor shoe via a cable to the motor and then via another cable
to a rotating contact on the axle, from where it flows to the wheel
and to the running rail. Thus the traction electricity is sharing
the running rails with any other circuits and arrangements have to
be made to stop it swamping these.

In a nutshell, to convert the train from one system to the other the
central conductor shoes are removed and the cable connected to newly
installed rotating contacts instead. These contacts are very much
like the commutator of a "traditional" electric motor but there is
only one segment.

Then what is the problem with Bakerloo line trains continuing further north,
assuming that they don't require any modifications?

They don't require any modifications (other than in the past those
already mentioned by others to do with insulation if they are older
trains). In these sections the central conductor rail is not
energised but is rather connected to the running rail which is used
for traction return. A "BR" train will "see" the traction voltage
between the (outside) conductor rail and running rail and work. A
"LT" train will "see" the same traction voltage between the outside
and central conductor rail and also work.

The difference is that on these sections the voltage difference is
between +600 odd volts and zero, while on "LT" lines the voltage
difference is between +400 odd volts and - 200 odd volts. The
voltage difference used to drive the motors is the same, but the
potential to the earth is different on the two systems. I have
deliberately used round numbers for the voltages, rather than the
nominal ones.



--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

On Sat, 29 Nov 2008 09:26:38 -0800 (PST) someone who may be MIG
wrote this:-

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,

A dangerous impression. If there was no (electrical) danger from the
central conductor rail then it would not be mounted on insulators.

Insulators are a give-away that something is energised.




--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

wrote in message

On hybrid sections (e.g. East Putney to Wimbledon) the 4th rail is at
+630v.


That would be the third (positive) rail. Otherwise the 'third rail' stock
wouldn't work too well...

Paul S

On Nov 30, 11:14*am, David Hansen
wrote:
On Sat, 29 Nov 2008 09:26:38 -0800 (PST) someone who may be MIG
wrote this:-

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,

A dangerous impression. If there was no (electrical) danger from the
central conductor rail then it would not be mounted on insulators.

Insulators are a give-away that something is energised.

Piecing it all together, I suppose it adds up, in that if a neutral
rail was to be used, it might as well the the running rails or
something earthed. So the fact that there's a special rail on
insulators means it can't be neutral. Never really thought it through
though.