On 18 Sep, 10:47, MIG wrote:
On 18 Sep, 09:44, "John Salmon" wrote:

"Sim" wrote Now let's be nice to each other! I did not know the details Charles

kindly provided, but it does make sense. Interestingly, not only does
fourth rail (however wired) exist as far north as Harrow, but the last
time I looked there was quite a lot left further on, although some of
it was lying rather dismally in the four foot rather than perched on
insulators. It was never formally removed, in other words, although
doubtless disconnected.

Previous discussions have suggested that it is still there because it
is still connected. *I think the running rails must be wired to the
fourth rail and then the fourth rail wired to earth in the way that
the running rails are in other third rail systems. *(Not technical,
but I assume that it saved connecting the running rails for one sort
of train and duplicating the connections for another sort of train.)

Question: was this the system at Euston and Broad Street etc, ie
positive to earth, rather than a bit positive to a bit negative?

Presumably for an LU train, the difference between the rails is all
that matters, whereas for a three-rail train it's the difference
between positive and earth that matters, so a NR train on conventional
LU track would only be at 420 V or something?

I really doubt that the old fourth rail is in circuit with anything.
Some is missing, and other sections are lying in the four foot. Not
much continuity there, I would have thought!

"Sim" wrote in message
...
On 18 Sep, 10:47, MIG wrote:
On 18 Sep, 09:44, "John Salmon" wrote:

"Sim" wrote Now let's be nice to each other! I did not know the details
Charles

kindly provided, but it does make sense. Interestingly, not only does
fourth rail (however wired) exist as far north as Harrow, but the last
time I looked there was quite a lot left further on, although some of
it was lying rather dismally in the four foot rather than perched on
insulators. It was never formally removed, in other words, although
doubtless disconnected.

Previous discussions have suggested that it is still there because it
is still connected. I think the running rails must be wired to the
fourth rail and then the fourth rail wired to earth in the way that
the running rails are in other third rail systems. (Not technical,
but I assume that it saved connecting the running rails for one sort
of train and duplicating the connections for another sort of train.)

Question: was this the system at Euston and Broad Street etc, ie
positive to earth, rather than a bit positive to a bit negative?

Presumably for an LU train, the difference between the rails is all
that matters, whereas for a three-rail train it's the difference
between positive and earth that matters, so a NR train on conventional
LU track would only be at 420 V or something?

I really doubt that the old fourth rail is in circuit with anything.
Some is missing, and other sections are lying in the four foot. Not
much continuity there, I would have thought!



But lying on the ground and connected to ground!?

DW downunder

On 18 Sep, 11:10, Sim wrote:
On 18 Sep, 10:47, MIG wrote:





On 18 Sep, 09:44, "John Salmon" wrote:

"Sim" wrote Now let's be nice to each other! I did not know the details Charles

kindly provided, but it does make sense. Interestingly, not only does
fourth rail (however wired) exist as far north as Harrow, but the last
time I looked there was quite a lot left further on, although some of
it was lying rather dismally in the four foot rather than perched on
insulators. It was never formally removed, in other words, although
doubtless disconnected.

Previous discussions have suggested that it is still there because it
is still connected. *I think the running rails must be wired to the
fourth rail and then the fourth rail wired to earth in the way that
the running rails are in other third rail systems. *(Not technical,
but I assume that it saved connecting the running rails for one sort
of train and duplicating the connections for another sort of train.)

Question: was this the system at Euston and Broad Street etc, ie
positive to earth, rather than a bit positive to a bit negative?

Presumably for an LU train, the difference between the rails is all
that matters, whereas for a three-rail train it's the difference
between positive and earth that matters, so a NR train on conventional
LU track would only be at 420 V or something?

I really doubt that the old fourth rail is in circuit with anything.
Some is missing, and other sections are lying in the four foot. Not
much continuity there, I would have thought!

It is still wired to the running rails, if you look from a train at
the other track, you can see the connections. The running rails were
never the continuous electrical return and were not bonded together.
The fact that the rail lies in the four foot doesn't matter as it is
earthed, missing sections will have been bypassed with cabling and
very little is actually missing if you look for it. If nothing else,
it has to be connected to something, due to possible induction from
the 25kV AC running along side.

On Fri, 18 Sep 2009 04:17:10 -0700 (PDT), Andy
wrote:

On 18 Sep, 11:10, Sim wrote:
On 18 Sep, 10:47, MIG wrote:


On 18 Sep, 09:44, "John Salmon" wrote:

"Sim" wrote Now let's be nice to each other! I did not know the details Charles

kindly provided, but it does make sense. Interestingly, not only does
fourth rail (however wired) exist as far north as Harrow, but the last
time I looked there was quite a lot left further on, although some of
it was lying rather dismally in the four foot rather than perched on
insulators. It was never formally removed, in other words, although
doubtless disconnected.

Previous discussions have suggested that it is still there because it
is still connected. *I think the running rails must be wired to the
fourth rail and then the fourth rail wired to earth in the way that
the running rails are in other third rail systems. *(Not technical,
but I assume that it saved connecting the running rails for one sort
of train and duplicating the connections for another sort of train.)

Question: was this the system at Euston and Broad Street etc, ie
positive to earth, rather than a bit positive to a bit negative?

Presumably for an LU train, the difference between the rails is all
that matters, whereas for a three-rail train it's the difference
between positive and earth that matters, so a NR train on conventional
LU track would only be at 420 V or something?

I really doubt that the old fourth rail is in circuit with anything.
Some is missing, and other sections are lying in the four foot. Not
much continuity there, I would have thought!

It is still wired to the running rails, if you look from a train at
the other track, you can see the connections. The running rails were
never the continuous electrical return and were not bonded together.
The fact that the rail lies in the four foot doesn't matter as it is
earthed,

It isn't deliberately earthed, it just isn't insulated from earth. The
"official" connection to earth is back at the substation. If the
fourth rail was removed it would increase the resistance of the path
through which the traction current flows with consequential wastage of
power.

missing sections will have been bypassed with cabling and
very little is actually missing if you look for it. If nothing else,
it has to be connected to something, due to possible induction from
the 25kV AC running along side.

ITYF connecting the end distant from the substation to earth would
actually aid any induction by turning it into one half of a
transformer. Counteracting this would require multiple earthing.

On Sep 18, 11:44*pm, Charles Ellson
wrote:
On Fri, 18 Sep 2009 04:17:10 -0700 (PDT), Andy
wrote:





On 18 Sep, 11:10, Sim wrote:
On 18 Sep, 10:47, MIG wrote:

On 18 Sep, 09:44, "John Salmon" wrote:

"Sim" wrote Now let's be nice to each other! I did not know the details Charles

kindly provided, but it does make sense. Interestingly, not only does
fourth rail (however wired) exist as far north as Harrow, but the last
time I looked there was quite a lot left further on, although some of
it was lying rather dismally in the four foot rather than perched on
insulators. It was never formally removed, in other words, although
doubtless disconnected.

Previous discussions have suggested that it is still there because it
is still connected. *I think the running rails must be wired to the
fourth rail and then the fourth rail wired to earth in the way that
the running rails are in other third rail systems. *(Not technical,
but I assume that it saved connecting the running rails for one sort
of train and duplicating the connections for another sort of train.)

Question: was this the system at Euston and Broad Street etc, ie
positive to earth, rather than a bit positive to a bit negative?

Presumably for an LU train, the difference between the rails is all
that matters, whereas for a three-rail train it's the difference
between positive and earth that matters, so a NR train on conventional
LU track would only be at 420 V or something?

I really doubt that the old fourth rail is in circuit with anything.
Some is missing, and other sections are lying in the four foot. Not
much continuity there, I would have thought!

It is still wired to the running rails, if you look from a train at
the other track, you can see the connections. The running rails were
never the continuous electrical return and were not bonded together.
The fact that the rail lies in the four foot doesn't matter as it is
earthed,

It isn't deliberately earthed, it just isn't insulated from earth. The
"official" connection to earth is back at the substation. If the
fourth rail was removed it would increase the resistance of the path
through which the traction current flows with consequential wastage of
power.


Earth was the wrong term, I should have said near earth potential, but
I was writing in a hurry. This, slightly dated, document has the
details

http://www.rgsonline.co.uk/Railway_G...lling%20Stock/
Other/GMTT0108%20Iss%201a.pdf, section 5.4 is especially relevant for
the 'disused' 4th rail north of Harrow & Wealdstone.

missing sections will have been bypassed with cabling and
very little is actually missing if you look for it. If nothing else,
it has to be connected to something, due to possible induction from
the 25kV AC running along side.

ITYF connecting the end distant from the substation to earth would
actually aid any induction by turning it into one half of a
transformer. Counteracting this would require multiple earthing.

See above, it is still better to have the rail 'in circuit' than not.

On Fri, 18 Sep 2009 17:25:54 -0700 (PDT), Andy
wrote:

On Sep 18, 11:44*pm, Charles Ellson
wrote:
On Fri, 18 Sep 2009 04:17:10 -0700 (PDT), Andy
wrote:


On 18 Sep, 11:10, Sim wrote:
On 18 Sep, 10:47, MIG wrote:

On 18 Sep, 09:44, "John Salmon" wrote:

"Sim" wrote Now let's be nice to each other! I did not know the details Charles

kindly provided, but it does make sense. Interestingly, not only does
fourth rail (however wired) exist as far north as Harrow, but the last
time I looked there was quite a lot left further on, although some of
it was lying rather dismally in the four foot rather than perched on
insulators. It was never formally removed, in other words, although
doubtless disconnected.

Previous discussions have suggested that it is still there because it
is still connected. *I think the running rails must be wired to the
fourth rail and then the fourth rail wired to earth in the way that
the running rails are in other third rail systems. *(Not technical,
but I assume that it saved connecting the running rails for one sort
of train and duplicating the connections for another sort of train.)

Question: was this the system at Euston and Broad Street etc, ie
positive to earth, rather than a bit positive to a bit negative?

Presumably for an LU train, the difference between the rails is all
that matters, whereas for a three-rail train it's the difference
between positive and earth that matters, so a NR train on conventional
LU track would only be at 420 V or something?

I really doubt that the old fourth rail is in circuit with anything.
Some is missing, and other sections are lying in the four foot. Not
much continuity there, I would have thought!

It is still wired to the running rails, if you look from a train at
the other track, you can see the connections. The running rails were
never the continuous electrical return and were not bonded together.
The fact that the rail lies in the four foot doesn't matter as it is
earthed,

It isn't deliberately earthed, it just isn't insulated from earth. The
"official" connection to earth is back at the substation. If the
fourth rail was removed it would increase the resistance of the path
through which the traction current flows with consequential wastage of
power.


Earth was the wrong term, I should have said near earth potential, but
I was writing in a hurry. This, slightly dated, document has the
details

http://www.rgsonline.co.uk/Railway_G...lling%20Stock/
Other/GMTT0108%20Iss%201a.pdf, section 5.4 is especially relevant for
the 'disused' 4th rail north of Harrow & Wealdstone.

It is interesting to see as much as 60v allowed WRT earth. With the
currents that might be caused by a train drawing full power this would
still seem to leave the opportunity for some damage to be caused by
accidental/unintentional contact between a local (true) earth and the
traction return rail via assorted metallic objects. I've seen the
damage that 50v (200A circuit fuse IIRC) can do to a screwdriver and
other assorted things that should not have been near a busbar and it
isn't pretty.

missing sections will have been bypassed with cabling and
very little is actually missing if you look for it. If nothing else,
it has to be connected to something, due to possible induction from
the 25kV AC running along side.

ITYF connecting the end distant from the substation to earth would
actually aid any induction by turning it into one half of a
transformer. Counteracting this would require multiple earthing.

See above, it is still better to have the rail 'in circuit' than not.