In article
,
D7666 writes
I'm probably wrong but I thought mirrors are an aid to driver to see
guard but DOO needs CCTV ?

DOO requires a clear view of the entire side of the train. This can be
done through mirrors, CCTV, or a combination.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is:

In article
,
Neil Williams writes
Might it be easier to fit CCTV to the train rather than the platform?

You still need to fit the cameras and the wiring. All it saves is the
cost of the monitors, but at the expense of a transmission system.

The Northern and Central Lines have this, but that's mostly because
there's no room to retro-fit the monitors in tube stations.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is:

In article , Steve Fitzgerald
] writes
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.

There are resistors (I've seen various figures between 1 ohm and a few
hundred ohms) connecting the power rails to earth. Because they are in a
2:1 ratio, in the absence of anything odd going on you get a small
current flowing through those resistors and thus a potential divider.
For example:

+----- +ve{630V supply}-ve -----|
| |
Outer Centre
rail rail
| |
|----[210ohm]---+---[105ohm]----|
| | |
|
Ground

In the idle state, there will be 630V across the pair of resistors which
add up to 315 ohms, meaning 2 amps will flow through them. Since the
central tap is grounded, the outer rail will be at +420V relative to
ground and the centre rail at +210V.

Suppose that something shorts the centre rail to ground, bridging out
the 105 ohm resistor. We now have:

+----- +ve{630V supply}-ve -----|
| |
Outer Centre
rail rail
| |
|----[210ohm]---+---------------|
| | (short) |
|
Ground

The centre rail is at ground potential and the outer rail is at 630V
because of the power supply. 3A now flows through the resistor. If,
instead, we short the outer rail to ground, a similar change happens.

Finally, what happens if some strange event causes a 210V potential
between the outer rail and ground? We get:

+----- +ve{630V supply}-ve -----|
| |
Outer Centre
rail rail
| |
|----[210ohm]---+---[105ohm]----|
| | |
|----{210V}-----|
| |
|
Ground

The difference between the 210V and the 630V supply means that the
centre rail will spring to -420V relative to ground. There will be a 1A
flow through the 210 ohm resistor and a 4A flow through the 105V
resistor.

In other words, the resistors keep the rails at the +420V/-210V position
*unless* something else happens to alter this. Hence "loosely tied". But
at all times (absent a major short) the two rails are 630V apart.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is:

On Tue, 2 Dec 2008 07:49:35 +0000, "Clive D. W. Feather"
wrote:

In article , Steve Fitzgerald
] writes
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.

There are resistors (I've seen various figures between 1 ohm and a few
hundred ohms) connecting the power rails to earth. Because they are in a
2:1 ratio, in the absence of anything odd going on you get a small
current flowing through those resistors and thus a potential divider.
For example:

+----- +ve{630V supply}-ve -----|
| |
Outer Centre
rail rail
| |
|----[210ohm]---+---[105ohm]----|
| | |
|
Ground

In the idle state, there will be 630V across the pair of resistors which
add up to 315 ohms, meaning 2 amps will flow through them. Since the
central tap is grounded, the outer rail will be at +420V relative to
ground and the centre rail at +210V.

Suppose that something shorts the centre rail to ground, bridging out
the 105 ohm resistor. We now have:

+----- +ve{630V supply}-ve -----|
| |
Outer Centre
rail rail
| |
|----[210ohm]---+---------------|
| | (short) |
|
Ground

The centre rail is at ground potential and the outer rail is at 630V
because of the power supply. 3A now flows through the resistor. If,
instead, we short the outer rail to ground, a similar change happens.

Finally, what happens if some strange event causes a 210V potential
between the outer rail and ground? We get:

+----- +ve{630V supply}-ve -----|
| |
Outer Centre
rail rail
| |
|----[210ohm]---+---[105ohm]----|
| | |
|----{210V}-----|
| |
|
Ground

The difference between the 210V and the 630V supply means that the
centre rail will spring to -420V relative to ground. There will be a 1A
flow through the 210 ohm resistor and a 4A flow through the 105V
resistor.

In other words, the resistors keep the rails at the +420V/-210V position
*unless* something else happens to alter this. Hence "loosely tied". But
at all times (absent a major short) the two rails are 630V apart.

They can be both at the same potential if there is a break between one
conductor rail feed and the substation (or only one of the pair of
switches feeding from the next section is closed) and you have a train
in section. There will be no voltage between the rails but they will
definitely be alive, leaving a trap for anyone who ignores the
standard "do not touch any rail" warnings,

On Dec 2, 6:55*am, "Clive D. W. Feather" cl...@on-the-
train.demon.co.uk wrote:
In article
,
Neil Williams writes

Might it be easier to fit CCTV to the train rather than the platform?

You still need to fit the cameras and the wiring. All it saves is the
cost of the monitors, but at the expense of a transmission system.

The Northern and Central Lines have this, but that's mostly because
there's no room to retro-fit the monitors in tube stations.


But don't the Electrostars have the cameras on the train, so there is
no transmission system needed, other than feeds into the train's own
systems.

You can see the cameras down the side of the train in this image:

http://www.railfaneurope.net/pix/gb/...377-432-mt.jpg

The Northern and Central lines are more complex of course, there
probably isn't space for cameras on the side of the cars.

On Dec 2, 6:55 am, "Clive D. W. Feather" cl...@on-the-
train.demon.co.uk wrote:

You still need to fit the cameras and the wiring. All it saves is the
cost of the monitors, but at the expense of a transmission system.

The Northern and Central Lines have this, but that's mostly because
there's no room to retro-fit the monitors in tube stations.

Jubilee Lines stock does as well.

--
Nick

On Dec 2, 6:53 am, "Clive D. W. Feather" cl...@on-the-
train.demon.co.uk wrote:

D7666 writes

I'm probably wrong but I thought mirrors are an aid to driver to see
guard but DOO needs CCTV ?

DOO requires a clear view of the entire side of the train. This can be
done through mirrors, CCTV, or a combination.


Thinking more on this since I made that comment perhaps my involvement
with underground infrastructure has confused me ... perhaps we have a
CCTV requirement that national railways do not.

--
Nick

On Nov 30, 4:47 am, Charles Ellson wrote:

relays drop out causing an alarm;

reset up to three times in
succession before someone is sent

Nope. Respond at first alarm with an emergency fault. At least thats
how we do it on ''my'' bit of underground infrastructure.

--
Nick

On Tue, 2 Dec 2008 13:17:18 -0800 (PST), D7666
wrote:

On Nov 30, 4:47 am, Charles Ellson wrote:

relays drop out causing an alarm;

reset up to three times in
succession before someone is sent

Nope.

Well, it's been a few years since someone demonstrated the process to
me.

Respond at first alarm with an emergency fault. At least thats
how we do it on ''my'' bit of underground infrastructure.

The same procedure for both unreproducable trips and persistent trips?

"Clive D. W. Feather" wrote in message
...
In article
, Neil
Williams writes
Might it be easier to fit CCTV to the train rather than the platform?

You still need to fit the cameras and the wiring. All it saves is the cost
of the monitors, but at the expense of a transmission system.

The Northern and Central Lines have this, but that's mostly because
there's no room to retro-fit the monitors in tube stations.

Less infrastructure and less cost?