Hi,

I was in leicester square tube station today on the northern line waiting
for a tbe a guy appeared to have droped something on the tracks on the first
ledge I think it was. My heart skipped a beat when I saw him jump down on
the track to get it. A tube was soon coming in 2 mins. He jumped back up and
said its ok the first track is not live! after scaring many people.

I have always thought all the lines are live?

No tube staff said or did anything one was a little up the platform, didn't
seem to bother him.

"mocha" wrote in message
...
Hi,

I was in leicester square tube station today on the northern line waiting
for a tbe a guy appeared to have droped something on the tracks on the
first ledge I think it was. My heart skipped a beat when I saw him jump
down on the track to get it. A tube was soon coming in 2 mins. He jumped
back up and said its ok the first track is not live! after scaring many
people.

I have always thought all the lines are live?

Only the ones with insulators under them are live - the train runs on on the
other two, and simply rubs the other ones to get its juice. One is +630v
and the other is -210v (or the +/- the other way round )

No tube staff said or did anything one was a little up the platform,
didn't seem to bother him.

That's not a good sign! I'd at least expect them to tell him not to do it

On Thu, 17 Nov 2005 19:20:45 +0000 (UTC), "mocha"
wrote:

I was in leicester square tube station today on the northern line waiting
for a tbe a guy appeared to have droped something on the tracks on the first
ledge I think it was. My heart skipped a beat when I saw him jump down on
the track to get it. A tube was soon coming in 2 mins. He jumped back up and
said its ok the first track is not live! after scaring many people.

He's right - the 3rd (outer electric) rail can switch from side to
side, and is normally placed on the opposite side from the platform at
stations. In other words, the rail nearest the platform is normally
not live. The exception is at stations (such as Arnos Grove) where one
track is served by 2 platform faces, where of course this is
impossible.

On a related note, on the episode of "The Tube" that just finished,
there was a shot of a track worker setting up a temporary speed
restriction sign on the track somewhere on the Met. He stood astride
one of the juice rails, bent over, and put a metal frame thing right
next to the rail! This appeared to be done (though the editing may
have been deceptive) while trains were running, in between trains, so
presumably the rails were live. How close do you have to get to the
rail for there to be a danger of arcing? What if he'd stumbled or
fainted?

On Thu, 17 Nov 2005 19:40:14 GMT, "d" wrote:



Only the ones with insulators under them are live - the train runs on on the
other two, and simply rubs the other ones to get its juice. One is +630v
and the other is -210v (or the +/- the other way round )

Isn't it 420v and 210v, making a difference of 630v (and I don't know
the polarity, either)?

"Ken" wrote in message
...
On Thu, 17 Nov 2005 19:40:14 GMT, "d" wrote:



Only the ones with insulators under them are live - the train runs on on
the
other two, and simply rubs the other ones to get its juice. One is +630v
and the other is -210v (or the +/- the other way round )

Isn't it 420v and 210v, making a difference of 630v (and I don't know
the polarity, either)?

The one in the middle is the negative and carries the lower voltage of 220v,
the one furthest from the platform is the positive and carries the higher
voltage of 440v for a combined total of 660v. The other two rails (the ones
not on insulators) do in fact have a slight electrical current in them to
work the signals (somewhere between 5 and 10 volts IIRC).
--
Cheers, Steve.
Change from jealous to sad to reply.

Steve Dulieu:
The one in the middle is the negative and carries the lower voltage
of 220v, the one furthest from the platform is the positive and
carries the higher voltage of 440v for a combined total of 660v.

Correct. Well, sources disagree on the exact nominal voltage: some
say -210, +420, net 630. The variation from time to time in practice
is likely to be more than a few volts anyway.

The train only sees the voltage difference; it'll run just as well
if one live rail is at 0 and the other is at 630 V. And the power
supply is designed so that this is exactly what happens if a short
develops between one side and the earth. (This is one of the
advantages of using two live rails.)

On tracks designed to be shared with trains from the 3-rail southern
network, like at Richmond, the live rails are always at 0 and +630.

The other two rails (the ones not on insulators) do in fact have
a slight electrical current in them to work the signals ...

Yep.
--
Mark Brader, Toronto | "We are full of digital chain letters and
| warnings about marmalade." --Matt Ridley

My text in this article is in the public domain.

...there was a shot of a track worker setting up a temporary speed
restriction sign on the track somewhere on the Met. He stood astride
one of the juice rails, bent over, and put a metal frame thing right
next to the rail! ... presumably the rails were live. How close
do you have to get to the rail for there to be a danger of arcing?

Practically touching, and also practically touching a ground (earth)
at the same time.

Depending on which rail it was, the voltage is only about the same
as you have in your house, or it's twice that. And you don't worr
about electric arcs coming out of your light sockets, do you?
What's different is the current (amperage) that the power supply
is set up to deliver without tripping a breaker, which will be much,
*much* greater. So if there is an arc, it's a much bigger deal.

What if he'd stumbled or fainted?

If the rails were live and the metal frame thing managed to touch a
live rail and also a running rail or something else earthed, both
at the same time, you might have seen some fireworks. If he managed
to do it with his own body, he could have been killed -- but he was
probably wearing enough clothing to make such a double contact
practically impossible.
--
Mark Brader, Toronto | There is no step function between "safe" and "unsafe".
| -- Jeff Janes

My text in this article is in the public domain.

In message , Steve
Dulieu writes
The one in the middle is the negative and carries the lower voltage of
220v, the one furthest from the platform is the positive and carries
the higher voltage of 440v for a combined total of 660v. The other two
rails (the ones not on insulators) do in fact have a slight electrical
current in them to work the signals (somewhere between 5 and 10 volts
IIRC).
The outside rail is positive and the centre rail is the negative with a
potential of 630v between them though there has been discussion on here
as to whether its a 420/210 split or a 630/0 split. The running rails
carry a.c. Voltages of between 5 and 10v but there can be up to about 30
track circuits at some places. So the running rails can potentially
carry a tingle at some points.
--
Clive