London Underground: middle rail

[willmow]

(Forgive me if this is the wrong forum)
I seem to recall seeing somewhere that the middle rail consists of separate sections that only go live in turn just before a train is due. Did I dream this?

 

[Elecman]

(Forgive me if this is the wrong forum)
I seem to recall seeing somewhere that the middle rail consists of separate sections that only go live in turn just before a train is due. Did I dream this?

Nope it is live at a couple of hundred volts all the time unless isolated for a possesion

 

[Mojo]

Nope it is live at a couple of hundred volts all the time unless isolated for a possesion

Or unless switched off for a short-term incident such as for certain types of unplanned works/incidents during the day or as per normal current procedures overnight!

 

[willmow]

Thanks all. I must admit to having weird dreams on occasion...

 

[CatfordCat]

(Forgive me if this is the wrong forum)
I seem to recall seeing somewhere that the middle rail consists of separate sections that only go live in turn just before a train is due. Did I dream this?

while i agree that the middle rail on the underground doesn't do this, the concept does exist -

wonder if you have seen something about stud contact, as used (with not a huge amount of success) on a few tram systems in the early 1900s (wikipedia article here) - or the 21st century equivalent in use in Bordeaux and a few other places (more here)

 

[eMeS]

... and here's me thinking that the centre rail is an earth return, at around 0V wrt earth. I wonder where I got that idea from.

 

[Dstock7080]

... and here's me thinking that the centre rail is an earth return, at around 0V wrt earth. I wonder where I got that idea from.

Most of the Sub-surface Lines (Circle/District/H&C/MET) are now 750vDC (+500v / -250v); the rest of the LU network is still 630vDC (+420 / -210v).

In this incident:
https://www.railforums.co.uk/threads/safest-way-to-rescue-someone-from-the-rails.170662/
the unfortunate rescuer had his hand severely burnt as he momentarily touched both the centre negative rail and one of the running rails at the same time.

 

[edwin_m]

... and here's me thinking that the centre rail is an earth return, at around 0V wrt earth. I wonder where I got that idea from.

Where Underground and third rail main line units use the same tracks, the middle rail is bonded to the running rails and all three sit at about earth potential, although not intentionally connected to earth because of possible problems with stray current.

 

[robbeech]

Where Underground and third rail main line units use the same tracks, the middle rail is bonded to the running rails and all three sit at about earth potential, although not intentionally connected to earth because of possible problems with stray current.

I assume this mean that mainline trains run at the reduced voltage for those sections of either 420 or 500? Or is the voltage at 750 on the third rail for this part?

 

[Dstock7080]

Or is the voltage at 750 on the third rail for this part?

At full 750vDC

 

[edwin_m]

I assume this mean that mainline trains run at the reduced voltage for those sections of either 420 or 500? Or is the voltage at 750 on the third rail for this part?

The third rail system, and also the system used where both types of train run, both have the third rail at 750V and the fourth rail at 0V, relative to the running rails. This means that on the transition to/from the "true" fourth rail system there must be a section with no conductor rails over a length at least as long as the spacing between shoes on LU motor cars, so that the bus cables between the shoes don't connect rails together that are of different potentials. LU only has shoes "bussed" together between the two ends of the same car, unlike the Southern where they are bussed through a unit, so as only LU trains pass through these transitions the gap only needs to be about one car length.

 

[cjp]

Hence the undergrounds instruction not to step on any rail?

 

[edwin_m]

Hence the undergrounds instruction not to step on any rail?

Quite easy to slip on a rail even if it's not live.

 

[Journeyman]

Quite easy to slip on a rail even if it's not live.

Yup, that's the primary reason. It's also the reason why you're supposed to only tread on ballast when walking along tracks - a frosty sleeper is absolutely lethal.

 

[Belperpete]

When I joined BR(S) I was given a safety leaflet telling me not to walk on the running rails, for the reasons described. And another one about not walking on the sleepers, for the reason stated. And another one about not walking on the ballast, which can shift under foot. Strangely enough, I wasn't given a leaflet telling me not to walk on the live rail!

 

[philthetube]

walking on the live rail is fine as long as you are not earthed, that is not to say that I recommend it.

 

[Teflon Lettuce]

When I joined BR(S) I was given a safety leaflet telling me not to walk on the running rails, for the reasons described. And another one about not walking on the sleepers, for the reason stated. And another one about not walking on the ballast, which can shift under foot. Strangely enough, I wasn't given a leaflet telling me not to walk on the live rail!

isn't it obvious why you wasn't? if the leaflets ruled out walking on the running rails, sleepers and ballast that only leaves the live rail to walk on