Great Western Coffee Shop

News report.

https://www.bbc.co.uk/news/business-45458222 (https://www.bbc.co.uk/news/business-45458222)

Early reports strongly suggest that this was an accident and NOT an attack or other deliberate action.
It would seem they were saved from any serious injury by the anti suicide pit between the rails.

Lucky also not to be electrocuted?

Why? The third rail is always on the far side, and the voltage on the (central) fourth rail should not be dangerous.

Yes, though dry clothing gives a good degree of protection. The middle rail is only live at about 220 volts to earth. This is potentially dangerous but relatively low risk compared to higher voltages.
The outer rail at about 440 volts to earth is a greater risk if touched, but being on the side distant from the platform is less likely to be touched by mistake.

I am old enough to remember 220 volt DC domestic mains, with which all sorts of risks were taken, but most people survived.

To clarify that, I was thinking that the fourth rail voltage was now quite low. However, on reflection I suspect broadgage is right - I thought there was a programme to increase the imbalance between third and fourth rail voltages, for safety reasons, but 2:1 may be all that is possible.

Am I right in thinking that the voltage on the Underground is +420V on the outer rail and -210V on the inner. Giving a potential difference of 630V? Am I also right in thinking that as this is only potential there is no current flow (amperage) unless there is a train in section drawing power?

I don't know enough about the power distribution, current flow and earthing of LU traction current. Could someone explain in layman's terms how it is possible to get a shock from touching the inner rail only. Can this happen with no train in section drawing power? Does touching the inner rail provide the means for current to flow from rail to earth via person, causing shock?

Basically, because the supply doesn't float - it is earthed, and forms in effect two supplies one at +420 and one at -210 V relative to the rails. The intended uses are all connected between the two, and not to earth, so all the current flows around through both halves of the supply. Ideally no current should flow through the earthing connection of the supply, though in reality all the leakage to earth can and that can be quite significant.

If it did float (i.e. was connected nowhere) it would be impossible to say with any certainty what the voltage on either rail would be - only their algebraic difference would be defined. The actual voltages would depend on the relative resistances of the leakage paths from either rail to earth.

Lots of knowledgeable people here, and I've certainly learnt a lot from all the detail given following my initial question,

Might have to read it a few more times before I understand it fully though  :D

Thanks.

Yes, the LUL system may be considered as two supplies one for each of the two live rails.
The inner rail, between the two running rails is kept at about 210/220 volts negative to earth.
The outer rail is kept at about 420/440 volts positive to earth.
The train is connected between the two and therefore receives about 630/660 volts.

Touching the inner rail whilst in contact with earth will result a 210/220 volt shock, somewhat dangerous.
Touching the outer rail and earth would result in a 420/440 volt shock, often fatal.

Either rail may be touched with impunity if the person touching it is completely isolated from earth.
A person dangling from a helicopter or balloon could touch either rail.
A bird may perch on either rail without getting a shock, this may be observed on above ground sections.

In theory a person may stand safely on a live rail if not in contact with earth.DONT EVEN THINK ABOUT ACTUALLY TRYING THIS. THE SLIGHTEST TRIP OR STUMBLE COULD BE FATAL. Or of course you might be hit by a train.

It might be more accurate to state that the voltage on the fourth/inner rail is less dangerous than that on the outer rail, or that found on most other electric railways.
By no means safe though.

The figures given predate the latest increases for S stock, on parts of lines where it is the only stock in use they are gradually raising the nominal voltage to 750 V, (+500/-250).  It can rise even higher when trains are regenerating, up to about 900V...

Paul

They were lucky, the risk of being struck by the train and the risk of electric shock ………………. just horrific.

This has been mainly on the surface areas, although the longer trend is I believe to increase the voltage to 750V nominal which NR has done on most of its network.  The raise in voltage is to do with energy efficiency (good old I²R)   It has to be born in mind that you just cannot raise one Substation rectifier to 750V and the next one be at 630V; the difference has to be graded over a number of Substation rectifiers.


LUL use a resistor potential divider to produce their reference to earth.  I would not get overly concerned whether the Voltage between the centre rail and earth is 220 or 250 as these values at DC can be lethal although factors like clothing, shoes also the concrete they were on all increase the resistance