Rail to earth voltages

[Phil]

I have just been reading on another site that the voltages of the posi and neg rails to earth are +420V and -210V respectively. I'm sure I read somewhere that at the start of four rail the neg rail was kept at 0V relative to earth. When did it change to the current ;D system and has it changed more than once during that time?

Reson for asking is that at 0V the neg would not be a hazard but at -210V it could be lethal, so why do it this way?

[compsci]

Doesn't it allow things to keep working, at least temporarily, if one of the two current rails is somehow earthed?

[Chris M]

The TfL site used to have a good FAQ on why LU uses 4 rails instead of 3, although I can't find it now. IIRC that basically said that it allows the service to continue if one of the power rails gets shorted (I think the example of a Coke can against the rail was used).
I'm no electrician, but I'd have thought that having a 4th rail at 0v relative to ground would have no benefit over having the ground return through the running rails (although I admit I've never fully grasped how this works).

Chris

[russe]

Tubeprune, at www.trainweb.org/tubeprune/tractioncurr.htm, gives a good explanation of the 'split potential' (+ve 420V, -ve 210V) system. The main reason for the 4th rail is that it provides an excellent low-resistance return electrical path. Creepage and clearance distances are of course slightly less for 420V or 210V than they are for 630V, although in electrical distance terms the differences are marginal, and the Southern 3-rail system dangles its 750V conductor rail ends far closer to the ballast (and therefore closer to rubbish and potential surface water etc) than LT does on either its positive or negative rails. In this respect, the split potential system is in electrical terms a more conservative approach, albeit a more expensive one, owing to the cost of the extra conductor rail - a significant cost factor between LT and the Southern, the latter having a far greater track mileage.

Having said that, anything north of Queen's Park on the Bakerloo, always something of a law unto itself electrically (a legacy from the LNWR and the original Bakerloo system), is I understand worked with having 630V potential on the positive rail, with the nominal 0V return path being the centre conductor rail bonded to the running rails.

Russ

[Phil]

Thanks for the link Russ- it always helps to know where to look! earth leakage makes a lot of sense for one rail not being at 630V, but can anyone confirm (or not) whether +630V to earth was used originally?

[russe]

Thanks for the link Russ- it always helps to know where to look! earth leakage makes a lot of sense for one rail not being at 630V, but can anyone confirm (or not) whether +630V to earth was used originally?

The original tube lines (City & South London, W&C, Central) were 3-rail, so had no choice but to have the whole of their potential difference to earth. (The voltage level for these lines was in the order of 500V.)

The first 4-rail system used a 630V potential difference, but I do not know whether the original electrical source feed featured a split potential arrangement. It is possible that the very early feeds were somewhat adhoc. The full commissioning of Lots Road power station would have rationalised the situation, but I do not know whether the full split potential arrangement was initiated with Lots Road coming 'on line' or later.

You're right to mention leakage currents - these are a significant design factor between 3- and 4-rail systems, and of a very high level in falling and torrential rain conditions. LT has to pay its electricity bill just like anyone else, but can afford to be far more prudent than the Southern.

Russ

[Deleted]

It is interesting that, as far as I know, LU is the only railway in the world to use a 4-rail system. This may be because it is also the only one to have significant mileage in very small cast iron tunnels.

I recall reading, many years ago, that at that time the voltages were nominally +600v on the outer rail and zero on the centre, but that they actually floated with respect to earth. This meant that, when a short developed between the outer rail and the tunnel lining, the trains were not affected: the outer became zero and the centre -600.