London Underground - Traction Current

[Baby Deltic]

I found this interesting video on YT. Who's going to be the first person to model a short circuit bar across the 3rd and 4th rails?

 

[L49]

It'll have to be Ron......

 

BANG!!!

[Xerces Fobe2]

I have 3rd/4th rail and a shorting bar a 2p coin for those emergencies where something goes wrong with the DCC and a train unexpectedly speeds off out of control! This was quite common on Croxley WRD awhile ago however I found that one of the PowerCab handsets had being damaged after an event! when I was away from the layout!

 

XF

[CloggyDog]

Plenty of historic footage there, 83TS on the Jub, unrefurb C69/77 Stock on the H&C and one of the A60s in a trial livery (blue doors).

 

I recall seeing that vid as part of a voluntary LU PWay course I did some years ago, which also included use of a SCD - I think we were shown one of those old folding wooden ones, as well as the more modern magnetic type, both now superceeded by an updated version of the latter with a plastic handle.

[bécasse]

Unlike the ex-Southern third rail network, on the London Underground neither the third nor the fourth rail was (is?) tied to earth. Normally the third rail had a potential of around +420 volts and the fourth rail had a potential of around -210 volts, but these values floated depending on the extent of the inevitable leakage to earth in any vicinity. If circumstances demanded either could be temporarily tied to earth using a bar as shown in the photo (and, I suspect but my memory has become hazy, by switchgear too.

 

Lines which were served by both Underground and SR trains (e.g. between Putney and Wimbledon) had the fourth rail permanently tied to earth (i.e. the running rails).

 

Oddly, now living in Belgium, I have had to get used to household electrical wiring where neither of the supply lines are tied to earth and, indeed, momentarily earthing either will bring the circuit breakers out.

[PhilJ W]

Unlike the ex-Southern third rail network, on the London Underground neither the third nor the fourth rail was (is?) tied to earth. Normally the third rail had a potential of around +300 volts and the fourth rail had a potential of around -300 volts, but these values floated depending on the extent of the inevitable leakage to earth in any vicinity. If circumstances demanded either could be temporarily tied to earth using a bar as shown in the photo (and, I suspect but my memory has become hazy, by switchgear too.

 

Lines which were served by both Underground and SR trains (e.g. between Putney and Wimbledon) had the fourth rail permanently tied to earth (i.e. the running rails).

 

Oddly, now living in Belgium, I have had to get used to household electrical wiring where neither of the supply lines are tied to earth and, indeed, momentarily earthing either will bring the circuit breakers out.

Third rail in the UK is DC IIRC at 750 volts. This is the voltage of the third rail with the fourth/running rail used for the return. In the case of 3 and 4 rail stock sharing tracks the fourth rail is electrically bonded to the running rails. 

[bécasse]

Third rail in the UK is DC IIRC at 750 volts. This is the voltage of the third rail with the fourth/running rail used for the return. In the case of 3 and 4 rail stock sharing tracks the fourth rail is electrically bonded to the running rails. 

The actual positive voltage on the third rail varies considerably according to circumstances, not least how far away one is from the nearest traction feed and how many trains are drawing current from that feed. The nominal maximum is normally quoted as +750 volts these days, it can be a bit higher but more typically the actual measurable value is lower. The running rails (and the fourth return rail in areas where it is tied electrically to the running rails) are nominally at 0 volts, but the earth tie isn't perfect and, especially during very dry weather, and with trains in section, a small but positive voltage can sometimes be measured in these to.

 

I should, perhaps, have been more explicit in quoting the actual nominal voltages on London Underground. Because of the way the step-down transformers in the supply are tapped, the nominal positive voltage in the third rail and the nominal negative voltage in the fourth rail don't have the same numeric value, being + 420 volts and - 210 volts respectively; in the absence of any earth tie, however, these values are pretty nominal - and the potential difference between them, 630 volts, is the nominal maximum available for traction. (I have amended my original post from "around +/- 300 volts".)

 

I don't advise, though, going out and trying to measure these voltages with your multimeter, whatever its calibration scales may say.

[Dan Hamblin]

You will also find that on certain shared sections with main line stock that the voltage will be a compromise between 630V and 750V dc. At some point in, 2015 I believe, the sub-surface network is due to switch to 750V dc, which means the positive rail will go up to 480V and the negative to 270V dc (nominal).

 

The Battery Loco fleet are currently being modified to work with this higher voltage (the charging circuit, for instance, will need to sense what voltage is coming from the collector shoes so that the correct charging profile is used for the traction batteries). Other trains like the AIT will get a modified MA regulator to prevent the MA output from gassing the train batteries.

 

Regards,

 

Dan

[New Haven Neil]

So who else was disappointed they didn't show them earthing a live section......???  TBH I would have thought you need to know what to expect, it won't be like earthing 12v!

[adanapress]

In passing,  in the very early morning after the great storm (1987 was it?)  a large sheet of corrugated had been blown onto the East London line, just south of Surrey Quays station.  The fireworks were very impressive indeed , and were still going on when I left the area, with other more urgent things to attend to.  (One of Mrs. T's privatisation  printing  projects!)  It seemed that the corrugated was very briefly spot welded to the live rail/s and then the wind would break the weld .. .. and again  ... and again.

[Edwin_m]

With the 750VDC system a partial short such as this does not draw much more current than a couple of trains accelerating, so it is sometimes difficult for the circuit breakers to detect it.  The lack of an intentional connection between the power or return rails and earth also means that a short to the earth will not always cut the power either.  With 25kV the earthing strategy is different and the much higher voltage means that there is no doubt at all when a short is taking place. 

[phil-b259]

Third rail in the UK is DC IIRC at 750 volts.

 

No it isn't!

 

When first introduced in the London area by the LSWR the voltage was 630V (exactly the same as the Underground had chosen if you take the difference between the negative rail value and positive rail voltage). ALL SR suburban units up to and including the 4SUB units had their electrical gear based on this value and prolonged running at anything more would quickly cause component damage.

 

Later extensions of the 3rd rail system (including the Brighton & Portsmouth main lines) used the higher voltage of 750V taking advantages of improvements in electrical knowledge (remember all LSWR & SR schemes required large manned rotary converters to transform the AC supplied by the electric companies to DC while the Brighton and later schemes used unmaned substations fitted with mercuary arc rectifiers)

 

When the 4 SUBs finally went in the mid 80s, BR took the opportunity to raise the voltage on the inner to 750V - However much of the London Underground stock still couldn't handle anything more than 650V so the Wimbledon - Putney Bridge, Richmond - Gunnesbury plus the LNWR suburban lines from Queens Park had to stay at 650V. When the new Underground S stock has finally seen off the last of the D stock on the district then the first two sections I mentioned can go higher, but given there is no date for the replacement of the Bakerloo line stock, 650V will still be around for many years to come.

[Southernman46]

The rotary convertor substations operated from an 11kV at 16 2/3hZ supply which was replaced during the period 1954-56 with traction substations (or new equipment within the existing buildings) using 33kV at 50hZ from the National Grid which had been set up in the intervening years.

[bécasse]

The real issue with traction voltages higher than 630v and the earlier Southern emu's, notably the 3- and then 4-SUBs, was that the lighting circuits were supplied directly from the traction supply and running on 750v would very rapidly blow the light bulbs. In later "traditional" units, the lighting supply came via motor-generator equipment which overcame this problem.

 

When Eastleigh started overhauling electric units, 4-SUBs could work there under their own power but the fuses had to be removed from the lighting circuits first.