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Could isolation transformers prevent electrocution on LU tracks?
On Sep 20, 8:37*pm, Denis McMahon
wrote: On 20/09/10 20:19, Dominic wrote: On London Underground the DC traction current circuit to and from the trains is via an insulated third live rail and an insulated fourth live rail, and the earthed running rails are not used for traction current. Would it be possible to use an isolation transformer to prevent the electrocution of a person who touched one of the live rails on this type of railway, or possibly on an AC version of it? My understanding, which I am sure will be corrected, is that an isolation transformer could prevent the electrocution of a person who connected one live rail to earth, but would not prevent the electrocution of a person who connected the third live rail and fourth live rail together. I would be very grateful to anyone who can explain further. Transformers don't work for DC, and aiui the 4th (return) rail exists to reduce the corrosion effects of stray currents in the rather moist environment of the tunnels. Correct. In particular, the use of fourth rails was the result of the use of cast-iron segments to line the tunnels, as these would have been very vulnerable to corrosion. To prevent the live rail - running rail shock risk, isolate the traction supplies from the running rails. However, I suspect that not having either side of the traction supply tied to earth brings back those stray current corrosion issues, and / or it may have other issues too, like affecting track circuits. Rgds Denis McMahon Not directly earthing the traction return system is the best means to prevent stray current corrosion. On a third-rail line, the running rails are mounted on insulated fastenings for this reason, and the negative busbar at a DC substation is insulated from earth. No third rail system can ever be immune to stray current corrosion, espacially at an AC/DC interface as the running rails at such points must be earthed, but it can be managed to a level that is ALARP (As Low As Reasonably Practicable). The drawbacks of a fourth rail system are (a) additional complexity for the pway and pick-up arrangements and (b) only one rail for the return circuit (on third rail systems, both running rails are used for traction return, thereby reducing the circuit resistance and allowing a small reduction in the number of substations). The Scarborough RT system in Toronto uses an ingenious fourth rail system where two shrouded rails are used, one above the other; imagine the DLR with two conductor rails one above the other and you get the idea. |
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