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    • br30453 posted
    Posted in Signalling and Infrastructure » Is this why Melbourne and Sydney suburban are electrified with overhead wires?

    It is often said that overhead electrification can be at a higher voltage than third rail. But I have heard of third-rail electrification as high as 1,500 volts, which is the same as Melbourne and Sydney, and the same as most DC overhead heavy rail electrification.

    Had electrification in these cities originally been by third rail, somehow I imagine that converting to 25kv AC supplied by catenary would have been a lot easier, with lines have both third rail for older trains, and overhead for newer trains during the transition.

    Could it be that overhead electrification was chosen for both Melbourne and Sydney because of level crossings?

    Myrtone

    The 1500v DC overhead was chosen as it was the best solution at the time when the Melbourne and Sydney systems were introduced.

    Nothing to do with level crossings.

    If those systems had been electrified with third rail then the clearances in tunnels and to overbridges would have been less than what now exists.

    So converting to overhead supply would have needed raising of all overhead structures to provide the necessary clearances.

    To convert from 1500v DC to 25,000v AC would be difficult as the "static clearance" for the former is, or was back when I was involved in these matters, 150mm, whereas for the AC system it is 300mm.

    Edit history

    Edited 03 Feb 2018 02:17, 3 years ago, edited by br30453

    It is often said that overhead electrification can be at a higher voltage than third rail. But I have heard of third-rail electrification as high as 1,500 volts, which is the same as Melbourne and Sydney, and the same as most DC overhead heavy rail electrification.

    Had electrification in these cities originally been by third rail, somehow I imagine that converting to 25kv AC supplied by catenary would have been a lot easier, with lines have both third rail for older trains, and overhead for newer trains during the transition.

    Could it be that overhead electrification was chosen for both Melbourne and Sydney because of level crossings?

    Myrtone

    The 1500v DC overhead was chosen as it was the best solution at the time when the Melbourne and Sydney systems were introduced.
    Nothing to do with level crossings.
    If those systems had been electrified with third rail then the clearances in tunnels and to overbridges would have been less than what now exists.
    So converting to overhead supply would have needed raising of all overhead structures to provide the necessary clearances.
    To convert from 1500v DC to 25,000v AC would be difficult as the "static clearance" for the former is, or was back when I was involved in these matters, 150mm, whereas for the AC system it is 300mm.