ABY       Collect your VC on the way out

Before I tell the story, I must explain that London Underground trains use two conductor rails that are both insulated from earth.  The principle is that a derailment that might short one live rail to earth does not shut the system down.  The current drawn by a train is hundreds of amps at, I believe, 650V DC.  I do not know whether the insulation is good enough to prevent electrocution if a person touched one live rail and earth.  I saw an example of how the insulation was good enough on the trolley-bus system.  See below.

I was subservient the the middle-level chap of our team.  I had no objections to that situation as I was very green in this new environment,.  On one occasion I went to a first-floor setup at an old substation near Clapham Common.  I can't remember the chap's name so I will call him Alan.  What our job was I can't remember, but Alan explained that the length of mock rail track on this first-floor landing was used to train tube-train motormen on how to proceed if there was an emergency that necessitated getting the  passengers off the train and onto the track.  The aim of the training was to familiarise the driver with the technique of shorting out the two live rails so as to trip the circuit-breaker at the substation.

Each driver's cab, so I was told, carried a shorting bar.  The device is made so that you can place one end on to one rail, and after closing you eyes you bang the other end hard down on the other rail and hold it there.  Alan quipped after the explanation by saying: "And you collect your VC at the door."

I don't know what current is likely to be drawn, but I am sure it must be well over a thousand amps.  Just imagine the arc that a thousand amps at 650V DC would cause if you were hesitant about making a good contact.  Alan then told me that most parts of the Tube system were fed from two substations in parallel.  And to add to the drama, there was (then) a piece of track near Turnham Green that could not be tripped with a short-circuit  --  the track resistance was just  too high.  I imagine that there was some sort of special notification to motormen about that piece of track.

For non-electrical folk, DC current will sustain a huge arc once struck, whereas an AC current arc will generally extinguish.  Just one of the advantages of AC.  But heavy traction is better with DC current.  I believe that modern trams rectify the juice on the tram whereas in the 50s, rectification was much more difficult than now.