21-06-2012, 06:05 PM
Quite a clever system to keep the phases apart. You get the live 25kv supply joining onto 100m of dead wire, then 100m of wire covered with ceramic beads, then 40m of dead wire (100m on bi di lines) followed by the live 25kv supply. On the ground are two sets of APC magnets (yellow boxed), one at the entrance to the neutral section, the other at the exit. The train has a sensor fitted at bogie level that opens an air blast or vacuum breaker on the power car when it runs over the first magnet, cutting off the supply to the power car. The second magnet cuts the power back in. Quite a simple system, but can be overidden by the driver using the reset button on the desk.
The more modern traction uses electronics to wind down the power as it goes over the magnets although it is always good practice to manuall power down on the approach. On the WCML, when we had AC locomotives, the 25kv feed went straight into an AC autotransformer with the output being controlled by a hand notched tapchanging switch. Power was taken by notching up or down the transformer outputs in order to achieve the desired control. Running down for a neutral section was usually initiated a mile from the neutral section, at 110mph you would be on the section in around 30 seconds, followed by shutting off just before the start of the section. We had a notorious one at Hatch End, the rule was to start running off the taps when you saw the white public house by the canal. One weekend it was repainted pink and a number of trains went though the section under full power, pulling the 25kv line breakers, extreme arcs from the pantographs, and in some cases causing severe locomotive and infrastructure damage. You just didnt mess with 25kv, it was unforgiving.
The more modern traction uses electronics to wind down the power as it goes over the magnets although it is always good practice to manuall power down on the approach. On the WCML, when we had AC locomotives, the 25kv feed went straight into an AC autotransformer with the output being controlled by a hand notched tapchanging switch. Power was taken by notching up or down the transformer outputs in order to achieve the desired control. Running down for a neutral section was usually initiated a mile from the neutral section, at 110mph you would be on the section in around 30 seconds, followed by shutting off just before the start of the section. We had a notorious one at Hatch End, the rule was to start running off the taps when you saw the white public house by the canal. One weekend it was repainted pink and a number of trains went though the section under full power, pulling the 25kv line breakers, extreme arcs from the pantographs, and in some cases causing severe locomotive and infrastructure damage. You just didnt mess with 25kv, it was unforgiving.
