19-04-2017, 06:18 PM
(This post was last modified: 19-04-2017, 06:29 PM by Terry.
Edit Reason: Added DC section!
)
Ah yes! Air Blast Breakers as used on the 25kV breakers on electric trains that rely on a blast of compressed air to literally blow out the spark!
I don't know if the ABB breakers get much practice these days but in the days of dual voltage overhead systems they got a lot. You could here them clearly if you sat anywhere near the Guard's Brake Van.
There was obviously some kind of voltage sensor permanently connected to the pantograph because what happened was that when the train encountered a neutral section, the breaker would drop out and when it reached the next energised section it would close again. The voltage sensor was supposed to be able to tell whether the supply was 6.25kV or 25kV and ensure that the equipment was switched appropriately but, apparently, it didn't always work correctly! One guard told me that if, when changing from 6.25kV to 25kV the sensor didn't quite get it right, the breaker would close "and then drop out again pretty damn fast!"
Of course, you don't need such high voltages at DC to cause a spark. When I was at secondary school there was something strange about the section gap at Upton Park station on the down side. Normally you would see a small arc as the shoes left and rejoined the conductor rails but this was different. In some trains every car was fitted with shoes whilst others were only equipped with shoes on alternate cars.
I can't remember which way round it was - this was 60 years ago! - but one configuration worked normally whilst the other resulted in a vivid arc about three feet long as every shoe left the rail. As the train had just started moving, this took something of the order of a second for each arc. I remember being fascinated by this process and walking down Green Street from the station looking up at the image of the arc still clearly visible among the trolleybus overhead as it gradually changed colour!
I don't know if the ABB breakers get much practice these days but in the days of dual voltage overhead systems they got a lot. You could here them clearly if you sat anywhere near the Guard's Brake Van.
There was obviously some kind of voltage sensor permanently connected to the pantograph because what happened was that when the train encountered a neutral section, the breaker would drop out and when it reached the next energised section it would close again. The voltage sensor was supposed to be able to tell whether the supply was 6.25kV or 25kV and ensure that the equipment was switched appropriately but, apparently, it didn't always work correctly! One guard told me that if, when changing from 6.25kV to 25kV the sensor didn't quite get it right, the breaker would close "and then drop out again pretty damn fast!"
Of course, you don't need such high voltages at DC to cause a spark. When I was at secondary school there was something strange about the section gap at Upton Park station on the down side. Normally you would see a small arc as the shoes left and rejoined the conductor rails but this was different. In some trains every car was fitted with shoes whilst others were only equipped with shoes on alternate cars.
I can't remember which way round it was - this was 60 years ago! - but one configuration worked normally whilst the other resulted in a vivid arc about three feet long as every shoe left the rail. As the train had just started moving, this took something of the order of a second for each arc. I remember being fascinated by this process and walking down Green Street from the station looking up at the image of the arc still clearly visible among the trolleybus overhead as it gradually changed colour!
