21-04-2017, 09:38 AM
6.25Kv on the railways was installed due to concerns over flashovers in areas of tight clearance ie. under bridges, narrow tunnels and structures within confined areas. Catenary is usually of the two wire contruct, lower contact wire on which the pantograph rubs, and the upper catenary wire with droppers to hold it all together and register arms on the end of insulators to set the wire stagger in order to spread out wear on the pan head. It was discovered that by skewing the catenary wire some 45 degrees or more from the vertical, the magnetic field around the catenary could be bent and would remove the flashover risk, so 6.25kv was eventually phased out and 25kv is the norm. The voltage sensing was somewhat erratic in operation, another reason to standardise.
Neutral sections, look trackside and you'll see a two small boxes at the entrance to the section and another pair going out. These are APC magnets, and if you look on the side of the power car or locomotive bogies you will see an APC sensor. The idea was that just before the neutral section the driver would shut off power. The APC would trigger and open the ABB or VCB on the roof, cutting off the power to the traction unit. The pan would then leave the copper contact wire, rub along approx 100yds of ceramic insulator, then over the neutral section separator itself, over another 40yds of ceramic insulator and then back onto the copper contact wire on which another APC magnet would activate the ABB or VCB and power would be restored. If this didn't happen, the SET/RESET on the drivers desk would close the ABB/VCB manually. Modern traction packages allow the driver to keep the power handle open when going through the neutral sections, the switching was done cleanly, but it's not recommended. The older locomotives could pull a prodigious arc when going through under full power and drop the breakers out at the substation on either side of the neutral section.
And, a good and. If somebody says the power is off on that road, you still can't touch it. If an adjacent road is still energised, anything up to 1kv can be induced onto the powered down catenary until earth clips are put on. Nasty surprise that.
Neutral sections, look trackside and you'll see a two small boxes at the entrance to the section and another pair going out. These are APC magnets, and if you look on the side of the power car or locomotive bogies you will see an APC sensor. The idea was that just before the neutral section the driver would shut off power. The APC would trigger and open the ABB or VCB on the roof, cutting off the power to the traction unit. The pan would then leave the copper contact wire, rub along approx 100yds of ceramic insulator, then over the neutral section separator itself, over another 40yds of ceramic insulator and then back onto the copper contact wire on which another APC magnet would activate the ABB or VCB and power would be restored. If this didn't happen, the SET/RESET on the drivers desk would close the ABB/VCB manually. Modern traction packages allow the driver to keep the power handle open when going through the neutral sections, the switching was done cleanly, but it's not recommended. The older locomotives could pull a prodigious arc when going through under full power and drop the breakers out at the substation on either side of the neutral section.
And, a good and. If somebody says the power is off on that road, you still can't touch it. If an adjacent road is still energised, anything up to 1kv can be induced onto the powered down catenary until earth clips are put on. Nasty surprise that.
