The 2nd panto is not for 6.25Kv, both pantos are connected to a common 25Kv bus bar on the roof which leads to the primary connection on the ABB, and thus to the transformer, there is a secondary tap off the 25Kv bus but right now l dont have acces to the drawings to find out where it goes.
6.25Kv was designed for stations and such, mind most of the GE London section was 6.25Kv and only changed over past Shenfield and on towards Colchester.
To change from 6.25 from 25Kv was done on the fly, upto 100mph at line speed, the panto takes 20-30 seconds to raise and lower to do this several times in the suburbs is prohibitive, thus there is no 6.25Kv Panto, if there were ?, where is it on EMUs ?, simple logic Holmes
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The 2nd panto was removed when all locos were converted to run with train air brakes ( locos had been running with loco air and train vac since inception ), the reservoirs are quite large so logic dictated the roof and the removal of the 2nd panto.
Note the pantos were fitted so that the knuckles were facing inwards at there respective ends, early tests found that the rear panto, knuckle facing fwd gave a better contact.
Drivers prefered to run with the aft panto at night as well, arching was near the rear of the loco and didnt effect vision as much as it did with the front panto up, few trains ran with both, maybe only on heavy trains where extra wire contact was required to take the higer current under load.
Switching is done internally and requires the use of the ABB, track side magnets are placed at the section gap and cause's the ABB to open, corresponding magnets, cause the loco to change over, past the section gap another magnet closes the ABB and the loco runs on 6.25Kv.
In the end even this was dispensed with and most 6.25Kv sections removed, in fact l dont belive the LMR even used them in anger, only the GE used them for awhile, on that region the change over was removed and the stock ran on reduced power in 6.25Kv sections.
Section gaps still exist, about every 20-30 miles depending on the route and station density, track magnets also exist and you can see the transducers mounted on the inner ends of the bogies to control the ABB through these gaps. Drivers are supposed to run down and during day time most do, however at night its possible to loose ones bearings and not run down in time, thus the ABB opens either under full load or partial load, the "splash" is quite effective at getting your attention.
Best run l had was on a 309 class that was late, driver never ran down for any section gap, running full power at line speed to make up for lost time ensured that the ABB worked overtime and the splash fair lit up the country side.
Colors, l have a color photo that shows AL3-4s in store at Bury in 1970, sadly the author has not annotated which members are which but it shows two class AL4s, one in BR blue and full yellow ends, the other in electric blue and small warning panel with white roof.
Class AL3 shows 3 locos all in electric blue, all with white roofs, one with white windscreen and small electric panel, the other two with full yellow fronts and red buffer beams.
Given the date l would say all carry pre tops numbers.
Hope that helps
Michael
