Power Signalling At Bristol
The three locking frames are the GRS type D, equipped with oak cabinets.
Bristol East - 368 lever spaces
Bristol West - 328 lever spaces
Bristol Loco - 32 lever spaces
They have levers spaced at 2 in. pitch, which due to the alternate upturned and downturned handles give a 4 in. handle spacing.
Each lever is equipped with a forced drop lock having an economising contact operated from the lever latch, the locks locking in the normal (A) and reverse (E) positions on point levers, and normal (A) and normal indication (B) positions on signal levers.
Two types of circuit controller are used; the air-break type for heavy point currents, operated from a rodded coupling attached to the end of the lever slide; and the rotary type used for signal controls and electric interlocking, operated by another rodded connection attached to the end of the lever slide. The rotary contacts are individually adjustable in 6 degree steps and can be accommodated to a maximum of 30 contacts per lever.
Where "Call On" signals are used, they are operated from a push button placed immediately above the respective Main Line signal lever, the push button selecting the "Call On" when the lever is pulled, there being no track control on "Call On" signals.
To avoid the multiplicity of indications displayed in many recent interlockings, it was decided to employ a more simple and arrestive type of indication, and "Correspondence" lights, which only light when the levers are in "correspondence" with their respective functions, are used.
Beneath the single row of correspondence lights is a row of "lock free" lights indicating when a lever is free to he pulled.
This arrangement has resulted in two lights only above each lever, the lamps being in every case the 12V 4W B.E.S.A. SL.5 type operated from 110V line circuits through individual lighting transformers mounted on the relay rack.
Special types of block instruments, block bells and telephone fittings were designed by the Railway Company to fit flush into the cabinet of the locking frame, which was specially adapted to receive them.
The Box to Box telephone system is a Central Battery Type with four multiple positions on the locking frame.
Each of the larger frames is equipped with three point machine ammeters and red cross protection lights, while the small Loco frame is equipped with one. These ammeters and cross protection lights are mounted with the block apparatus.
All the frame wiring is terminated on a terminal board equipped with double ended insulated headed O.B.A. terminals mounted at the bottom and running the entire length of the frame, thus there is no need for the cabin wiring to be brought up in between the moving parts of the frame.
In both the West and East frames are eight levers equipped with cranks and rodding to directly actuate mechanically operated detonators placed on the tracks almost immediately outside the cabin. This arrangement was found to be much neater than installing eight small ground frame levers at the end of the locking frame.