Power Signalling At Bristol
The new permanent way is laid with 95 lb. B.S. rails to G.W.R. standard practice. Points are operated by G.R.S. model 5A direct current machines fed from a 120 volt battery situated in the cabin.
These point machines are self-contained units comprising a motor, operating mechanism, point lock, motor control contacts and detector contacts.
A special feature of the machine is the provision made to bring its mechanism to rest without shock, by means of a "dynamic snub" - a current generated by the motor, after the machine has performed its work, flowing through the motor and control wires on closed circuit. The energy in the moving parts is expended in generating this current, and the machine brought to rest without shock.
Individual A.C. detection circuits ar e employed, no additional wires being required for the purpose. The transformers employed to energise the detection circuits are designed to provide current for one detection relay only, so affording protection from the effects of crossed wires, etc.