Power Signalling At Bristol
The track circuits are the condenser fed type having a 4 1/2 Mfd. condenser, adjustable in steps of I, I 1/2 and 2 Mfd's, in series with the 110 volt primary winding of the track feed transformer. The secondary winding connected to the rail is 10 volt with a tap at 7.5 volt, capacity of each transformer being 10 V.A.
The track relays are the well known Model 2 Form A type, equipped with 10 front and 2 back contacts. Being rotor operated they are most efficient and economical in operation. The local winding is energised directly from the 110 volt A.C. supply. The track winding is arranged so that the same relay can be used with safety on either a short track circuit having high ballast insulation resistance, or on long tracks having low ballast insulation resistance. The general efficiency of the type of track circuit installed at Bristol is such that all track relays have been brought to and housed in the Cabins, an ideal arrangement from the Maintainer's point of view. Relay terminals are provided with G.R.S. insulated heads.
Track feed and track relay leads to the rails are single 7/.029 in. Ancalite insulated wires carried in trunking mounted at sleeper level and brought out to the rail terminal bonds through wooden trunking risers.
The actual connection to the terminal rail bonds is loosely wound round it for two or three turns before being soldered, thus preventing fractures which may be caused by a directly connected soldered joint due to rail vibrations.