Power Signalling At Bristol
The power supply for the signalling installation is taken from the Bristol City electricity supply at Temple Meads Railway station sub-station and an alternative supply from the City being taken at the East box. A ring main is provided between the Railway station sub-station and the East and West signal boxes and works at 400 volts 50 cycle A.C., this being transformed at the East and West boxes to 110 volt A.C. which is used for all signal circuits.
The point operating current is taken from the 120 volt 200 ampere hour batteries trickle charged from the A.C supply, a D.C. ring main being provided between the East and West boxes.
In the event of a complete failure of the Bristol City supply a 110 horse power Diesel engine set with push button start can be brought into use and by means of interlocked switch gear can supply the 400 volt A.C. ring main.
The lighting for the three signal boxes at Bristol is normally taken from the 110 volt A.C. signalling supply. In the event of a power failure throw-over switches are provided and the lighting connected to the 120 volt D.C. supply, the switches for this purpose being situated in a convenient place for operation by the signalman.
Two main switch boards are provided, one in the power house at the West box which also houses the Diesel standby plant, and another in the power house at the East box. Switching on the ring mains can therefore be done at the two ends of the Yard.
The 110 volt A.C. and D.C. supplies are brought on to operating switchboards fixed in the relay rooms of each cabin.
The operating switchboards supplied by the G.R.S. are the wall mounted type equipped with a duplicate set of double pole air-break overload circuit breakers, moving iron voltmeters, leakage indicators and double pole single throw knife switches, one set for the A.C. supply and the other for the D.C. supply.
The A.C. ammeter only is mounted on the switchboard, the D.C. ammeter being mounted on the locking frame, A.C. load being fairly constant is for use of the linemen whereas the D.C. ammeter will only indicate when a point machine is moving and is therefore of more use to the signalmen to observe the operation of the points.
Both the operating switchboards and the locking frames are equipped with red lights which are illuminated if a cross occurs on any point operating wire.