Part 1 of this series introduced the drawings detailing the electrical controls at Spring Vale Sidings Box. There's a brief description of this signal box here. It's helpful to keep in mind the layout, shown below. Clicking on any diagram gives an enlarged image which can be downloaded or printed.
In this instalment we'll look at Block Control on the Up and Down Lines (shown on sheet 2 of the drawing set).
Introduction to Block Control:
Early block instruments were purely communication devices. The safe working of trains depended upon the signalman only giving 'Line Clear' for a train to approach when it was safe to do so. Various improvements were introduced, like 'Sykes Lock and Block' and 'Interlocking Block' but these were partially mechanical systems, not completely foolproof and were not universally adopted. Over the years, successive accidents revealed weaknesses which could be guarded against by adding electrical controls.
Signal Proving:
One source of accidents was trains running away because signals were not displaying a 'Stop' aspect when required, either through a Signalman's direct error or because the position of the signal arm did not correspond with the position of the controlling lever in the signal box. At night (or in conditions of low visibility) a signal lamp 'out' could similarly result in running away.
Normally, 'Line Clear' was given to the box in rear before permission had been obtained for the train to proceed into the section ahead. This was perfectly safe provided that the Home signal was at danger so as to stop the approaching train. In addition, the Distant signal should be at Caution, as advance warning to the driver of the need to stop at the Home signal.
Let's summarise the electrical interlocks required:-
(1) Prove signal arms 'on': The mechanical interlocking on the lever frame responds to the position of the lever - there is no information about the actual position of the signal arm controlled from the lever.
(2) Prove Home signal lever 'Normal' and electric lever lock not energised: This ensures that the Home signal levers are correctly Normal at the time 'Line Clear' is given to the box in rear.
At Spring Vale, Block Control was provided on both the Catchems Corner and Deepfields block instruments to ensure that the relevant Home and Distant signals were 'On' before 'Line Clear' could be given to the box in the rear. The detailed circuit is described below.
Catchems Corner direction:
On the Up Main, the criteria for accepting a train offered from the Catchems Corner direction were:-
- Up Distant Signal Arms (19a/19b) proved 'On'.
- Up Home Signal Arms (levers 20 & 23) proved 'On'.
- Up Home Signal Levers (20 & 23) proved 'Normal' and electrically locked.
On the right of the diagram above is the block instrument to Catchems Corner with two polarised galvanometers and the commutator switch. The right hand side of each galvanometer connects to line and thence to the block instrument in the Catchems Corner direction via contacts on the Block Switch ('BNC'). The Block Switch contacts are closed when Spring Vale Sidings box is 'Open'. On the Up Main, separate batteries are provided for giving 'Line Clear' and 'Train on Line'. Negative 'Line Clear' battery is fed via contacts: '20LCC' (Lock Proving Contact on 20 lever lock, proving lever normal and lock not energised), '23LCC' (Lock Proving Contact on 23 lever lock, proving lever normal and lock not energised), '20RGCR' (contact on Red Signal Proving Relay for signal 20), '23RGCR' (contact on Red Signal Proving Relay for signal 23),'19YCR' (contact on Yellow Signal Proving Relay for signals 19) to the 'LC' pole of the commutator. If the control conditions are met when the signalman places the Block Instrument Commutator to 'Line Clear', current flows from the battery, through the proving contacts, through the commutator at 'Line Clear', through the closed front contact of relay 'T1TPS' (track Circuit 1 Track Repeating Stick Relay), through the coil of polarised relay 'UP MAIN BS' (Block Stick Relay) and via a current limiting resistor to the positive of the 'Line Clear' battery 'RET'. A stick relay is one which, once operated, self-maintains. The 'UP MAIN BS' contact shorts out the chain of proving contacts, allowing the signalman to obtain 'Line Clear' from the box in advance and clear his signals, without losing the 'Line Clear' to the box in rear.
Can we come back to a proper discussion of 'T1TPS' in a later installment? For now, assume that track circuit T1 (the Up Main 'Berth' track circuit) is clear and that 'T1TPS' is energised.
Note, in passing, that the 'DN MAIN BCR' (Block Proving Relay) is a polarised relay which will energise only when 'Line Clear' polarity is being received from Catchems Corner direction. Energisation of this relay is used to release the down signals at Spring Vale Sidings.
Deepfields direction:
On the Down Main, the criteria for accepting a train from Deepfields direction were:-
- Down Distant Signal Arms (37, 36a/36b) proved 'On'.
- Down Home Signal Lever (35) proved 'Normal' and electrically locked.
Similarly, on the right of the diagram above is the block instrument to Deepfields. On the Down Main, separate batteries are provided for giving 'Line Clear' and 'Train on Line'. Negative 'Line Clear' battery is fed via contacts: '35LCC' (Lock Proving Contact on 35 lever lock, proving lever normal and lock not energised), '36a/b YCR' (contact on Yellow Signal Proving Relay for signals 36a/36b), '37YCR' (contact on Yellow Signal Proving Relay for signal 37) to the 'LC' pole of the commutator. If the control conditions are met when the signalman places the Block Instrument Commutator to 'Line Clear', current flows from the battery, through the proving contacts, through the coil of a polarised relay 'DOWN MAIN BS' (Block Stick Relay) and via a current limiting resistor to the positive of the 'Line Clear' battery 'RET'. The 'DOWN MAIN BS' contact shorts out the chain of proving contacts, allowing the signalman to obtain 'Line Clear' from the box in advance and clear his signals, without losing the 'Line Clear' to the box in rear.
Note, in passing, that the 'DN MAIN BCR' (Block Proving Relay) is a polarised relay which will energise only when 'Line Clear' polarity is being received from Deepfields direction. Energisation of this relay is used to release the Up signals at Spring Vale Sidings.
In the next instalment, we'll talk about Lever Locks.
