Previous parts:-
Part 1 Introduction: Drawings.
Part 2 Relay room: cable terminations, relay rack and power supplies.
Part 3 Colour Light Signal 6: Control
Part 4 Colour Light Signal 6: Lighting and Indication
Working the box.
Simplified and approximate diagram. Clicking on any diagram displays a higher-resolution version which can be downloaded or printed
Introduction:
Parts 3 & 4 of this series described colour light signal 6. This time, we'll look at the relay circuits on part of Sheet 2 of the Contractor's Drawings, titled '19 Signal Selection, Control, Lighting & Indication Circuits'. Some of the principles we've already introduced when we looked at signal 6 in Parts 3 and 4 but this time we need to consider Approach Locking and Route Release.
A train proceeding in the Up Direction, after passing the last colour light signal controlled by Wolverhampton Power Box approached Princes End signal 19, a 3-aspect colour light. Lever 19 controlled the change from Red to Single Yellow then the aspect of the next signal (semaphore signal 18, giving access to the single line to Wednesbury) controlled the change from Single Yellow to Green.
Approach Locking and Route Release:
On plain double track line, trains follow one another in the same direction. The main risk is a train running into the rear of a preceding train. Ensuring that the line is clear to the next red signal, plus an extra overlap distance, before a train is allowed to proceed provides the necessary safety.
But where there are junctions, there is greater chance of conflict between two moving trains and additional safeguards are provided. At Princes End, double track became single so the potential conflict was between an Up train overrunning signals 19 and 18 and a Down train leaving the single line. The need for Approach Locking on signal 19 is based on the assumption that, having cleared signal 19 for an approaching Up train, the signalman may replace it (accidentally or in an emergency) before the train has passed the signal. Depending on the position of the train when the signal goes back to Red, the driver might not be able to stop before the signal. With purely mechanical interlocking, once the signalman had replaced lever 19, he would immediately be able to clear the signals for a train leaving the single line, creating the risk of an accident if the Up train was unable to stop. Approach Locking mitigates this risk using relays and a timer which still allowed the signalman to place signal 19 at danger in an emergency but enforced a suitable time delay before signals could be cleared for another movement.
Approach Release:
If the signalman wanted to clear signal 19 to allow a train to approach but was not in a position to clear signal 18 giving access to the single line, it was important to prevent the train from approaching at speed. In mechanical days, this was achieved by keeping a signal at danger until the train was close to it, called 'bringing a train under control'. With electrical locking, as described here, this was achieved by preventing signal 19 from clearing until track circuit 462 had been occupied for a suitable time.
Timer Relay:
The controls at Princes End used the Westinghouse 'PTT1' Thermal Timer for route release and approach release. This used the principle that if two dissimilar metals are welded together in the form of a strip and then heated, the strip will bend due to the differing expansion of the two metals. Two electrical contacts were placed so that a circuit was made to one contact at room temperature (the 'COLD' contact) and to the other when hot (the 'HOT' contact). An insulated coil of wire was wrapped around the bimetal strip to heat the strip when required. The time period was altered by adjusting the position of the 'HOT' contact. A slow-to-release relay and two diodes were placed in the same case as the timing device. When the 'HOT' contact closed, the slow-to-release relay energised and latched over a 'stick' contact. A contact on the 'slow to release' relay disconnected the heater coil so that the bimetal strip cooled. A second relay externally normally formed part of the timer circuit. This second relay was used to prove that the 'COLD' contact was closed before timing started and re-closed at the end of the timing. The diodes were suppression or commutating diodes wired across the coils of the internal and external relays to minimise arcing on the contacts of the Thermal Timer.
Colour Light Signal 19 Control:
Here's the circuitry controlling signal 19:-
Because Approach Locking is provided on signal 19, the first stage is to 'prove the route' by relay '19UCR' (19 route proving relay) in the relay room.
19UCR relay:
A number of conditions must be satisfied for '19UCR' to energise:-
- Track circuit 463 must be clear proving that the line is clear to signal 18. A contact on '463TS' stick relay is used, to prove that lever 18 has been replaced after the previous train.
- Track circuit 464 beyond signal 18 must be clear, to provide an overlap.
- Either a single line token must have been obtained and signal 18 cleared OR the Approach Release circuits must have brought the train under control.
- The signalman must have reversed lever 19.
(There's also a lock proving contact '19LCC' included but at present I'm puzzled by this).
So, if the signalman has 'got the road' and reversed levers 19 and 18 with clear track circuits, '19UCR' will operate immediately. If the signalman has not 'got the road', provided the two track circuits beyond signal 19 are clear and lever 19 is reverse, '19UCR' will operate when track 462 has been occupied for long enough for '462TJR' and '462TS' to operate (we'll look at the detail of this timing circuit shortly).
Control of Signal Aspect:
Two relays (in Location Case PE2/2 adjacent to signal 19) directly control the signal aspect:-
- '19HR': When released, the signal displays Red, when energised, the signal displays a Yellow (or Green).
- '19DR' When energised, '19DR' produces a Green.
The detailed controls for these two relays are shown below.
19HR relay:
'19HR' is fed with 50 volts from the relay room 'Double Cut' as follows:-
- '19AJS' (signal 19 Approach Timing Stick Relay) must be energised.
- '19AJR' (signal 19 Approach Timing Relay) must be energised.
- '19ALR' (signal 19 Approach Locking Relay) must be released.
- '19UCR' (signal 19 Route Proving Relay described above) must be energised.
19DR relay:
'19HR' is fed with 50 volts from the relay room 'Double Cut'as follows:-
- '19(RB) Circuit Controller in the signal box
- '19HR' in location case PE2/2.
- '19H1PR' in location case PE2/2.
Colour Light Signal 19 Indication:
Latching relay '19GCR' in the relay room provides indication of the signal in a similar way to the indication of signal 6, already described in Part 3.
463TS:
As soon as the train passes signal 19, track circuit 463 becomes occupied. '463T1PR' releases and, in turn, '463TS' and '19UCR' release, replacing signal 19 to danger. Even when track 463 clears, '463TS' will not operate until the signalman replaces lever 19 to 'normal', when '19(NA)' circuit controller will energise '463TS'.
3-aspect signal head:
Detail of 3-aspect signal head
The diagram above details the actual 3-aspect signal head. Details are similar to the 4-aspect signal head already described in Part 4 of this series.
Signal 19 Lighting and Filament Failure Indication:
19 Signal Lighting and Filament Failure
The terminals on the right hand side of this diagram connect to the signal head (see Fig. 8 above). The lighting is derived from a 110 volt a.c. supply in the location case adjacent to the signal (BX110 and NX110). The arrangements are similar to those for signal 6, already described in Part 4 of this series.
19 R/H/D ECR:
This relay will always be energised if one of the three main filaments or one of the three auxiliary filaments is drawing current. The circuit is similar to that for signal 6, already described in Part 4 of this series.
19 EC1PS (M):
This relay indicates failure of any of the main filaments to the signalman. The circuit is similar to that for signal 6, already described in Part 4 of this series.
Approach Control Timing Circuits:
19 Approach Control Timing Circuits
Both timing circuits above use the 'PTT1' Timer Relay. The upper circuit details the route release whereby, if signal 19 was prematurely replaced by the signalman, an electric lock prevented the lever from being placed fully normal (allowing another movement to be set-up) until after a suitable delay. The lower circuit shows the approach release circuit which delayed the clearance of signal 19 if the next signal (signal 18) was not 'off'.
19AJR:
'19AJR' (19 Approach Timing Relay) was energised provided that the 'COLD' contact of timer relay '19AJS' was closed. So '19AJR' released at the start of timing and a normally closed contact was used in the feed to '19AJS' coil which energised when the 'HOT' contact was made. A contact on '19AJS' disconnected the heating element so that, at the end of the cooling cycle, the 'COLD' contact was made, re-energising '19AJR'.
19AJS:
'19AJS' is fed with 50 volts from the relay room on pin 12 to initiate the timing sequence provided the following conditions are met:-
- '19UCR' released. This is the case if the signalman is attempting to replace lever 19 to 'normal'.
- '463T1PR' operated. The track immediately beyond signal 19 must be clear.
- '19(B)' circuit controller must be closed. This is the case if the signalman is attempting to replace lever 19 to 'normal'. The electric lock will prevent the lever from being fully replaced until Route Release has been obtained.
- '19JN' (19 Timer Manual Switch) must be closed. This is the case if the switch on the Block Shelf in the signalbox is not operated.
When 50 volts is applied to pin 12 of '19AJS', power is applied to the heating element through normally closed contacts on '19AJS' and '19ALS' which should both be released. After the heating cycle, the 'HOT' contact closes, applying power to the coil of '19AJS' (via the normally closed contact of '19AJR'). A contact on '19AJS' disconnects power from the heater element but '19AJS' remains energised over its 'stick' contact, as long as '19ALS' remains released. At the end of the cooling cycle, the 'COLD' contact closes and '19AJR' re-energises. Now both '19AJS' and '19AJR' are energised, 50 volts is fed to the coil of '19ALS' which energises and remains energised through its 'stick' contact, as long as '19UCR' is released. '19AJS' (intentionally slow to release) now releases because the normally closed contact on '19ALS' opens to remove 'N50' from the '19AJS' coil circuit. A normally open contact on '19ALS' applies the 12 volt supply from the relay room to 19(B)L lever lock, allow the signalman to replace lever 19 normal and free the mechanical locking for another movement.
19AJR:
'462TJR' (462 Track circuit Timing Relay) was energised provided that the 'COLD' contact of timer relay '462TJS' was closed. So '462TJR' released at the start of timing and a normally closed contact was used in the feed to '462TJS' coil which energised when the 'HOT' contact was made. A contact on '462TJS' disconnected the heating element so that, at the end of the cooling cycle, the 'COLD' contact was made, re-energising '462TJR'.
462TJS:
'462TJS' (462 Track circuit Timing Stick relay), in conjunction with '462TJR', enforced a delay before signal 19 could clear if signal 18 was at danger. 'B50' was applied to '462TJS' when an approaching train occupied track circuit 462 and a normally closed contact on '462T1PR' closed, provided track circuit 463 (the track immediately beyond signal 19) was clear. After the heating cycle, the 'HOT' contact closes, applying power to the coil of '462TJS' (via the normally closed contact of '462TJR'). A contact on '462TJS' disconnects power from the heater element but '462TJS' remains energised over its 'stick' contact. At the end of the cooling cycle, the 'COLD' contact closes and '462TJR' re-energises. Now both '19AJS' and '19AJR' are energised, it is possible for '19UCR' to be energised (as described above) and signal 19 may be cleared.
Next:
In the next instalment, we'll tackle something a bit more straightforward, from Sheet 3 of the Contractor's Drawings, 'Incoming & Outgoing TPR Circuits'.
