Previous instalments:-
Part 1 Introduction: Drawings.
Part 2 Relay room: cable terminations, relay rack and power supplies.
Part 3 Colour Light Signal 6: Control
Working the box.
Simplified and approximate diagram. Clicking on any diagram displays a higher-resolution version which can be downloaded or printed
Introduction:
We're continuing to study the relay circuits on part of Sheet 1 of the Contractor's Drawings, titled '5(N) Lock & 6 Signal Selection, Control, Lighting & Indication Circuits'.
4-aspect signal head:
Detail of 4-aspect signal head
Each lamp was fed from its own step-down transformer. The primary of each transformer (terminals 1 - 6 on the diagram above) was fed, via control relay contacts in the adjacent location case, with nominal 110 volts a.c.
The signal lamps were dual-filament low voltage bayonet-fitting type. Note that the 'Auxiliary' filament was fed from a lower voltage than the 'Main'. Each 'Main' filament was in series with a lamp proving relay mounted in the signal head ('ER1' to 'ER4'). If current was being drawn by the 'Main' filament, the appropriate lamp proving relay was energised and the 'Auxiliary' filament was disconnected. Release of the lamp proving relay connected the 'Auxiliary' filament via a normally closed ('back') contact.
Further contacts on the lamp proving relays were wired to indicate the status of the 'Main' filaments (terminals 7 - 9 on the diagram above). The contact circuit is drawn rather oddly but the effect is that the signal head presented a closed circuit between terminal 7 and terminal 8 (which was linked to terminal 9 externally) provided any of the relays 'ER1', 'ER2', 'ER3' were energised. The indication contact on 'ER4' (the second Yellow lamp used for the 'HH' 'Double Yellow' Aspect) was shorted out by the external link and thus not used. We'll return to this in '6 EC1PS(M)' below.
The signal head was a rectangular die cast box closed by a hinged door at the rear secured by a padlock. The front of the head had four apertures fitted with projecting sheet steel hoods to minimise the effect of overhead sunlight. Each aperture had a clear cast glass lens (the colour filtering was behind this lens). To project an intense beam visible at a distance, a Fresnel lens was used where the rear of the lens was shaped into a series of stepped rings. Because the Fresnel is so efficient, it could be difficult for a driver stopped close to the signal to confirm the aspect. In early signals of this type, a small second aperture was provided aligned to face a train waiting at the signal. This aperture was also provided with a small hood. Because of the shape of this hood, the device was often called the "pig's ear". However, by the time of the West Midlands Resignalling, a simpler arrangement was in use. The section of cast Fresnel Lens between around 'four o'clock' and 'five o'clock' intentionally had a different profile, which deliberately scattered light towards a waiting train, rather than projecting it forward as part of the main beam.
The signal head was then mounted in a variety of ways, so as to give the driver the best possible 'sighting' of the signal. Straight tubular posts, brackets, massive cantilevers or gantry bridges were used, depending upon the geography. Most of the colour lights on the West Midlands Resignalling were right-hand brackets and, as far as I remember, Princes End signal 6 was of this pattern.
Signal 6 Filament Failure Indication:
Here are the filament failure and indication circuits for signal 6 (468TS coil circuit shown here has already been discussed in Part 3):-
The terminals on the right hand side of this diagram connect to the signal head (see Fig. 7 above). The lighting is derived from a 110 volt a.c. supply in the location case adjacent to the signal (BX110 and NX110). BX110 is fed via the unit '6 R/H/D ECR O/J' and then switched by relay contacts to power the R, H and D lamps. A separate BX110 feed is taken via relay contacts to power the HH lamp. NX110 is connected to terminal 6 (common for the R, H and D lamps) which is linked to terminal 2 (common for the HH lamp).
6 R/H/D ECR:
The '6 R/H/D ECR O/J' unit is detailed below:-
The unit comprises an adjustable resistor (a rheostat) and a bridge rectifier. Alternating current to the R, H and D lamps flows through this resistor, developing an a.c. voltage across the resistor. The bridge rectifier full-wave rectifies this voltage, giving an unsmoothed d.c. voltage which is applied to the coil of relay '6 R/H/D ECR' (6 red/yellow/green lamp proving relay). The black square drawn inside the relay coil indicates the the relay is 'slow to release', preventing relay contact 'chatter' because of the unsmoothed d.c. and drop-outs as the signal changes from one aspect to another.
Provided the adjustable resistor is correctly set, relay '6 R/H/D ECR' will always be energised providing one of the three main filaments or one of the three auxiliary filaments is drawing current. Note that that the HH lamp current is not monitored. If the second yellow lamp fails, it merely means that when the signal should be displaying 'Double Yellow', it will display 'Single Yellow'. Since a 'Single Yellow' is more restrictive, the failure is safe.
6 EC1PS(M):
'6 EC1PS(M)' is concerned with failure of main filaments, hence '(M)'. In 'Detail of 4-aspect signal head' above it was stated that "the signal head presented a closed circuit between terminal 7 and terminal 8 (linked to terminal 9) provided any of the relays 'ER1', 'ER2', 'ER3' were energised. B50 from the location case is fed via these contacts together with N50 from the location case down two wires to the relay room. Here, slow-to-release stick relay '6 EC1PS(M)' is energised through either of two parallel contacts - its own stick contact or a contact on relay 'CANCEL NR'. We'll discuss 'CANCEL NR' in a later part but note the designation 'NR' means 'switch relay'. Failure of a main filament which should be lit will remove the coil voltage from 'CANCEL NR' and '6 EC1PS(M)' will release. Even when the failure has been repaired, '6 EC1PS(M)' will not energise until the signalman has acknowledged, energising 'CANCEL NR'. The stick contact on '6 EC1PS(M)' will then keep the relay energised until the next main filament failure.
6 EC1PR:
'6 EC1PR' (6 lamp proving first repeating relay) in the relay room is concerned with 6 signal 'out'. Provided either a main or standby filament is operating, '6 R/H/D ECR' will be energised at the location case by the signal. B50 and N50 is 'double-cut' at the location case by normally open (front) contacts on '6 R/H/D ECR' and fed down two wires to operate relay '6 EC1PR' provided the signal is lit.
Signal 5 Lever Lock:
Even when the mechanical interlocking allows signal 5 to be cleared, the lever remains locked normal until the electric lock '5(N)L' is energised. This lock is powered from a 12 volt supply in the relay room under the control of '467T2PR', '6G1PR' and '6EC1PR'. In addition, pushing down the catch handle of the lever prior to moving the lever closes an electrical switch '5N'. As the lever is reversed this switch opens to save power. Track circuit 467 is immediately ahead of signal 5 so a 'front' contact on '467T2PR' proves that the track circuit is clear before the lever lock can be energised. Finally, its necessary to prove that there's a lit signal ahead of signal 5. Normally, '6EC1PR' will be energised and a front contact in series with the lever lock proves 6 signal is lit before signal 5 can be cleared. As we discussed in an earlier instalment, trains are allowed to move towards a colour light which is 'out', provided the control relay proves that the signal should be 'off'. Thus, a front contact on '6G1PR' can allow the lever lock to be released even when signal 6 is 'out'.
Next:
In the next instalmen (here) we'll tackle more of the relay circuits, from Sheet 2 of the Contractor's Drawings.
