Part 1 of this series introduced the drawings detailing the electrical controls at Princes End after it became a Fringe Box to Wolverhampton Power Signal Box. Part 2 described the relay room with its cable terminations, relay rack and power supplies. There's a brief description of this signal box here. It's helpful to keep in mind the track layout, shown below.
This simplified and approximate diagram shows Princes End with the single line to Wednesbury on the left and the double track to Tipton Curve Junction (controlled by Wolverhampton PSB) on the right. Clicking on any diagram displays a higher-resolution version which can be downloaded or printed
Introduction:
This time, we're going to look at part of Sheet 1 of the Contractor's Drawings, titled '5(N) Lock & 6 Signal Selection, Control, Lighting & Indication Circuits'. A train proceeding in the Down Direction needed the semaphore home signal lever 5 to leave the single line and then colour light lever 6 to proceed into the Wolverhampton Power Box area. Signal 6 was a 4-aspect colour light. Princes End controlled the change from Red to Single Yellow then the aspect of the next signal (WN186) controlled the change from Single Yellow to Double Yellow or Green. The different aspects have code letters as follows:-
R: Red, H: Single Yellow, HH: Double Yellow, D: Green.
Colour Light Signal 6 Control:
Here's the circuitry controlling signal 6:-
Three relays (in Location Case PE2/1 adjacent to signal 6) directly control the signal aspect:-
- '6HR' is controlled from Princes End signal box and relay room. When this relay is released, the signal displays Red, when energised, the signal displays a Yellow (or Double Yellow or Green).
- '6HHR' is controlled from the Location Case in the vicinity of signal WN126. When energised, it produces a Double Yellow (provided '6HR' is also energised).
- '6DR' is controlled from the same Location Case. When energised, it produces a Green, provided '6HR' and '6HHR' are also energised.
We'll now look at the controls on these three relays in more detail.
6HR relay:
'6HR' is fed with 50 volts d.c. from Princes End relay room. Note that both positive and negative feeds to '6HR' coil are switched by contacts on the controlling relays, avoiding a common earth return. This arrangement is called 'Double Cut' and prevents spurious voltages flowing in the earth from incorrectly operating the relay. This is particularly important on electrified railways, where large traction currents are flowing.
'6HR' is energised if all the following requirements are met:-
- '468TS' is energised (meaning the track immediately beyond Signal 6 is clear)
- '469T1PR' is energised (meaning the overlap beyond 'WN186' is clear)
- 'WN186GCZR' is energised (this is the Signal Proving Special Relay for WN186)
- Two contacts on '6(RE)' circuit controller in the signal box are closed.
- '469T1PR' is energised when track 469 is clear. It is a repeater relay for the actual track circuit relay ('469T'). '1P' indicates 'first repeating'.
- 'WN186GCZR' is controlled from a 50 volt supply at signal WN186. Again, the circuit is double cut. Provided'WN186 R/H/D EC1PR' is energised, 'WN186GCZR' will be energised. 'WN186 R/H/D EC1PR' is a Lamp Proving 1st Repeating Relay for signal WN186. Colour Lights are 'proved' by measuring the current flowing through the lamp filament (this was before the days of Light Emitting Diode signal lamps). The 'R/H/D' means we don't care what colour is being shown, as long as it's lit. Should the main filament fail, there's a standby filament (as we'll discuss later). But, it's possible for both the main and standby filaments to fail, meaning the signal is 'out'. We then resort to 'double-blocking' to keep traffic moving. If the controls for signal WN186 are trying to display a 'proceed' indication, it's safe to clear signal 6 to 'Single Yellow' because the track is clear at least to the signal after WN186. So, in parallel with the contacts on 'WN186 R/H/D EC1PR', there are normally open contacts on 'WN186HR' and 'WN186H1PR'. Thus, 'WN186GCZR' in Princes End relay room is energised if either WN186 is lit or WN186 is off.
- Circuit Controller 6(RE) is mounted on the lever frame in Princes End box and controlled by the movement of lever 6. Levers don't just have 'Normal' (N) and 'Reverse' (R) positions. Signal engineers allocate five intermediate positions so the full list is N, A, B, C, D, E, R. So when contacts on a circuit controller are labelled '6(RE)' that means the contacts close just before the lever is fully reversed in the frame and remain closed in the reverse position.
6HHR relay:
'6HHR' is controlled from a 50 volt supply at signal WN186. The feed is double cut and requires 'WN186 R/H/D EC1PR', 'WN186HR' and 'WN186H1PR' at the location case adjacent to signal WN186 energised. In addition, at the location case adjacent to signal 6, '6HR' and '6H1PR' are proved energised.
6DR relay:
At the location case adjacent to signal WN186, the double-cut feed to '6HHR' is fed via contacts on 'WN186HHR' and 'WN186HH1PR' to feed '6DR'. In addition, at the location case adjacent to signal 6, '6HHR' is proved energised.
Colour Light Signal 6 Indication:
The traditional method of indicating the position of semaphore signals to the signalman was by a polarised galvanometer in the signal box fed via a single wire from the signal with earth return. The system was powered by a battery at the signal, fed through a contact box attached to the signal arm. The arm 'On' sent current of one polarity to the box, 'Off' reversed the polarity and anything in between sent no current.
The indication of signal 6 used a similar philosophy, but the circuit used the 50 volt supply at the signal location case and the circuit was 'double-cut' by contacts on '6HR' and '6H1PR'. If the 'Back' (normally closed) contacts on the relays were made, one polarity was sent to line, indicating the signal at Red. If the 'Front' (normally open) contacts were made, the opposite polarity was sent to line, indicating the signal 'Off'.
6G1PR:
'6G1PR' (signal 6 1st Repeating Relay) in the relay room at Princes End was a special relay - basically two relays side by side (called 'N' and 'R') joined by a mechanical latch. The operating coils are wired in parallel and are polarised by a permanent magnet so that the 'N' relay operates for one polarity, the 'R' relay for the opposite polarity. When the 'N' relay operates, it will latch in the operated position (with 'R' released) even with power removed. However, when 'R' operates, it will latch operated and release 'N'. Thus, even without power, the relays will retain the 'last known state'. This type of relay is commonly used for point indication but it is also used for signals to prevent unsafe conditions arising in the case of power or other failure which might inadvertently allow a conflicting or dangerous move to be set-up.
468TS:
The circuit controlling the coil of relay '468TS':-
Track circuit 468 is immediately past signal 6, so it's clear the track circuit needs to be unoccupied before the signal may be cleared. As soon as the train passes signal 6, track circuit 468 becomes occupied and relay 468TS releases, replacing the signal to danger, even though the lever may still be reversed in the frame. The 'stick' relay ensures that, even when the train has gone and track circuit 468 clears, relay 468TS does not 'pick up' until the signalman has replaced lever 6 to 'normal'.
If lever 6 is correctly replaced after a train circuit controller contact '6(NA)' will be closed. As soon as track circuit 468 clears, relay '468TS' will energise via the normally open ('front') contact on '468T2PR' and remain energised through its normally open 'stick' contact even after the signalman reverses lever 6 for a following train. '468TS' will only release when the following train occupies track circuit 468 and the normally open contact on '468T2PR' opens.
Next:
In the next instalment (here) we'll tackle more of the relay circuits on Sheet 1 of the Contractor's Drawings.
