Time Release Units
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By Andrew Dickinson
The time release units that would have been installed in Exminster are very well illustrated in Adrian Vaughan's 'Signalman's Reflections' on page 156 (both plates). These units are the device with the round glass cover. They were used by the GW S&T in the 1940s and may have been used from as early as the 1930s (Challow - 1931) up to the last days of the GWR. Certainly, they were installed at Banbury South and Banbury Junction (1944) and these units were in use up to 1978 when new colour light signals or removal of existing signals forced their demise.
The description of the operation of these units described in Newsletter Issue 3, relating to 'Signalman's Twilight' is quite correct in that signalmen operated them manually when required. They were not automatically activated, unlike later time release units installed from the early days of BR(WR) onwards. The text in the top paragraph of 'Signalman's Reflections', page 157, also gives a description of the operation of the timer unit and the principal of approach locking itself.
These timer units were (unusually for equipment used on a British Railway) manufactured by an American company.
The signalman activated the timer by turning the knob on the front of the unit anti-clockwise. This action started a spindle rotating inside the unit. The unit was entirely clockwork and the only electrical connections to the unit were to the switch contacts it actually controlled. The clockwork part of the instrument controlled the rotation of the spindle, and the position of the spindle controlled the making and breaking of the contacts. The unit made a ticking noise when it was timing out. When the unit had reached the end of the time delay period, the spindle speeded up, and then closed the contacts at the end of the timing cycle. The purpose of the speeding up was to enable the contacts to close quickly and hence make a clean 'make'. If the spindle had operated the contacts when it was running at its slower timing out speed, then there would have been a slow 'make' of the contacts with possible sparking due to the electrical load of the lock coil and hence possible damage to the contacts.
The knob has held in its 'Normal' position by the pressure of a separate spring. To activate the unit, the signalman turned this knob to overcome the spring pressure and then the unit started to time out. The timer spring was wound up (ready for the next time it was required) when the signalman reset it after it had timed out by turning the knob clockwise until it 'latched' under the spring pressure described above.
This 'latching' action was also important in that it closed the contacts at the end of the timing cycle. The closure of these contacts proved that the unit was set at the start of the timing cycle and that, when operated, the unit would provide the time delay as designed.
This is one of the safety features that signalling engineers incorporated into equipment and circuit design. If the unit had developed a mechanical fault and only a small amount of the designed time delay was available, this would be a dangerous situation in that the safety of the approach locking would only be provided for a short time or, worse, would not be provided at all.
These circuits were wired into the Normal lock circuit of the lever(s) working the approach locked signals. So that Exminster, if the signalman had used the time release for signals 65/79 after changing his mind about running a goods train along the Down Main (having cleared and replaced signal 79, allowing the two minutes timeout, then fully replacing 79 and reversing 61), then he could not clear 65 until he reset the timer. Also, by incorporating these timer contacts in the Normal lock circuitry the integrity of the timer was proved every time the signalman pulled the signal.
The contacts in this application are called 'back' contacts and this description applies to any relay contact that is closed when a relay is de-energised. Any contact that is closed when a relay is energised is called a 'front' contact. The opposite end of any contact is the pivot or 'arm'. All these contacts were wired to the terminals on the relay itself, and are identified on wiring diagrams as '1B', '2F', '2A', and so on. Relay coil terminals are identified as 'R1', 'R2'.
The equipment letter used for any time release unit is 'J' and for any relay (regardless of its application) is 'R'. So a timer unit is identified in the diagrams as 'JR'. At Exminster, therefore, the units would be identified as 46/47JR, 65/79JR and so on.
Later (from BR(WR) days onwards), timer units were relays that were installed in the relay cupboard downstairs. They were fully automatic in operation and were electrically activated when the signalman placed the lever of the approach locked signal in the 'B' position.
Thank you to Andrew Dickinson for this technical information.