Approach Locking
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By Andrew Dickinson
I would suggest that approach locking was installed on all those signals that applied to facing points that were more than a specified distance away from the signal box, and because of that the signalman would have had a restricted view of these points, if any at all. By definition, all signals applying to facing motor-worked points would fill these criteria. In addition, points that were not readily visible, such as those obstructed by station buildings or bridges, could also be a candidate for approach locking.
I’m not completely sure where the idea has originated that signals 63/64 were provided with approach locking. The Box Instructions state [published in Issue 3] that “the Down Goods Line Starting Signals had a time release instrument provided in connection with the working of the approach locking in the Down Goods Line”, but these signals were surely signals 46/47, not 63/64. [Agreed. DS.]
I would suggest that signals 63/64 never had approach locking provided on them. If this locking had been provided, then there would undoubtedly have been a track circuit through points 59 in the Down Goods Line as part of the working of the approach locking circuitry, but as was originally stated [back in Issue 3], there were no track circuits in that area.
Interestingly, the lever frame order sheet [available from the Society website] omits 46 in the list of levers equipped with NB locks. This is the signal for moves out onto the Down Main. Lever 46 would certainly have had an NB lock fitted to it as approach locking was provided on both signals 46 and 47.
Another item of equipment that was installed in the overall provision of control circuitry and equipment for signals with approach locking was another relay. This was an essential part of the working cycle of the approach locking circuitry, and was called a ‘Stick Relay’, or ‘SR’ using equipment letters. At Exminster, they would have been identified as 46/47SR and 65/79SR.
The purpose of a stick relay is to store an ‘event’ (such as the passage of a train) or the granting of a permission, for use later by the equipment, but for one operation only (such as the block release on the section signal for ‘one pull’ only).
This was achieved by wiring a feed to the coils of the relay through the front contact on the same relay. When the relay has ‘picked’ (usually through other equipment operating for a short time to put a ‘pick up’ feed onto the relay, it will hold up through that contact. But if the feed to the coils is cut when other relays or equipment operates, the relay drops and remains down even if the feed is then restored. For a stick relay in the circuitry to operate in general requires a ‘make’ circuit to ‘pick’ it, and a ‘break’ circuit to cause it to drop. It is in effect an electric ‘latch’.
In practice, with levers 65 and 79 normal, 65/79SR would normally be ‘picked’ or ‘up’. An NB band on lever 79 (and 65) circuit controller was wired into the feed circuits for 65/79SR, so that when lever 79 was pulled beyond the ‘B’, then this relay dropped down and remained down.
When returning 79 to the normal position, it would be checked at the ‘B’ position by the electric lock. There were two ways that the lock would pick, either a) through the passage of a train past signals 65/79 (requiring 65/79SR to be ‘up’), or b) through the operation of time release unit 65/79JR.
For a) above, occupation of 61T was required to prove the passage of the train and to cancel the approach locking for that train. To pick 79 ‘B’ lock required 65/79SR picked. This was achieved through the operation of 61T repeat relays, which caused 65/79SR to pick and stick. When 61T cleared, 65/79SR remained picked. In other words, this relay had ‘stored’ this ‘event’ for future use. If the signalman was slow in replacing lever 79 and the train had cleared 61T, he could still fully replace the lever due to 65/79SR being picked.
If there were no passage of a train, 65/79SR remained down and 79 ’B’ lock would not pick.
The signalman then had to resort to b) above to fully replace the lever.
Our thanks go to Andrew Dickinson for another most enlightening instalment.