| THE JOY OF INTERLOCKING Chris Turnbull |
The author, on his three-year mission to build a new exhibition
layout to try out new methods and new ideas, has boldly gone where he has feared to go before and has installed working colourlight signals
on his layout of Cromer. As well as two starters, the prototype Cromer boasts two SPAD (Signal Passed At Danger) indicators a few hundred yards
in advance of the starters. On the model Cromer, it was intended to have these as non-working but this was about to change.
"Now you've successfully installed working starters why not have the SPAD indicators working as well?" suggested Roger Murray, signalling guru and builder of excellent colourlight signals. "I can supply them if you like". Hmm, this was not part of the original concept; I had not catered for these in the design of the wiring, not least in the lack of interbaseboard connections. Nevertheless, Roger had made such a splendid job of the starters that I was persuaded to take the plunge. It seemed churlish not to, and a cheque was despatched. A couple of months later the SPAD indicators arrived and they were every bit as good as the starters; Roger had excelled himself again. As well as the signals themselves, Roger also provided the control boxes, multi-pin baseboard connectors and advice on how to wire them. For those not familiar with SPAD indicators they are three-aspect colourlights, all red. The centre light is constant, the outer two flash; of course, I had never seen them working, this being something usually only seen by an unfortunate driver. |
Cromer is where trains from Norwich to Sheringham reverse in a remnant of the former Cromer Beach station, each of the two platforms being signalled for bidirectional working. Several criteria had to be satisfied:
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Interlocking with the SPAD indicators
Fig 1 shows the arrangement of signals and detectors that satisfy items 1 to 3, only one line being shown for clarity. There are three detectors, which may be either reed switches or infrared detectors, one to arm the system, one to trigger it, and a third to disarm the system. A train entering the station disarms the system (although the system is already disarmed by the previous departing train) and, upon reaching the arm detector, arms the system, this last action lighting an LED on the control panel. Although the train passes over the trigger detector when entering the station, this has no effect at this time as the system must be armed first. Upon leaving the platform the trigger detector detects the train and, since the system is now armed, will operate the SPAD indicator and isolate the track - but only if the signal is showing a red aspect. |
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Initially reed switches were installed; these worked very well but relied on the stock having a magnet fixed to the underside.
This was fine as long as there was no visiting stock running but this was not going to be the case, it being precisely this occasion when
a SPAD was most likely to occur. Infrared detectors have now been installed and, whilst all stock is now detected, there is one disadvantage.
Assuming one magnet per train or locomotive, only one reed switch can be operated at any one time -- unless the reed switches are placed
too close together, which is not advisable. |
Thus the arm and trigger relays cannot be activated simultaneously. With infrared detectors the
whole train is detected, a long train spanning both the arm and trigger detectors thereby activating the SPAD indicator and isolating the track.
It is necessary, therefore, to ensure that the distance between the arm and trigger detectors is not less than the maximum train length.
Other than this, positioning of the detectors is not critical although the trigger detector should, naturally, be just to the rear of the starter.
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Fig 2 shows how the system is wired. The essentials are three detectors, either infrared or reed switches, two latching relays,
one to arm the system, one to trigger it, one ordinary relay and the SPAD indicator operating unit. If, like me, you have never heard of
latching relays, a word about these may be in order. Like conventional relays, the relay operates upon the passing of a current through
the coil, but then latches (as the name implies). When the current ceases, contact continues to be made until unlatched. Thus the passing
contact of a reed switch or detector is converted to the continuous current required to operate the SPAD indicator. The detectors used
were IRDOT-2 from Heathcote Electronics and they have six terminals wired as shown in Fig 3. Nos 1 and 6 are the 12V DC supply, the positive
being looped across to terminal 4 thus providing the switch operating the relay. The LED can either be left in-situ on the unit or mounted
in the control panel this providing the indication of platform occupation required. The SPAD operating unit comes from Roger Murray as part
of the overall package and it, too, has six terminals. Nos 1 and 2 are the 12V DC supply from the red signal aspect, 3 and 4 the trigger
circuit whilst 5 and 6 are the arm circuit and should be wired together.
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When a train enters the station it first passes over the disarm detector which unlatches both relays. This breaks the trigger circuit
at the arm relay, supplies the track with power and restricts the starter to a red aspect although, in practice, this state already exists
since the previous departing train has had the same effect. Upon reaching the trigger detector nothing happens as the latching circuit is
fed via the arm relay, contact only being made when that relay is latched. Once the arm detector is reached the arm relay is latched,
thereby allowing the trigger relay to operate the next time a train passes over it. In addition, the starter may now be changed to green.
Upon leaving the platform the trigger detector latches the trigger relay but only if the train has departed against a red aspect,
the latching circuit to the trigger relay also being fed by a relay dependent on the signal aspect. If the starter is red, the SPAD
indicators operate and power to the track is cut off, the system being reset by a push button on the control panel allowing the train
to proceed. If you want to be really nasty this reset button could be hidden under the baseboard! If the starter is green, the trigger
relay does not latch, the track is not isolated and the train continues on its journey, activating the disarm detector which unlatches
the whole system and returns the signal to red, ready for the next incoming train.
Right: Looking from Gasworks Bridge towards the station, showing the two crossovers. The scenery is in the construction stage and is slowly advancing towards the platform. On this layout the signalling has been installed at the same time as the track, not as an afterthought. |
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TB34, the Sheringham line starter. When off, the theatre displays either N for Norwich or S for Sheringham depending on the setting of the facing crossover. If the trailing crossover is set to the curve, the signal cannot display green. | ||||
 TB34 SPAD indicator
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Interlocking between starters and track
The operation of the prototype Cromer and the corresponding track layout is, nowadays, relatively simple. Two tracks enter the station, one from Norwich and one from Sheringham, each being operated on the 'one engine in steam' principle (what an anachronism). There are two crossovers allowing a train on one line to reach the other -- and that's it. Trains enter the station on the straight and depart via the relevant crossover, thus allowing entry into the station without having to first slow down to traverse a 15 mile/h crossover. When departing, trains struggle to attain 15 mile/h before the crossover is reached, and so there is no reduction in journey time. Figs 4 and 5 show the diagrammatic and schematic arrangements that satisfy criterion 4 - that both starters should not simultaneously display green nor conflict with the setting of the crossovers, it not being possible to depart (or arrive) with the trailing crossover set to the curve. Taking TB32 as an example, this controls the line to Norwich, access to the Sheringham line being via the facing Norwich--Sheringham crossover, the trailing crossover being the Sheringham-- Norwich. For TB34, the starter controlling the Sheringham line, the situation is similar but opposite. The feed, 12V negative in this case, is fed to one of the centre terminals of the two double-pole double-throw switches that control the starters. Whilst they are both switched to display a red aspect the logic is straightforward; once switched to green the interlocking comes into play, the power being fed to a centre terminal on the TB34 switch. If TB34 is already switched to green the power is fed back to the red feed for TB32, but if TB34 is red the power goes to a spare pole on the switch controlling the trailing, Sheringham - Norwich, crossover. Since this will result in a derailment if set to the curve, the power is returned to the TB32 red aspect if this is the case, otherwise TB32 changes to green. There is similar interlocking between the setting of the crossovers and the track feed to the fiddle yard, but without the complication of signalling, preventing an exit that will result in a derailment. |
 
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Conclusion and acknowledgements
This first attempt at interlocking has been great fun to devise and construct, although it would not have happened without the valued assistance of Roger Murray of Roger Murray Colour Light Signals and Roger Kingstone, who supplied a great deal of prototype information, to both of whom I am most grateful. As with all modelling projects the finished article belies an abundance of failed attempts and blind alleys, and this was no exception. The final design underwent many changes, usually when something did not work as expected, and it was at these times that Roger's sage advice was appreciated. And did I have any problems with the number of ways on the inter-baseboard connections? As luck would have it, no - but this was only after I had realised the wiring could be considerably simplified and I had made and installed additional connectors. Now, perhaps, I can finally get on with the scenery. |
The underside of the baseboards showing on the left-hand board the two SPAD indicator operating units and timber protection to one of the Tortoise point motors, and on the right-hand board the two disarm infrared detectors and inter-baseboard connector. |
A continuation of photo 6. On the left-hand board from left to right, two pairs of latching relays, one the trigger, one the arm, two ordinary relays (on foam pad to reduce vibration), the two trigger, infrared detectors and another inter-baseboard connector plus, of course, lots of wiring. On the right-hand board the printed circuit boards and connectors for the two platform starters which were all provided by Roger Murray, as were the SPAD operating units in photo 6. |
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