FPL interlocking

Chris Mitton
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FPL interlocking

Postby Chris Mitton » Wed Dec 04, 2013 11:53 am

While doodling an interlocking diagram for Stowe Fen the other night, a question crossed my mind which at first sounds idiotic, but on second thoughts....

Would the lever for an FPL be interlocked in the frame with the turnout it locks? The more I thought about it, the (counterintuitive) fail-safe answer ought to be no. If the lock mechanism on the turnout failed for any reason (broken bolt, fastening bolts working loose, whatever), the signalman would find it impossible to pull the turnout lever, being locked by the tappets to the FPL lever, and therefore believe that the turnout was locked when in fact it could (and probably eventually would) move under a passing train. Therefore he should depend solely on the FPL at the business end to tell him whether the points could be pulled or not. The interlocking of turnout and signals would then prevent conflicting signals clearing unless the FPL bolt was firmly pushed into the stretcher bar.

Similar reasoning suggests that signals should not be interlocked with an FPL lever except by the detectors at the lineside. Can the S&T engineers on this forum tell me if I've got this right? or am I being excessively paranoid? If this is the case, it will greatly simplify my lever frame, at the expense of having to dream up some form of working FPL (mechanical or electronic).....

Regards
Chris

John Palmer
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Re: FPL interlocking

Postby John Palmer » Wed Dec 04, 2013 3:35 pm

To my very limited knowledge it is customary for the FPL lever to be interlocked in the frame with the lever for the points that it bolts. In the majority of cases the lock plunger stands out when its related lever stands normal in the frame, so the usual sequence is to work the points and then the related FPL lever to bolt them. That is the reverse of the sequence your post seemed to me to be suggesting, so maybe I have misunderstood what you are saying.

A common practice is to locate the FPL lever adjacent in the frame to lever for the points that it bolts. The locking required in the frame for such an arrangement simply consists of a single dog actuated by its contiguous tappets.

If you start making equipment that is remote from the frame part of the locking you increase the scope for the installation being compromised by external agencies such as weather and vandalism – Lichfield, New Year's Day 1946 suggests ways in which such a situation might come about.

Detection should 'prove' the position of the lock plunger and thus prevent clearance of signals over a route other than that for which the points are set and locked. Lt. Col. Woodhouse decided to discount four railwaymen's evidence that Lichfield No. 1's Up Fast Home signal had been cleared because the points from Up Fast to Up Slow stood reversed, and with the points in this position he concluded that the detection arrangements made it impossible for this signal to have cleared.

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Tim V
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Re: FPL interlocking

Postby Tim V » Wed Dec 04, 2013 4:53 pm

The locking diagrams I have seen show that the interlocking is in the frame. The FPL on the point merely detects the position of the switch blades. Don't forget there is a huge amount of leverage on the lever in the frame, easily capable of destroying a FPL mechanism.
Tim V

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grovenor-2685
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Re: FPL interlocking

Postby grovenor-2685 » Wed Dec 04, 2013 5:18 pm

The locking diagrams I have seen show that the interlocking is in the frame.

Yes, I have never seen any exception to this. Also it is mandatory for running signals to be released by the fpl lever (for the more common arrangement where the points are bolted with the fpl lever reverse. Where the points are bolted with the fpl lever normal then the signals will lock the fpl lever) Shunt signals may or may not be released by the fpl.
The FPL on the point merely detects the position of the switch blades.
Not at all, the FPL on the points does what it says and locks the points in position, detectors are different mechanisms.
Don't forget there is a huge amount of leverage on the lever in the frame, easily capable of destroying a FPL mechanism.
Facing point locks are strong bits of kit, the lever locking will stop anyone trying to move the points when bolted but if it were possible I would certainly not expect the FPL to be destroyed, there are a number of weaker links between lever and FPL that would give first even if the signalman could exert enough force!

Mechanical detectors are connected to the point blades and the fpl so that the signal slide can only be pulled through the detector if the points are in the correct position and bolted, this is the protection against broken point linkages allowing the levers to be correctly moved without the points or fpl following correctly. Electrical detectors make the same check but would control an electric lock on the signal lever giving a more positive result.
Regards
Keith

Chris Mitton
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Re: FPL interlocking

Postby Chris Mitton » Wed Dec 04, 2013 8:19 pm

Thanks gents, that's sorted that one in my mind....at least I don't have to build working FPLs, I'll leave that to the Howard Boltons of this world!

Regards
Chris

martin goodall
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Re: FPL interlocking

Postby martin goodall » Wed Dec 04, 2013 9:39 pm

I took this question to mean - does the FPL lever when locked stand out of the frame (Reversed) or in the frame (Normal)? The answer to that questiuon is that the practice of different companies varied.

On the GWR, the FPL was locked when its lever was Reversed, and was released when the FPL lever was Normal in the frame. There were other companies, though, (but I can't recall offhand which ones) where the FPL was locked when the FPL lever was Normal, and the lever had to be pulled (reversed) to release the FPL.

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grovenor-2685
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Re: FPL interlocking

Postby grovenor-2685 » Wed Dec 04, 2013 10:27 pm

Martin you are correct that the locked position of an FPL lever could vary, not just between companies but within companies. I mentioned the two possibilities in my post above, but it was NOT the question originally asked, which I thought Chris explained very clearly.
Regards
Keith

Terry Bendall
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Re: FPL interlocking

Postby Terry Bendall » Thu Dec 05, 2013 7:19 am

The Leighton Buzzard Narrow Gauge Railway has a lever frame which includes a facing point lock which I use quite often. The lever for the lock has to be pulled forward (reversed) to unlock the lever that operates the points (it is a cross-over), then the points are moved, and then the FPL lever is returned to normal - i.e back to lock the blades. As Keith says it is a fairly substantial bit of kit - the locking bar is about 30mm x 15mm so it is unlikely to break.

Terry Bendall

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Russ Elliott
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Re: FPL interlocking

Postby Russ Elliott » Thu Dec 05, 2013 11:20 am

If a running signal lever is released by an fpl lever, presumably the releasing of that running signal lever is also conditional on the turnout lever being in a particular position?

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grovenor-2685
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Re: FPL interlocking

Postby grovenor-2685 » Thu Dec 05, 2013 11:36 am

Depends if the point concerned can be locked both ways or not, if the locking requirement was for the normal position only then the fpl lever would lock the point and the signal be released by the fpl.
If the points may be locked in either position then the signal over the points normal has to lock the points and be released by the fpl. The signal reading over the points reverse would be released by both points lever and fpl lever.
Regards
Keith

John Palmer
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Re: FPL interlocking

Postby John Palmer » Thu Dec 05, 2013 5:47 pm

Just to demonstrate that there were exceptions that would prove pretty well any rule, the layout at Burnham-on-Sea had some fairly bizarre interlocking arrangements. These included two separate FPL's, both of which engaged into the same stretcher between a pair of switch tongues. One bolted the points normal, the other reverse. So far as we have been able to ascertain, conventional detection equipment, or a device based upon similar slotting principles, was used to implement the interlocking between the station's signal box and the ground frame that worked the points mentioned above. Similar devices were also used as the means for releasing various ground-worked point levers in the layout.

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grovenor-2685
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Re: FPL interlocking

Postby grovenor-2685 » Thu Dec 05, 2013 9:37 pm

John Palmer wrote:Just to demonstrate that there were exceptions that would prove pretty well any rule, the layout at Burnham-on-Sea had some fairly bizarre interlocking arrangements. These included two separate FPL's, both of which engaged into the same stretcher between a pair of switch tongues. One bolted the points normal, the other reverse.
That is certainly unusual :)
So far as we have been able to ascertain, conventional detection equipment, or a device based upon similar slotting principles, was used to implement the interlocking between the station's signal box and the ground frame that worked the points mentioned above.
Mechanical release of ground frames in this way usually used a device more akin to an fpl than a detector, same in principle but more robust. Usually the slide worked by rod from the ground frame and bolted by rod from the cabin.
Similar devices were also used as the means for releasing various ground-worked point levers in the layout.

Regards
Keith


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