(11-01-2011, 02:41 AM)ivanutd Wrote: Hello PJW,
Also on 1998 Layout, there is a connection line between platforms 2 and 3 which is operated by a GFR - i have never done a CT for a GFR, can you please let me know what would be required for this in the exam and how to approach it?
I don't think that candidates have ever yet been asked to do a Control Table for a Ground Frame but there is always a first time.
Basically a GF release is a "cheap and nasty' point; the points are locked normal except when the signaller at the controlling signalbox gives a release. This permits local operation (often the train movements are controlled by handsignals / radio, but there can be signals which are also operated from the same GF- or indeed once the local operator has got the points Reverse, occasionally it is arranged that the signals are cleared by the signaller at the main signalbox).
The key elements are:
1. Once the release has been given, then there has to be a corresponding restriction on what the signaller can do (so that the possibility of conflicting movements is prevented).
2. Once the local operator has finished with the GF and restored the associated points to their normal position, then the release is surrendered and therefore the points become locked in their pre-defined position. The signaller now regains the ability to signal over the points in their normal lie.
Hence there are two elements to a GF Control Table-
a) those conditions which must be satisfied before the release can be given; broadly list those signals that must be Route Normal / have route locking free to prove that any movements previously authorised by the signaller over the GF's points have been completed
b) those conditions that must be satisfied before the release can be returned to the signaller from the local operator; broadly any signals controlled locally must have been restored fully normal and the points must have put back to where the signaller would expect them- where applicable this is: set into position, locked by FPL and detected. Also the GF itself must be proved locked- this may be by proving a mechanical key has been restored and locked into its default position and thus placed 'beyond use', or where the release is directly via an electric lock then by proving that lock de-energised and engaged.
Generally where there is a track circuit over the GF points, then this would be proved clear both in the release being given and the release being surrendered. There is a caveat however; sometimes there is actually no track circuit over all ends of the points and indeed there are some 'in section Ground Frames' where the GF controlled points are for a siding connection in the middle of a long block section on a rural line- in this case the track control is the occupancy of the section for a long time, since the intention is to prove that there is a train on the line in a position to be able to be shunted into that siding.
So think of the GF and its operator combined as a form of point machine- the conditions for its release are broadly that of the associated points being able to move from Normal to Reverse. The signaller offers a release of the GF to the operator on site that has to take it up- this unlocks the points but the points don't actually move until another action is taken (often the mechanical pulling of a lever but there are Ground Switch Panels which use electrical operation of points but once really has to wonder why such are provided nowadays since there is little financial saving compared with direct signalbox operation and with attendant disadvantages).
Obviously in routes which read over those points there is no 'availability' test for the points; the GF must always* be released or restored prior to the route being set, but the aspect level does effectively set, lock and detect the points, directly or inherently via the GF release. [* OK there are always exceptions- there are some GFs for which a release is automatically offered by the setting of a route that reads over the associated points Reverse but these are rather exceptional and probably worth ignoring to avoid confusion].
I hope that I have given enough information that you can attempt a GF Control Table for the example in this layout; have a go and we'll see how you get on!
