early circuits



   railway pages

     block instrument wiring

       LNER instruments

       simple block

       early circuits

       ex-GNR mechanical

circuit 1A theory

       circuit 1B theory

       circuit 2 theory

       circuit 1A examples

       circuit 1B examples

       circuit 2 examples

       circuit 1+2 hybrid

       Welwyn release

       some questions

    It appears that the earliest control applied to 3 wire, 3-position block was that which requires the home signal to be on before line clear can be pegged - i.e. home normal control. This requires a single stick relay. There are two versions of the circuit.

    (Puzzled by written wiring diagrams? try this quick guide)

    The operation of the circuit is the same in either case. At Line Blocked, no current is sent to the block line. At Train on Line, negative battery is sent to line.

    Now suppose we want to peg line clear. BCS will be down (de-energised), and so the proving circuit must be closed, i.e. home signal at danger (the proving circuit may also check the distant signal, and that the relevant signal lamps are alight). On moving the commutator to Line Clear, BCS picks up, closing its top contact. The home signal can now be cleared, and BCS will stay up, fed through its own top contact. BCS drops when we move the commutator to Line Blocked.

    The two circuits differ only in whether BCS is by-passed by contact C1. This makes no difference as far as home normal control is concerned. It became significant later on, when this circuit was used as a component of more sophisticated controls - see Circuits 1A and 1B. Eventually form B was preferred.

    The later circuits had a characteristic style of construction (lettered screw terminals, way contacts were added to block instrument, etc), so I have assigned to 'early circuits' any example of home normal control which appears to be different!

    Example 1

    Richard Pike sent me this nice early drawing showing home normal control applied to the down main block at Heckington (large file - 941 kb). The diagram also shows the down main home lever locked by a track circuit.

    Example 2

    This GCR instrument has seen better days - the needle is broken, for a start! - but is still interesting. It is reputedly from the Leicester area, and is shown below with the cover removed:

    Contact C1, which opens when the commutator is moved to line clear, is of a different form to that found on instruments with the later, standard, circuits. The contact is shown below in the open position:

    On the back of the instrument, we find a small metal box. Most back boxes on LNER instruments were made of wood. The bright green paint was added recently.

    Open the back box, and we find a single relay. Enough wires remain to show that this is an example of home normal control, version A.