circuit 2 examples



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       circuit 1A examples

       circuit 1B examples

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       Welwyn release

       circuit 1+2 hybrid

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    Circuit 2 is associated with the Tyer 'Black Box' instruments. Wiring diagram SWD 301A clearly shows one of these instruments, but an identical commutator is found on many of the 'North Eastern' double line blocks.

    Unlike Circuit 1, the relays to provide the block control functions were in a separate unit outside the block instrument. Nevertheless, the instruments are not without interest:

    The bell is located on the back of the instrument:

    Example 1 - block bell relay

    These instruments have the option to incorporate a block bell relay, so that the solenoid for the bell striker is operated by a local power supply, and not directly connected to the block line. Here's an example of the relay:

    Example 2 - shorting plug instead of relay

    Some instruments were fitted with shorting plug instead, so that the bell striker solenoid was directly connected to the block line:

    Note also the red lettering on the upper needle unit, and the white line down the centres of the pointers - these features are not usually present.

    Example 3 - free block (?)

    The next example has neither bell relay nor shorting plug - so the bell has been disconnected (of course, the relay or plug could have been removed after it was taken out of service). The bell itself is still present on the rear of the instrument, and the bell tapper was obviously used (terminals 1 to 3 have remains of wire attached to them).

    Unfortunately, whoever removed the instrument did a very thorough job, and cut the wires so short that no wiring labels remain - a pity, as these might have revealed its origin! But they didn't remove all evidence: look carefully and you will see that terminals 7 and 8 are linked, also there is no connection to terminal 4. This shows the block was not used with the BZR1 relay unit. In fact, the fact there is only one connection to terminal 6 could indicate this instrument was used without any controls at all (compare with SWD 302 with the Elsecar Junction block circuits showing free block to Darfield Main).

    Example 4 - Blackmoor Crossing Block Circuits

    The Tyer Black Box instruments were very common on the eastern side of the Woodhead Route, from Rotherwood to Woodhead; most of the signal boxes, of which I have seen interior photographs or other evidence, had this type of instrument with the characteristic rectangular Welwyn release nearby on the block shelf.

    Here are the wiring diagrams for Blackmoor Crossing: block circuits and block bells. These diagrams actually refer to the relay unit as BZR and not BZR1; it is not clear to me what the difference is.