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It looks like a RCD, it feels like a RCD and it smells like a RCD. Crabtree BS4293.

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  • is it the only RCD ? it would be OK as the front end of a TT, if the 30mA protection is done circuit by circuit further down, to see a 500mA or 300mA is a common starter RCD for some of the older farms round here before any submains or boards, serving to provide some fire prevention and a last ditch protection if something bad happens further in.

    Of course if you cannot make it fire on a proper RCD test, you should treat it as faulty & replace.  It should fire on LN imbalance, just like a modern one.

    On some early current mode RCDs,  they were re-using bits from the voltage operated models so the test button is not (I think) introducing a fault like a modern RCD - it is just operating the firing solenoid, so they may past self test even if the pick up winding comes  detached. Is that flat enamelled  wire on the core or a trick of the camera and  lighting ?

    I suspect JP will want you to post it to him as a museum  of interesting parts  if it is surplus to requirements


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  • is it the only RCD ? it would be OK as the front end of a TT, if the 30mA protection is done circuit by circuit further down, to see a 500mA or 300mA is a common starter RCD for some of the older farms round here before any submains or boards, serving to provide some fire prevention and a last ditch protection if something bad happens further in.

    Of course if you cannot make it fire on a proper RCD test, you should treat it as faulty & replace.  It should fire on LN imbalance, just like a modern one.

    On some early current mode RCDs,  they were re-using bits from the voltage operated models so the test button is not (I think) introducing a fault like a modern RCD - it is just operating the firing solenoid, so they may past self test even if the pick up winding comes  detached. Is that flat enamelled  wire on the core or a trick of the camera and  lighting ?

    I suspect JP will want you to post it to him as a museum  of interesting parts  if it is surplus to requirements


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