I finally found a use for it yesterday.
Let's start off with picture 1.
This is going to be a dry liner box in the ceiling with a CO detector over it. Power comes in via the T&E and goes out to the 3 core and earth to the CO detector in the kitchen. The 3 core offers an interlink should I decide to use it.
Now in the 6 weeks since I first fixed the place the kitchen ceiling has been patched and reskimmed and the cable in the kitchen is now missing.
I am not going to pretend that I had no idea where the missing cable was. Now to make this work I had to connect up the 3 core and earth cpcs to give the 3 core the required earth.
I made the following video.
and although X marks the spot I knew that the cable should have been in line with the trunking for the break glass so I drilled slightly off from the marked spot.
Have a look at picture 2
and X was where the end of the cable was resting on the ceiling.