However, what if the CU is 16th Edn or earlier and is plastic? There are no exposed CPs to be connected.

But the supply circuit still needs an Earth - since it needs to provide that facility to downstream circuits - just like the c.p.c. in a socket circuit where the sockets are plastic and on plastic boxes (and with plastic faceplate screws or plastic cover caps if you insist). I've no problem calling it a c.p.c. even if were supplying a plastic CU that supplies an all insulated lighting circuit (c.p.c. run to all points, but no exposed-conductive-part to connect it to). Maybe the definitions could do with a bit of modernising though.

   - Andy.

However, what if the CU is 16th Edn or earlier and is plastic? There are no exposed CPs to be connected.

But the supply circuit still needs an Earth - since it needs to provide that facility to downstream circuits - just like the c.p.c. in a socket circuit where the sockets are plastic and on plastic boxes (and with plastic faceplate screws or plastic cover caps if you insist). I've no problem calling it a c.p.c. even if were supplying a plastic CU that supplies an all insulated lighting circuit (c.p.c. run to all points, but no exposed-conductive-part to connect it to). Maybe the definitions could do with a bit of modernising though.

   - Andy.

Of course - I have never said that the cable D could be omitted, but if the CU is non-conducting there are no exposed CPs in that circuit so the definition of a CPC does not fit well.

Actually we are quite happy with a CPC to an all plastic socket or fused spur that does not need it - but things that connect beyond do. I see this as the same, but clearly others do not ;-)

But I agree it is not a nice question - and we all know what the wire is for.

Mike.