Adding a cpc

[I_spark]

Hello,
If a lighting circuit has been installed without a cpc in the CU.
But everywhere else in every switch and light there is a cpc.
Would it be alright to add a 2.5mm cpc in the CU and runnit to any switch or light?
Because I tried it with a wonder lead and I am getting a Zs on the lights.
Thanks in advance.

 

[davy_owen_88]

A separate earth core must be 4mm² unless it is mechanically protected.

Can't you run a new length of 1.5mm² t&e to the first junction box/ceiling rose?