Good day

I am new on this forum. Would like to ask a simple question regarding AC wiring of a normal house. Most older houses do not have earthing for ceiling lights.

Therefore, is it allowed to run a single 1.5mm green/yellow wire [House Wire] from light to light without using conduiting, to earth the metal ceiling fans in each room and then connect it properly to cold water pipe ? This is to avoid costs of using twin and earth/surfix as the house is large.
Alternatively- what is the compliance requirements for this earthing circuit ?