Can I use a 4 core wire for a 2 core purpose?

[wonderwire]

I have a 3 core and earth in the ceiling used to wire a PIR switch to lights, but want to replace the PIR with a pull switch (2 core only).

Can I simply cut and tape the unused cores to make them safe, or do I need to find the junction box to disconnect them at the other end too and replace it with a 2 core wire? I think it's maintenance free so not likely accessible.

Thanks

 

[flameport]

Put terminal blocks on the unwanted wires. Do NOT cut them off or use tape.

 

[ban-all-sheds]

Some say best practice is to connect all unused cores to earth.

 

[PBC_1966]

If it's a PIR there at the moment the third core is likely a neutral. Just put into a "choc block" terminal or similar to make it safe and leave tucked in the back of the new switch box. Then it will still be there should you ever wish to change again or extend from that point for some other lighting.

 

[Taylortwocities]

Again

Even though the Pull switch itself only required two wires to function, you must still connect the earth (cpc) of the cable at the supply end and run it through to the switch.
Why?
so that it can perform its function as the circuit protective conductor (cpc) - especially needed if the switch is metal (or is changed) to a metal switch or other device that will need to be earthed.