the switch must be connected to the neutral , it should not be connected to the phase wire. connecting to the phase wire is dangerous and may cause electrical accident.
Switch to the main line can be added through the main meter from where the electricity is coming into the premises. This would serve as the main switch and provide safety.
Break your feed where ever you want to install swich. Joint all nuetrals back together and install switch between the two black conductors to be controlled
Swith off the main power line by removing cut-off bridge. Now you can cut the main line wire, cut the tip at both the ends and connect it to the Single Pole Single Through (SPST) switch. Take care to fasten the wire properly.
