Immersion Heater Timer Switch

[keithlbw]

HELP
I've just bought a timer switch for my immersion heater.The instructions say connect the live from the SUPPLY to terminal 2 and the live from the HEATER to terminal 1 . Neutral wire is not a problem as both go to terminal 3. Earth is not a problem either.

When I take my old on/off switch off the wall how do I know which live is supply and which is to the heater.

[dipee65]

See answer in How it Works section
Alan

[keithlbw]

Thanks Alan
Regards Keith