I have a limited wattage supply and rather than remembering to switch of the heating for a couple of minutes to boil the kettle would like to fit a relay in order to "break" the feed to the heater and remake the feed after the kettle has boiled using the plug on/off switch.what type and rating of relay should I use? input 230 volts 16 amp RCD approx 3680 watts. Heating max 2000 watts and kettle 3000 watts. Thank you
You could use a current transformer with its primary in series with the kettle and its secondary connected across the AC coil of the relay/contactor you're going to use to kill the heater while the kettle's plugged in.
The relay contacts would be normally closed and they'd open when you plugged in the kettle.
Caveat: There'll be an instant when the kettle and the heater would both be on until the relay contacts opened.
More important, if you don't know what you're doing and/or you're not familiar with working with the mains, you could easily kill yourself, so my advice would be to hire a licensed electrician to do the work.
There are also devices called "current switches" or "current relays" which will do exactly what you want, so you may want to take that course instead. Search on Google for lots of hits.