Defy Spiral Plate Stove Problem - Help !

It sounds as though the switch is not working, its contacts now stuck closed. A replacement switch should be available at reasonable cost, and unless the wires to the switch are burnt, the change-out should be straightforward.

You could remove the heater element and check the resistance across the ends (you'll need an ohm meter that can measure low values of resistance). Then compare that reading with the other similar burners on the stove that are still working OK, to see if the readings are similar. An overheating element would have a very low ohms reading.

You can check, too, by swapping the suspected coil with one of the other similar coils to see if the 2nd coil also overheats when plugged into the same location.

These 2 steps should allow you to see if it's the heater coil or the switch.

I was going to suggest "Start placing your kettle on a different element", but you two guys gave much better suggestions.