A printed circuit "plate" heater 10 comprises an insulated steel disc having conductive tracks (18 etc.) deposited thereon. Live 11 and neutral 12 terminals are formed in the centre. Towards the circumference of the heater, is an unheated conductive ring 15 connecting eight discrete thermal fuses 16 designed to blow at 180° (ie. serious overheat condition). Conductive track 17 links the fuses to seventy-two heater cells 22 connected in parallel and formed of 1000Ω/square resistivity material or tortuous tracks of 10Ω/square material having a high PTC of resistance. In the event of localised overheating of a heater cell, its resistance will increase, leading to a decrease in its heat output. Serious overheating will result in a flow of heat to one or more fuses 17 which will then below.