a planar heating element for an electrically - heated water boiling vessel comprises a stainless steel plate having a printed heating element formed on one side thereof , the printed beating element being sandwiched between electrically - insulating layers of glass . a control for switching off the element in an overtemperature situation has a primary actuator in the form of a bimetal which directly accesses the printing element through a window formed in the outermost glass insulating layer , and the bimetal is held in a secondary actuator in the form of a thermally - collapsible carrier for the bimetal which contacts the glass insulation around the window . by virtue of this arrangement the primary and secondary actuators are subjected to different temperatures which ensures that the secondary actuator is not susceptible to nuisance operation .