Water boiling vessel immersion heater with primary and secondary thermal cut-out protection

In combination with a water boiling vessel, an electric heating element and an element protector and steam sensing unit for switching off the power supply to said heating element in the event of element overheating occasioned by said element being switched on without there being sufficient water in said vessel and also when water boils in said vessel, said heating element comprising a resistance heating portion for heating water in said vessel and an element head portion serving for mounting said heating element in said vessel, and said element protector and steam sensing unit comprising a first part mounted adjacent said element head portion so as to be subject to heat developed at said head portion by virtue of the powering of said element, and a second part arranged to be subject to steam vented from the interior of said vessel when water boils in said vessel, said first part comprising first and second bimetallic elements in close thermally coupled relationship with said element head and first and second pairs of switching contacts coupled with respective ones of said first and second bimetallic elements via respective electrically-insulating movement-transferring couplings for transferring the respective bimetallic element movements into switching movements of the respective switching contact pair, said first and second pairs of switching contacts being connected electrically in series with each other and with the resistance heating portion of the heating element, and said second part comprising a third bimetallic element subject to heating by steam vented from said vessel and movement-transferring means associated with said third bimetallic element for transferring movement therefrom to at least one of said first and second pairs of switching contacts for initiating switching movements thereof, said first bimetallic element being set to an operating temperature such as to determine a mean maximum operating temperature of said heating element and the operating temperature of said second bimetallic element being set above that of said first bimetallic element such that said second pair of switching contacts opens in the event that the heating element temperature increases significantly above said maximum operating temperature in consequence of failure of said first bimetallic element to open said first pair of switching contacts.

In combination with a water boiling vessel, an electric heating element and an element protector unit for switching off the power supply to said heating element in the event an element overheating caused by said element being powered when there is insufficient water in said vessel, said heating element comprising a resistance heating portion arranged for heating water in said vessel and an element head portion serving for mounting said heating element with respect to said vessel, said resistance heating portion of said element having cold tails projecting through said element head portion and including a return portion contacting said element head portion on one side thereof, and said element protector unit being assembled to said element head portion on the other side thereof from said resistance heating portion of said element and comprising primary and secondary safety cut-out switches connected in series with each other and said resistance heating portion of said element and serving for disconnecting said heating element from said electrical supply in the event of element overheating, said secondary cut-out switch having a higher operating temperature than said primary cut-out switch and serving as a back-up for said primary cut-out switch in the event of failure to operate of the same, each of said primary and secondary cut-out switches comprising a pair of switching contacts comprising a first contact engaged with a respective one of said cold tails of said resistance heating portion of said heating element and a second contact engaged with a respective one of a pair of power supply terminals provided in said element protector unit for the coupling of an electrical power supply thereto, one of said first and second contacts being mounted on a moving contact carrier which is coupled via an electrically-insulating movement-transferring member with a respective one of first and second snap-acting bimetallic members respectively constituting the prime movers of said primary and secondary cut-out switches, said first and second bimetallic members each being mounted in close heat transfer relationship with said other side of said element head portion, an electrically-insulating wall portion of said element protector unit being interposed between said first and second bimetallic members and said primary and secondary switching contact pairs and said electrically-insulating movement-transferring members extending through apertures defined in said wall portion.