A kettle boils water with a hidden heating plate, and a thermostat switches the current to hold it near boiling. Behind that everyday control sits a one-shot safety switch that cuts the power for good if the kettle ever overheats.
An electric kettle boils water with a heating plate in the base. That plate is simple and reliable, but any heater that never switches off is dangerous, so the kettle carries two separate temperature devices. The first is the everyday thermostat, which cycles the power to keep the water near boiling.
The second is the thermal cutout, a one-shot safety switch wired in the same circuit. It does nothing at all until something goes wrong; then it breaks the circuit once and stays broken.
Outside, a kettle is a sealed jug with a switch. Hiding the housing reveals the heating plate, the thermostat and the cutout, and the magnified panel shows how the cutout's alloy, spring and contacts actually work.
The key idea is fail-safe design: the thermostat handles normal use thousands of times, while the cutout is a deliberate one-time sacrifice that guarantees the kettle turns off even if the thermostat is stuck on.
When you switch the kettle on, mains current flows from the cord, through the base connector, and into the resistance element in the heating plate. The plate warms up and starts passing heat into the water above it.
The hot plate heats the water, which rises and carries the heat away in a convection current. Near 100 °C the water boils, holding that temperature while it turns into steam.
All this time the thermostat has been working. Its bimetallic strip bends as it warms, opening the circuit when the water boils and closing it again as things cool, so the kettle switches on and off by itself.
A thermostat that sticks closed cannot switch off. The water boils dry, the plate and the base keep getting hotter, and the temperature rises past the point the normal thermostat would ever allow.
Inside the cutout, a low-melting alloy holds a spring back. When the runaway heat melts the alloy, the spring is released and snaps the moving contact away from the fixed one. The circuit breaks, the current stops, and because the alloy cannot re-form, the cutout stays open for good.
It is a one-shot safety switch. If the normal thermostat fails and the kettle keeps heating, the cutout melts and breaks the circuit before the base can overheat dangerously.
The thermostat switches on and off all the time to hold the temperature steady. The cutout stays closed for its whole life and opens only once, at a much higher temperature.
Usually no. Its sensing alloy melts, so the break is permanent. Once it has tripped, the appliance is meant to be repaired or replaced, not reused.
A stuck thermostat can keep the element on forever. Two independent devices — one resettable, one sacrificial — make the kettle fail safe.
A small spring is held back by the solid alloy. When the alloy melts, nothing holds the spring any more, so it snaps the moving contact away from the fixed contact.
It protects against any overheat that reaches its trip temperature, including a kettle boiled dry, as long as that heat reaches the cutout.