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As an additional long-stop safety measure to prevent a possible fire if other measures fail, some kettles have an element which is designed to burn out before it gets dangerously hot. Unfortunately, such elements are prone to burning out needlessly. In others, a plastic is used in the switch mechanism which will soften with excessive heat, releasing the spring which holds the contacts closed and so switching the current off. You then have a dead kettle, but at least it didn't burn your house down.