A
fusible plug is a threaded metal cylinder usually of
bronze,
brass or
gunmetal, with a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point that flows away if a pre-determined, high temperature is reached. The initial use of the fusible plug was as a safety precaution against low water levels in
steam engine boilers, but later applications extended its use to other closed vessels, such as
air conditioning systems and tanks for transporting
corrosive or
liquefied petroleum gasses.