The I²t value of a fuse is a measure of the energy which is required to open the fuse element and to interrupt the current. It is determined by factors such as the design of the fuse element, the type of filler, etc. The I²t value is also a direct measure for the let-through energy which is exposed to the item being protected by the fuse in case of an over-current situation. It is a function of current squared and time, hence it is expressed in A²s (amps x amps x seconds).