When fuses are subjected to current pulses, cyclic stress is induced owing to temperature variations. Damage in the fuse elements is gradually accumulated and results in resistance increase. Consequently, current-time characteristics shift to effect early operation during the normal service period. Under such circumstances, proper protection required for other equipment will not be achieved. Reliability concerns for after sales service arise owing to these facts. The paper presents a method for predicting lifetimes of fuses for cyclic loading. The physical model is based on the description of elastic, plastic and creep deformation during a series of current pulses. In comparison with existing methods and IEC recommendations, it has the advantage of requiring a very limited number of tests. Results from the physical model can be used to evaluate practical fuse designs and provide advanced information for fuse replacements. The method has been demonstrated for commercial time-lag miniature fuses, lowvoltage fuses with M-effect, low-voltage fuses for semiconductor protection, and high-voltage fuses for motor protection.

Fuses are designed to operate under fault conditions. However, because of ageing processes, fuses also operate in practice under normal load conditions. This article presents a method to determine how fuse characteristics change depending on specific load conditions. Based on metal fatigue laws, the process of ageing is described and compared with measurements. A method is proposed how to formulate general ageing behaviour from a limited number of measurements under specific conditions. The method is demonstrated for miniature fuses but also showed to be valid for fuses for the protection of motors and semiconductors.