A fuse is a sacrificial device for the protection the rest of the circuit in an over-current situation. There are situations we need to know with some reasonable confidence what the actual maximum current is for a given fuse without destroying it. This is beneficial when the specifications for the fuse at hand is unknown (e.g. with cheap no-name brand fuse), or the circuitry to be protected is so critical that the interrupt current level must be specified precisely.
Obviously for resettable fuses (such as PPTC fuse or polymetric PTC fuse), the rated current can be easily tested repetitively. Thus the focus here is for glass fuses and HRC fuses.
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