When should a fuse be replaced?

A fuse serves as a protective device in electrical circuits, designed to break the circuit and prevent excessive current from causing damage. When the current flowing through the fuse exceeds its rated limit, the fuse element melts, interrupting the circuit and stopping the flow of electricity.

Replacing a fuse is necessary when it has no continuity, which indicates that it has blown and can no longer provide the necessary protection. A fuse with no continuity means that it is no longer functional and must be replaced to restore electrical safety and proper circuit operation.

In contrast, having continuity indicates that the fuse is still intact and functional, thus it should not be replaced at that time. The other options given do not represent the correct criteria for replacing a fuse. For example, there's no standard requirement to replace a fuse once a year, as its lifespan depends on the electrical load and conditions under which it operated. Similarly, a fuse should not be replaced based solely on its resistance reading, especially if it's still functioning properly, and a fuse that has continuity is not needed to be replaced at all.