How can measuring the resistance of a fuse, tell you that it is defective?

A fuse breaks a circuit when current is too high. A fuse must be replaced when it breaks a circuit.

Slowpoke4 years ago
A fuse is a self - sacrificing protection device. Simple fuses are non-resetable, so when they blown they are destroyed and must be replaced.
It is unusual for a fuse to actually blow unless there is a fault with in the circuit that it is protecting. Excess current flows within the fuse causing localised heating which melts the fuse element (usually thin tinned copper wire) thus protecting the circuit or more correctly removing the power to the circuit and preventing further damage or fire. A fuse generally tests good at a very low resistance. If it has a high resistance or near infinite resistance, it is faulty (obviously since it cannot pass power).
Fuses are a cheap & simple means of providing protection but they do have limits.
They can fail for other reasons. Corrosion, metal fatigue and electromagnetic stress (similar to metal fatigue). If it is a glass encased fuse with metal ends; the solder connecting the caps to the fuse link can go high resistance (metal fatigue and or dissimlar metal corrosion) - The fuse look perfect but doesn't pass any current.
Generally, if a fuse has blown, there is something wrong with the circuit that it is connected to. Replacing the fuse with a higher rated one can lead to some nasty and expensive problems. Always use the correct rated fuse and the correct type. Some fuses are quick acting, some are slow acting (surge resistant) and all are rated by voltage and current.

NachoMahma4 years ago
.  When blown, a fuse is an open circuit and will have infinite resistance. Before it blows, it has very close to zero resistance.
.  See Slowpoke's reply for how fuses work and why you have to replace a blown fuse (plus more).
Home work ?