Herculese mose fuse

[Puggie]

Protect the device from what? The device needs a fault to draw more current to blow the fuse. If you have a power transistor go and you blow an internal fuse, yes it will protect the power supply from becoming the next bit to let the magic smoke out, but the transistor is already destroyed.
If you try driving some silly impedance speakers you might blow a power supply rail fuse before the transistor cooks, but that is the only situation I can think where a fuse can protect a device from damage. Most cases they are there to reduce the extent of damage once something has failed.

 

[sktn77a]

Err...... how about a power surge in the mains?

 

[mindlagoon]

The fuse is not a surge protection device. It is a fault-detection device and damage-limitation device. Surges or shorter fast transients won't necessarily blow a fuse. Surge and FT protection devices protect from very short but very high voltage spikes (e.g 2kV), which could cause damage but have insufficient energy to blow a fuse.

 

[davidsrsb]

The fuse prevents a fire after the surge has damaged the equipment or a SPD fails short circuit

 

[Puggie]

Err...... how about a power surge in the mains?

A fuse won't do anything about a power surge (you would need a TVS diode or similar for that). But if the surge (over voltage) blows something up, the fuse will prevent further damage to the power supply.

A 3A fuse will pass 6A for 30sec without blowing, they are not a super fast high precision limiting device, more a last ditch sacrificial thing when it goes really wrong.