Fast-Acting Fuse (F Type)
The fuse characteristic that reacts to overcurrent in the shortest time. In a short circuit it interrupts the growth of the current early and protects the cable and connections. On the other hand it also treats the transient pulses of motor starting as a fault and leads to nuisance blowing.
The fusing wire is thin and has low thermal mass; for that reason it melts within milliseconds at a few times overcurrent. It is the right choice for circuits with no starting pulse, such as electronic boards and signal and control supplies.
In motor circuits, however, you meet this picture:
- The system works fine a few times, then the fuse blows in cold weather or when the load gets heavier.
- Trying to solve it by increasing the fuse rating leaves the cable unprotected.
- The right solution is to move to a slow-blow characteristic that accommodates the starting profile.
Automotive blade fuses generally have a fast-acting characteristic. In 12 V and 24 V motor driver installations this is the most frequently encountered cause of nuisance blowing.
Whenever a fuse blows, look for the reason first: it may be mechanical binding, a short circuit, reverse connection or a current rise caused by a dirty commutator. Fitting a new fuse of the same rating and carrying on hides the fault.
The context in which this term is used: DC Motor Driver Selection Guide