Slow-Blow Fuse (Type T)
A fuse characteristic that can pass short overcurrent pulses without blowing while still safely interrupting a sustained overload. It is obtained by increasing the thermal inertia of the fusing wire. It is preferred in circuits with high current pulses lasting seconds, such as motor starting.
In motor circuits the starting current rises to typically 5-8 times the nominal value and lasts 0.5-3 seconds depending on the load. A fast-blow fuse blows on this pulse; the slow-blow characteristic passes the pulse and interrupts only if it lasts longer.
Points to watch when choosing:
- Rating: Changing the characteristic is no substitute for increasing the rating. First calculate the correct ampere rating, then choose the characteristic.
- Cable protection: A slow-blow fuse also interrupts a little later in a short circuit; the cable cross-section must be chosen to withstand this extra time.
- Marking: On glass tube fuses the letter T indicates the slow characteristic and F the fast one.
Automotive blade fuses generally have a fast-blow characteristic; if you experience nuisance blowing in a motor application, moving to a slow-blow type is a better solution than increasing the rating. Do not raise the fuse rating above what the cable can carry.
The context in which this term is used: DC Motor Driver Selection Guide