As an engineer, you often encounter such a problem when facing the selection of fuses, should you choose Fast-Acting or Slow-Blow?
The following are some specific judgment methods and practical examples for this problem:
Basis for judgment：
| Characteristic | Fast acting fuse | Slow acting fuse |
|---|---|---|
| Fusing speed | Rapid fusing (millisecond level) when overcurrent | Allows short-term inrush currents to pass through (tens of milliseconds to seconds) |
| Applicable scenarios | Surge-free sensitive circuits (e.g. ICs, LEDs) | Circuits with surge or inrush currents (e.g., motors, power supplies) |
| Key parameters | Low I²t value (low fusing energy) | High I²t value (high surge energy tolerance) |
| Typical load characteristics | The steady-state current is stable with no instantaneous spikes | The current rises significantly when starting or switching |
Example of fusing speed comparison: Slow-acting fuses vs. fast-acting fuses
0ABB（slow acting） C2F（Fast acting)
2. Selection steps
-
Analyze the load characteristics of the circuit, such as whether there is an inrush current?
For example, scenarios such as motor start-up, capacitor charging, and power supply power-on will generate instantaneous currents that are several times the rated current. -
Is the load sensitive?
For example, semiconductor devices (MOSFETs, ICs) need to be quickly cut off from overcurrent to prevent damage. -
Match the fusing characteristics
There is a surge → choose the slow breaking type: avoid the surge causing the fuse to blow by mistake.
No surge → Optional quick break type: Quickly cut off the fault current and protect sensitive components.
- Verify the I²t value
The I²t value (blowing energy) of the fuse needs to be greater than the I²t value of the inrush current (to ensure that the fuse does not blow during the surge).
3. Specific examples
Example 1: LED driver circuit selection
Assumed operating current: 1A (stable, no surge)
Since LEDs are sensitive components, they need to respond quickly to overcurrent or short circuits.
Choose a quick-acting fuse (such as C1F) with a rated current of 1.5A (1.25~1.5 times the working current).
Blows the fuse in milliseconds to protect the LED.
Example 2: Motor control circuit
Assuming operating current: 2A (steady state).
Inrush current: 15A at motor start-up (150ms lasting).
Since the inrush current is much higher than the steady-state current, the fuse needs to withstand short-term overcurrent.
Choose a slow-acting fuse (e.g. C1T) with a rated current of 3A (1.5 times the operating current).
When the motor starts, the 15A current lasts for 150ms, and the fuse does not blow;
If the fault causes the current to continue ≥ 3A, the fuse blows within seconds.
Fourth, summary
Quick-break type: used in scenarios where there is no surge and fast protection is required (such as electronic board and battery protection).
Slow-breaking type: used in scenarios where there is a surge and short-term overcurrent needs to be tolerated (such as motors, power supplies, and inverters).