Under what circumstances should I choose a slow break fuse?

Under what circumstances do fuses choose to break slowly? Comprehensive Analysis and Selection Guide
1、 The core applicable scenarios of slow breaking fuses
1. Capacitive and inductive circuit (with startup surge)
Motor and compressor:
Compressors in air conditioners, refrigerators, and washing machines can generate surge currents of 5-10 times the rated current during startup, lasting for hundreds of milliseconds. Slow breaking fuses can tolerate this surge and avoid accidental melting.
Example: In the air conditioning compressor circuit, a slow break fuse with a rated current of 3A is used, which can tolerate an instantaneous current of 15A when starting, but it will melt when continuously overloaded.
Power adapter and inverter:
When the power input terminal is connected, capacitor charging will generate an instantaneous current of 2-3 times the rated current. Slow breaking fuses can ensure that the equipment starts normally.
Example: The input terminal of a mobile phone charger uses a slow break fuse, which tolerates surge currents when turned on and protects the circuit from damage.
2. It is necessary to tolerate brief overcurrent scenarios
Industrial control equipment:
Devices such as PLCs and servo drives may experience brief overcurrent during startup or sudden load changes. Slow breaking fuses can prevent misoperation due to instantaneous overcurrent.
Example: In industrial motor control circuits, slow break fuses tolerate surge currents during motor start-up to ensure production continuity.
Household Electric Appliances:
For example, electric drills, vacuum cleaners, etc., the current during startup is much higher than the steady-state value. Slow breaking fuses can ensure the normal start-up of equipment and provide protection against continuous overload.
Example: In the electric drill circuit, a slow break fuse with a rated current of 5A is used to tolerate an instantaneous current of 20A during startup.
2、 Technical parameter support
1. Current time characteristics
I-t curve: The current time characteristic curve of a slow breaking fuse is gentle, and the melting time decreases slowly with increasing current.
Example: At 200% rated current, the melting time is usually within 1-2 minutes (as specified by UL standards).
Melting energy (I ² t value):
The I ² t value of a slow breaking fuse is higher, which can withstand larger energy pulses and avoid misoperation during surges.
Formula: The fuse energy I2 × t needs to be greater than the surge energy that may occur in the circuit.
2. Typical parameter range
Rated current: usually selected slightly higher than the normal operating current of the circuit (such as a circuit operating current of 2A, optional 3A slow break fuse).
Response time: At 10 times the rated current, the melting time is usually within 0.01~0.1 seconds (as specified by IEC standards).
3、 Practical application cases
Case 1: Air conditioning compressor startup protection
Scenario: A 15A surge current is generated when the air conditioning compressor starts, lasting for 500ms.
Selection: Use a slow break fuse with a rated current of 3A (such as Littelfuse 3AB series), which can tolerate starting surges but will melt when continuously overloaded.
2. Case 2: Power adapter input protection
Scenario: When the mobile phone charger is powered on, the capacitor charging generates an instantaneous current of 10A.
Selection: Use a slow break fuse with a rated current of 2A (such as Bel Fuse 0ZCF020) to ensure normal start-up and provide protection against short circuits.
3. Case 3: Industrial Motor Control Circuit
Scenario: When the servo motor starts, it generates a surge of 8 times the rated current for 200ms.
Selection: Use slow break fuses with a rated current of 5A (such as Schurter T9S series), tolerate starting surges, and ensure production continuity.
4、 Selection criteria and steps
1. Core principles for selection
Circuit characteristics: Confirm the presence of start-up surges or transient overcurrent in the circuit (such as capacitive inductive loads).
Rated current: Choose a fuse slightly higher than the normal operating current of the circuit to ensure reliable melting in the event of continuous overload.
I ² t value matching: Ensure that the I ² t value of the fuse is greater than the surge energy that may occur in the circuit to avoid accidental melting.
2. Selection steps
Determine circuit type:
If it is a capacitive inductive circuit (such as a motor or power adapter), choose a slow break fuse.
If it is a pure resistive circuit without surges (such as electric kettles, LED lighting), choose a fast break fuse.
Calculate rated current:
The rated current In should meet the requirement of In ≥ I working × 1.5 (considering safety margin).
Verify I ² t value:
Calculate the potential surge energy I surge 2 × t surge in the circuit, ensuring that the I ² t value of the fuse is greater than this value.
5、 Summary
Advantages of slow breaking fuses:
Strong anti surge capability, avoiding misoperation due to startup surges or brief overcurrent.
Suitable for capacitive and inductive circuits, such as motors, power adapters, industrial equipment, and other scenarios.
Selection suggestion:
Priority should be given to scenarios with slow breaking fuses: circuits with startup surges and need to tolerate brief overcurrent.
Combining current time characteristics, I ² t values, and other parameters to ensure that the fuse matches the circuit characteristics.
Based on the above analysis, the key role of slow break fuses in capacitive and inductive circuits with startup surges can be clarified, and detailed guidance can be provided for specific selection.