Does a blown fuse mean cutting off the power supply?

Does a blown fuse mean cutting off the power supply? Comprehensive analysis and special case explanation
1、 Normal situation: A blown fuse is equivalent to cutting off the power supply
1. Basic principles
Fuse mechanism: The fuse is made of low melting point metal wire. When the current exceeds the rated value, the metal wire heats up and melts, forming an open circuit and cutting off the circuit.
Circuit status: After melting, the current cannot pass through the fuse, the circuit is completely disconnected, and the power supply stops. For example:
Home circuit: After the fuse is blown, the appliance stops working, indicating that the power supply has been cut off.
Industrial equipment: After the fuse is blown, the equipment will be powered off to avoid further damage.
2. Standard verification
UL/IEC standards: Fuses must undergo rigorous testing to ensure that the circuit is completely disconnected after melting. For example:
Breaking ability test: Simulate short-circuit current (such as 1500A/240V AC) to verify whether the fuse can reliably cut off the circuit.
Temperature rise test: Under full load, the temperature rise should be less than 50K to ensure that there is no residual current after melting.
2、 Exception: The power may not be completely cut off after the circuit is blown
1. Improper installation
Poor contact: If the fuse is not installed correctly (such as mechanical damage or oxidation), there may still be contact between the two ends after melting, forming a weak current.
Case: User feedback: The fuse of the water heater was not completely disconnected after being plugged in, and the fuse needs to be replaced and the circuit checked.
Solution: Regularly check the installation status of fuses to ensure good contact and avoid using damaged or aged fuses.
2. Capacitive discharge effect
Residual charge: In circuits containing capacitors, after the fuse is blown, the capacitor may store charge and discharge briefly, generating instantaneous current.
Impact: Instantaneous current may cause arcing or brief power supply, but it is not sufficient to maintain normal circuit operation.
Response measures: Avoid using fuses or parallel discharge resistors in capacitor discharge circuits to accelerate charge release.
3. Characteristics of self-healing fuses
High resistance state: The self recovering fuse (PPTC) becomes a high resistance state during overcurrent, and although there is still a very small current, it is not enough to maintain the operation of the circuit.
Standard definition: IEC 60269-4 stipulates that self recovering fuses should return to a low resistance state after troubleshooting, and the period of melting is considered as cutting off the power supply.
Application scenario: Suitable for scenarios that require repeated protection (such as USB interfaces), but attention should be paid to their aging characteristics.
4. Circuit design defects
Parallel circuit problem: If the fuse only protects part of the circuit (such as the parallel branch), other branches may still have power.
Case: The main switch is installed after the safety device, and when the main switch is disconnected, the right side of the blown fuse may still be charged.
Solution: The main switch must be installed before the safety device to ensure that the circuit is completely cut off after the fuse is blown.
3、 Summary and Suggestions
1. Core Conclusion
Under normal circumstances, a blown fuse is equivalent to cutting off the power supply and completely disconnecting the circuit.
Special circumstances: Poor installation, capacitor discharge, self-healing fuse characteristics, or circuit design defects may result in incomplete disconnection, but these are considered abnormal or design issues.
2. Safety advice
Correct installation: Regularly check the contact status of the fuse to avoid mechanical damage or oxidation.
Select qualified products: Prioritize the use of UL/IEC certified fuses to ensure reliable melting performance.
Circuit design optimization: Place the main switch before the safety device to avoid residual current caused by parallel circuits.
Capacitor treatment: In circuits containing capacitors, avoid using fuses or parallel discharge resistors in the discharge circuit.
Through the above measures, it can be ensured that the fuse reliably cuts off the power supply when it melts, ensuring the safety of the circuit.