A fuse is a commonly used overcurrent protection component. Its function is to promptly cut off the circuit when an overcurrent occurs, preventing damage to electrical equipment or wiring. The selection of a fuse is an important factor affecting the safety and reliability of the circuit. Therefore, it is necessary to comprehensively consider the various parameters and performance of the fuse according to the characteristics and requirements of the circuit, and choose the appropriate fuse model and specifications. This article will introduce the basic principles, parameters, performance, and selection methods of fuses.
1. Rated Voltage: Must be greater than or equal to the actual application voltage. Fuses generally have ratings such as 24V, 32V, 58V, 63V, 125V, 250V, etc.
2. Standards: The fuse standard is determined according to the safety certification required by the complete machine, such as UL standard or IEC standard.
3. Structural Dimensions: The size of the fuse is determined by the space available in the circuit design, such as length, diameter, and whether it has leads.
4. Breaking Capacity: The fuse's breaking current should be greater than the maximum fault current in the circuit.
5. Rated Current: Determined with reference to the following items:
(1) Normal operating current, running under 25°C conditions.
The rated current of UL specification fuses ≥ normal operating current / 0.75;
The rated current of IEC specification fuses ≥ normal operating current / 0.9.
The rated current of PSE specification fuses ≥ normal operating current / 0.8.
(2) Ambient Temperature: The current carrying capacity test of the fuse is conducted at an ambient temperature of 25°C. The higher the ambient temperature, the shorter the fuse's lifespan and the lower its carrying capacity. Therefore, ambient temperature should be considered when selecting a fuse. The effect of ambient temperature on current carrying capacity is shown in the following figure:
(3) Fast-acting fuses are suitable for pure resistive circuits, with the maximum I2T value in the datasheet being more appropriate; slow-blow fuses are suitable for capacitive or inductive circuits, with the minimum I2T value in the datasheet being more appropriate.
(4) Pulse: Pulses generate thermal cycling and mechanical fatigue that affect the fuse's lifespan. In design, the pulse I2T, considering the pulse factor, should be less than the fuse's rated melting energy I2T.
Fuse rated I2T > actual pulse I2T / Pf.
Pf: Pulse factor, varies depending on the number of pulses the fuse can withstand. Specific values:
Surge cycle resistance capability.
When the measured pulse I²t value is 22% of the fuse's melting I²t value, the fuse can withstand up to 100,000 pulse waves.
When the measured pulse I²t value is 29% of the fuse's melting I²t value, the fuse can withstand up to 10,000 pulse waves.
When the measured pulse I²t value is 38% of the fuse's melting I²t value, the fuse can withstand up to 1,000 pulse waves.
When the measured pulse I²t value is 48% of the fuse's melting I²t value, the fuse can withstand up to 100 pulse waves.
6. Testing
The samples selected through the above procedures need to be tested in the actual circuit to verify whether the chosen fuse is appropriate. This verification should include tests under normal and fault conditions to ensure that the selected fuse provides protection in the protected circuit.
