In practical use, it is not enough to select fuses only according to UL specifications.
1. Several issues to be considered in choosing
On the basis of knowing the parameters of the circuit used, the fuse is selected according to the following aspects: normal current, fuse current (current and time to be fused), open-circuit voltage, short-circuit current, impulse current (current, time and times not cut off), ambient temperature.
(1) Normal current: First of all, we need to know the normal current flowing through the fuse in the circuit used. Usually we need to set a reduction in advance, and then choose according to the following principles: normal current is less than the product of rated current and reduction factor.
(2) Fuse current: According to UL specifications, the fuse should be fused rapidly when rated current is 2 times. But in most cases, in order to ensure reliable fusing, we recommend that the fuse current should be 2.5 times larger than the rated current. In addition, if the fuse time is important, we should also refer to the fuse characteristic diagram provided by the manufacturer to make a judgment.
(3) Open-circuit voltage: Open-circuit voltage should generally be selected to be less than the rated voltage. For example, when the fuse with rated voltage of DC24V is used in the circuit of AC100V, it may ignite or break the fuse.
(4) Short-circuit current: We call the maximum current flowing through the circuit short-circuit as short-circuit current. For all kinds of fuses, the rated circuit breaking capacity is prescribed. When selecting fuses, we should pay attention not to make short circuit current exceed the rated circuit breaking capacity. If the fuse with small breaking capacity is selected, it may cause the fuse to break or cause fire.
Impulse current: Observe the waveform of impulse current (pulse current waveform), and calculate its energy with I2t value (Joule integral value). The impact on the fuse varies with the magnitude and frequency of the impact current. The ratio of I2t value of impulse current to I2t value of single pulse fuse determines the number of times that the fuse can withstand impulse current.