Step 1: Choose the size and shape of the fuse

 

The choice of fuse with size and shape features mainly depends on the space for the user to design the circuit and the installation requirements of the circuit.

Note: For the size and shape characteristics of the fuse, please refer to the specific engineering drawings of the fuse in each specification.

 

Step 2: Select the required safety certification

 

Which safety certification fuse to choose depends on the safety certification that the user's product needs to pass.

Note: For the safety certification passed by the fuse, please refer to the specific safety certification passed by the fuse in each specification.

 

Step 3: Choose the rated voltage of the fuse

 

The rated voltage of the fuse must be greater than or equal to the effective voltage used in the circuit, that is, the fuse can only be used in a circuit that is less than or equal to the effective voltage used. Specifically: a fuse with a rated voltage: 125V can only be used in a circuit less than or equal to 125V, not 250V, a fuse with a rated voltage: 250V can only be used in a circuit less than or equal to 250V, or Used in 125V circuits.

 

Step 4: Select the rated current of the fuse (In)

 

The rated current of the fuse In = the maximum stable current of the circuit/(safe attenuation rate of the fuse * attenuation rate of the operating environment temperature).

 

(1) The safety attenuation factor of the fuse

Due to the difference between the actual circuit use of the fuse and the laboratory test conditions (such as: the difference between the fuse box used by the customer to install the fuse, the contact resistance generated by the fuse clip, and the cross-sectional area of the wire in the circuit), ensure that the fuse is in It can work continuously and reliably under the condition of 25±5℃. When choosing a fuse, you need to consider the safety attenuation factor of the fuse. Please refer to the following simple calculation formula.

- American standard products (UL standard): Fuse rated current (In)=loop maximum stable current/0.75

- European standard products (IEC standard): Fuse rated current (In)=loop maximum stable current /0.9

- Japanese-standard products (JIS standard): Fuse rated current (In)=loop maximum stable current /0.85

 

(2) Use ambient temperature attenuation rate

When the fuse is used at an ambient temperature of 25±5℃, the rated current value of the fuse is not affected by the ambient temperature; when the fuse is used at an ambient temperature greater than 30℃, the ambient temperature has a great influence on the rated current carrying capacity of the fuse. Therefore, the influence of ambient temperature on the fuse must be considered when selecting the fuse. (See Figure 1):

 

 

Curve A: Very fast-blow, fast-blow and slow-blow fuse curves.

 

The rated current In of the fuse is calculated above. When selecting a fuse, a fuse with the best rated current value greater than or equal to In should be selected.

 

Step Five: Consider the current pulse

 

Current pulse refers to those instantaneous currents with large peak currents and short duration (less than 10mS) (such as the inrush current, starting current, inrush current and transient current generated by the power-on moment, voltage change and other factors). If the fuse is required to be able to withstand the current pulse under normal requirements without fusing, when selecting the fuse, the impact of the pulse on the fuse must be considered.

First of all: Calculate the I2t value of this pulse according to the maximum pulse generated in the actual circuit:   

Usually, the I2t value of the pulse is calculated according to the measured pulse waveform and the most approximate waveform corresponding to the following table.

 

 

Next: Using the pulse cycle endurance capability, the customer selects the number of times that the pulse can be endured. (See picture 3)

 

 

Curve U represents: the ratio of the pulse I2t value to the I2t value of the required fuse:

Ability to withstand cyclic pulse:

When U=22%, it can withstand 100,000 pulses

When U=29%, it can withstand 10,000 pulses

When U=38%, it can withstand 1,000 pulses

When U=48%, it can withstand 100 pulses

Note: There must be a sufficient time interval (10 seconds) between the pulse and the pulse so that the heat in the previous process can be dissipated.

Finally: to facilitate the pulse cycle tolerance and the pulse I2t value in the circuit, calculate the I2t value of the required fuse:

(I2t value of the required fuse = pulse I2t value / U)

U = I2t value of pulse / I2t value of the required fuse

Calculate the required fuse I2t value and the fuse itself rated melting heat energy I2t value comparison:

Choose the corresponding specification whose rated melting heat energy I2t of the fuse is greater than or equal to the I2t value of the required fuse.

 

Step 6: Test

 

After selecting the specifications of the fuse, according to the samples provided by the company, it must be installed in the actual circuit for testing to verify whether the selected fuse is reasonable; this verification should include tests under normal working conditions and fault conditions to ensure the selected fuse The specification works normally in the protected circuit.