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The purpose of this guide is for user to properly select a fuse which provides protection of components, circuit or equipments. The following flow is considered for fuse selection.

Factors taken into consideration
The safety approvals required for fuse shall be determined according to the safety approvals required on the end product. It is the safety approval that determines initially the selected fuse designed to IEC standard or UL standard.

a. The space limit of circuit in design.
b. Mounting mode.

The voltage rating of the fuse shall be greater than, or equal to the available circuit voltage.

The interrupting rating of the fuse should exceed the Maximum Fault Current of the circuit.

Does there exist "turn-on current" in a circuit when the end product turns on or off? The "turn-on current" is normal for some circuit and requires the slow-blow fuse or medium slow-blow fuse.

The overload current and time in which a fuse must function (It may be specified on the specific protection needs of circuit by design engineer.) Referring to the Time-Current curve, the maximum current rating which meet the requirement would be taken as the upper limit for current rating A1.

a. Steady state current through a fuse (based on the specific circuit).
b. The difference of current rating for fuse designed to IEC standard and UL standard, refer to STEADY STATE CURRENT.
c. Effect of ambient temperature on current-carrying capacity of fuse, refer to AMBIENT TEMPERATURE.
d. Effect of pulse (including surge currents, turn-on currents, in-rush currents and transients) on life of fuse, refer to PULSE.
e. "Turn-on current" and duration should be compared toTime-Current curve of relevant fuseWhen A1≥A2, any rating can be selected from the range of A2 to A1.
When A1＜A2, it is advised to select another type fuse.

The sample shall trial-operate in the actual circuit.

　

 

　
STEADY STATE CURRENT
　
 There exist the different conditions between the actual appliance and test conditions, such as,
 A. Fuse-holder
 B. Connecting cable size.
 C. Contacting resistance between fuse clip and fuse, etc.
 The above factors should be taken into consideration when selecting a fuse in a 25℃ ambient.  To compensate for the variation, the following conditions shall be melted to ensure the fuse operating continuously and properly.
 Fuse designed to IEC standard: current rating (In)=steady state current of circuit/0.9
 Fuse designed to UL standard: current rating (In)=steady state current of circuit/0.75
　

AMBIENT TEMPERATURE
　
 The current carrying capacity tests of a fuse are performed at 25℃ and will be effected by changes with the ambient temperature. The higher the ambient temperature is, the shorter fuse life will be, and the lower the current carrying capacity will be. So the ambient temperature shall be considered for proper fuse selection. Refer to the following charts showing the effect on the current carrying capacity of all kinds of fuse.
　
(I) Effect on rating and opening time in 5In of traditional slow-blow and medium slow-blow fuse
(II) Effect on rating and opening time in 5In of fast-acting and slow-blow fuse

PULSE

 Pulse produces thermal cycling and mechanic fatigue which could affect the life of fuse. In design the selected fuse should have a I2T value much greater than the I2T value of pulse. Refer to TABLE A showing the relationship between the life of fuse (the endurable times of pulse shock.) and U ( ratio between pulse I2T value and fuse I2T value ).The I2T value of a fuse presented in this catalog may be for your reference. The I2T value of a pulse can be approximated from the following formulas for a typical wave shape, refer to TABLE B.

　
Endurable times of pulse shock
U( ratio)
100,000 times
20%
10,000 times
30%
1,000 times
40%
Note: Adequate interval (5s-10s) must exist between pulse events to allow the heat from the previous event to dissipate
TABLE A

　

 

TESTING
 The selected sample should be tested in the actual circuit to verify the selection .The testing should be include the tests under normal and fault conditions to ensure that the fuse will operate properly in the circuit.

 In all kinds of electrical protection doubts, contacting our engineers are welcome Possibly we will work with you making some special recommendations to help you solve these problems.

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