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An important technical consideration during fuse selection is to choose a model that is not prone to incorrectly blowing during its system life and that protects a circuit by quickly blowing in case of any problem. A correct fuse selection method is described below for designers of electronic equipment and systems.
To use a fuse properly, it is necessary to find out the folowing about the circuit beforehand.
1.Derating for steady-state current
2.Derating for ambient temperature
3.Derating for pulse current
4.Rush current
5.Rated Voltage and Breaking capacity
6.Clearing characteristics
7.Failure rate
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1.Derating for steady-state current
In the case of a steady-state current load, the fuse is recommended to be derated to less than 70% of its rated current.
Compact chip-type micro fuses are easily affected by nearby components and those that generate heat. A fuse operating environment below 50% of a rated current is recommended as a steady means of derating.
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2.Derating for ambient temperature
When the fuses are used at a higher ambient temperature, the current rating should be derated by the coefficient shown below.
This is caused by the fast blowing time due to ambient temperature, that is, because the rated current is reduced.
For example, the temperature derating factor will be 0.92 when Daito's SBL fuse is used in environments where the temperatures are 60 degrees celsius inside the power supply unit.
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Code |
Fuses |
(a) |
P(0.3A-1.3A), P4H(0.4A-1.3A)
MP(0.32A-1.6A), HP(0.32A-0.5A) |
(b) |
GP, P(2A-5A), PL(7.5A-15A), P4H(2A-10A)
UP, UPK, MP(2A-7.5A), HP(1A-6.3A), EP
DM, LM, HM, VM, SVM, SBM, CM, KMC, KMD
BD, DCP, BE, BL, SBL
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(c) |
SP(all) |
(d) |
SMP(all) |
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3.Derating for pulse current
The load current of a fuse is measured in terms of an effective value. When a fuse is used in a circuit with a relatively long pulse interval, the fuse is sometimes damaged if the peak value of a pulse is large, despite a relatively small effective current.
The following illustration shows an example of derating to a current emitted by a Daito P , PL alarm indicator fuse. It shows that a large derating factor is required for a large and sharp pulse current if the period exceeds 0.1 seconds, despite the small effective value.
The derating factor will be 0.6 for a period of 1 second and pulse width of 0.01 second (duty factor is 0.01).
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You can determine the permissible steady-state current for a long service life, by multiplying the current rating by the derating coefficients discussed in paragraphs 2.1, 2.2 and 2.3.
Permissible steady-state current <
[Rated current] * 0.7 * [Temperature derating coefficient] * [Pulse current derating coefficient] |
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4.Rush current
An inrush current often flows to the fuse in circuits where the power circuit and capacitor of a DC-DC converter are located in the secondary stage. If the Joule heat generated by an inrush current repetitively passes a fuse, the associated fuse element will develop metal fatigue due to oxidation deterioration, expansion and shrinkage, eventually causing blowing of the fuse itself.
The durability of a fuse is measured by substituting an inrush current for heat energy (Joule-integral) and by comparing it with the Joule-integral value at which the fuse element blows. The heat radiation quantity reverts to zero in a short-time blowing region of less than about 10ms. The Joule-integral value of fuse blowing becomes a certain value dictated by the material and sectional areas of the fuse wire.
The diagram below shows an example of the durability of a Daito micro fuse exposed to an inrush current. The axis of ordinates plots the Joule integration ratio (level of heat energy) of an inrush current and fuse blowing and the axis of abscissas, cyclic durability of the fuse to the iterative passing of a current.
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Code |
Fuses |
(1) |
P4H(More than 7.5A), PL(More than 10A), SP, SMP |
(2) |
UP, UPK |
(3) |
P(Less than 1.3A), P4H(Less than 1.3A), GP, MP, HP,
EP |
(4) |
P(More than 1.3A), P4H(1.3A-5A), PL(7.5A) |
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Code
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Fuses
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(1) |
DM, HM, LM, VM, CM, BD, DCP
BL(1A-6.3A), BE(1A-1.6A) |
(2) |
SVM, SBM |
(3) |
SBL |
(4) |
KMC, KMD |
(5) |
BL(More than 8A), BE(More than 2A) |
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5.Rated Voltage and Breaking capacity
5.Rated Voltage and Breaking capacity
An arc is generated by the blown element the moment a fuse blows. The higher the circuit voltage, the more the arc energy increases and larger it becomes, rendering it difficult for the fuse to blow. The boundary between the circuit voltage and current that can correctly cut off the circuit can be specified as rated voltage and breaking capacity.
A fuse with a rated voltage higher than the circuit voltage and with a breaking capacity exceeding the foreseeable short-circuit current must be selected. |
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6.Clearing characteristics
To enable a fuse to correctly isolate a faulty circuit and minimize faults in case of circuit failure, the damage characteristics of devices and circuits to be protected must be understood correctly and compared with fuse breaking characteristics.
If the circuit voltage exceeds 100V, the generated arc energy is sometimes large and will extend beyond several milliseconds, despite the fact the voltage is below the rated level. The breaking performance of a fuse must be checked to strictly ensure circuit protection.
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7.Failure rate
The failure rate of a fuse in terms of measurement of its lifespan is generally indicated by FIT (failure units).One FIT represents one failure in total operation hours of 1 billion [pieces x hours]. Long-term continuous current carrying tests for blows, including micro fuses and conducted by Daito, have resulted in few FITs, signifying that Daito fuses have high reliability. |
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