Datasheet
6
Calculation using Formula 1 :
Rated current ≥ 30/(0.90×0.94) = 35.5A
The above calculation result shows that the fuse with rated current of 35.5A or more should be selected for this circuit.
Type JHC, with rated current of 50 A and 40A can be selected.
3. Calculation from overload current
3–1 Measurement of overload current
Using oscilloscope or equivalents, measure overload current that needs to break circuit.
Example : Effective overload current = 140 A
3–2 Calculation from overload current
Determine rated current so that overload current can be 2.5 times larger than rated current.
Use Formula 2 to calculate rated current of fuse.
Rated current of fuse ≤ Overload current/2.5 ... Formula 2
Example : Overload current = 140 A
Use Formula 2 to calculate the rated current.
Rated current ≤ 140/2.5 = 56 A
The above calculation result shows that the fuse with rated current of 56 A or less should be selected for this circuit.
Type JHC, with rated current of 50 A and 40A can be selected.
4. Calculation from inrush current
4–1 Measurement of inrush current waveform
Using an oscilloscope or equivalent, measure waveform of inrush current of actual circuit.
4–2 Creation of approximate waveform
Generally, waveform of inrush current is complicated. For this reason,
create the approximate waveform of inrush current as shown on Fig. C to
simplify calculation.
4–3 Calculation of I
2
t of inrush current
Calculate I
2
t (Joule integral) of approximate waveform.
The formula for this calculation depends on the approximate waveform.
Refer to Table A.
Example : Pulse applied = 2 ms, Peak value = 300 A
Approximate waveform = Triangular wave
Since the approximate waveform is a triangular wave, use the
following formula for calculation.
I
2
t of rush current = 1/3 × Im
2
× t ... Formula 3 (Im : Peak value, t : Pulse applying time)
Use Formula 3 to calculate I2t of the inrush current:
I
2
t = 1/3 × 300 × 300 × 0.002 = 60 (A
2
s)
* Following formula is generally used for calculation of I
2
t as i(t) equal to current.
2
t＝∫
0
ｔ
ｉ
2
（t）dt
4-4 Search of load ratio
①Set up number of cycles to withstand.
Name
Waveform
I
2
t
Name
Waveform
I
2
t
Sine wave
（1cycle）
Trapezoidal
wave
Sine wave
（half cycle）
Various
wave 1
Triangular
wave
Various
wave 2
Rectangular
wave
Charge/
discharge
waveform
JOULE-INTEGRAL VALUES FOR EACH WAVEFORM
PULSE RESISTANCE CHARACTERISTICS
2ms
300A
Fig. C：Inrush current waveform
Red line : Actual measurement waveform
Black line：Approximate waveform
Table Ａ
m
t
0
1
2
t
m
0
0
t
2
m
0
t
m
0
-t
τ
i (t) = m e
-t/τ
m
0.368 m
1
3
m
2
t
m
2
t
1
2
m
2
t
1
2
m
2
t
t
2
t
3
m
0
t
1
t
2
0
1
t
2
0
t
1
t
3
2
1
m
2
τ
1
2
1
2
t + (
1
-
2
)
2
t
1
3
2
1
t
1
+{
1
2
+ (
1
-
2
)
2
}
(t
2
-t
1
)+
2
2
(t
3
-t
2
)
1
3
1
3
1
3
1
3
m
2
t
1
+ m
2
(t
2
-t
1
) +
m
2
(t
3
-t
2
)
1
3