1. Basic Fuse Selection

2. Basic Fuse Selection 1 Choice of the standard [UL, IEC...]
2 Choice of the rating voltage
3 Choice of the breaking capacity
4 Choice of the fuse type [5x20mm, TR5..]
5 Calculation of the operating current
6 Considering the current definition factor
7 Considering the temperature factor
8 Choice of the rating current
9 Considering the inrush current
10 Considering the pulse factor
Approval VN
IBC
Ioperate max FI FT IN
I2tpulse
FP, I2tmelt
Basic Fuse Selection

3. Europe America Japan Worldwide No Approval
Where do you intend to approve your equipment?
Which agency approvals do you require?
Choice of the standard
Europe
America
Japan
Worldwide
No Approval
Basic Fuse Selection

4. Voper. = ? VN  Voper.max What is the maximum effective operating
voltage of the circuit to be protected?
Choice of the rating voltage
Voper. = ?
VN  Voper.max
Basic Fuse Selection

5. IBC > Ifault max What maximum fault currents can occur?
Choice of the
Breaking Capacity
Ifault1,Tfault,max1=?
Ifault2, Tfault,max2=?...
Ifault max = MAX (Ifault1,Ifault2..)
Circuit
Power source
IBC > Ifault max
If the maximum fault current (short-circuit
current) exceeds the rated breaking capacity
of the circuit protection component, a safe
blowing is no longer guaranteed (explosion, fire).
Basic Fuse Selection

6. VN  Voper.max IBC > Ifault max Fuse Type Approval FI
What kind of fuse can we use?
Choice of the fuse type
VN  Voper.max
IBC > Ifault max
Fuse Type
Approval FI
Type of fuse (SM/TH)
Basic Fuse Selection

7. I rms max =  [Ieff1 + Ieff2 + Ieffn]
What is the maximum effective operating current (rms)
flowing through the overcurrent protection component?
Calculation of the
operating current
Irms max = ?
3,5
Appendix A 3
I rms max =  [Ieff1 + Ieff2 + Ieffn]
2,5 2
I (A)
1,5 1
0,5 2 4 6 8
t (sec)
Mains voltage fluctuations should be considered when determing the maximum effective operating current (rms).
Basic Fuse Selection

8. FI = 1/fI (FI see data sheet)
Consideration of the current definition factor
Consideration of the current definition
FI (rated current definition factor)
Typical mean time-current characteristics
t (s)
Irms < FI x IN, 23°C
FI = 1/fI (FI see data sheet)
1A UL
1.000
1A IEC
100
IN (1) = Irms max x FI 10 1
Example:
FI = 1/ 0.75 = 1.33 for Type 198 (UL fuse)
0,1
If the maximum, permissible continuous
current of the overcurrent protection component
is exceeded, a premature failure of the component can be expected.
0,01
0,001 10
100
I (Amps)
Basic Fuse Selection

9. Tambient = ? Consideration of the temperature factor FT =
Consider temperature
F = quick-acting T = time-lag
I rated (derating %)
What is the maximum ambient temperature in your equipment, close to the overcurrent protection component? FT 20
Appendix D 10
0,91 F
-10
1,10
Tambient = ?
23°C T
-20
1,25
-30
FT =
-20 20 40 60 80
Ambient temperature (Tamb)
If higher ambient temperatures are
not considered, a premature blowing
of the overcurrent protection
component may occur.
Example:
FT = 50° for a time lag fuse
Basic Fuse Selection

10. IN (1) = Irms max x FI x FT IN  Irms max x FI x FT
Calculate the rated current
IN (1) = Irms max x FI x FT
Choice the rated current...
IN  Irms max x FI x FT
...in kind of using the next higher current value from the data sheet
Basic Fuse Selection

11. IN  IP /40 What inrush currents will stress the overcurrent
protection component?
Consideration of the inrush current
0,005
0,01
0,015
0,02
0,025 2 4 6 8 10 12
t (sec)
Ioper (Amps)
ÎP =
IN  IP /40
This check formula considers the metallurgical effect of the pulse peak value on the melting
element. Contact WICKMANN if your inrush peak
current exceeds 40 x IN.
Interim summary: At this point, it is often not yet determined if a fast-acting or time-lag component is required.
Basic Fuse Selection

12. Are there any inrush current pulses that will stress the over-current protectioncomponent?
Consideration of the inrush pulses
Example: Inrush pulse caused by a capacitor
installed in an input circuit
I²tmelt ³ I²tPulse
Correlation between the energy
content of the pulse and the required energy to melt the metallic element
Ioper (Amps) 12 R
IP = 10
I²t V C
= ?
Pulse 8 6 4 tP 2
If inrush pulses are not considered, a premature blowing may occur.
Appendix B
0,005
0,01
0,015
0,02
0,025
t (sec)
Example: I²tPulse=1/2 x IP² x tP=0.4A²s
Basic Fuse Selection

13. I²tmelt > I²tPulse X FP
Pulse factor: Fp
Consideration of the pulse factor
I²tPulse / I²tmelt Fp
I²tmelt > I²tPulse X FP
When selecting the minimum I²tmelt
of the overcurrent protection com-
ponent, it should be determined
whether to choose a fast-acting
or time-lag characteristic. 1
0,9
Appendix C
0,8
0,7
fast-acting
0,6
0,5
time-lag 2
0,4
0,3
0,2 5
0,1 10 1 10
100
1.000
10.000
Quantity of Pulses (nP)
For a quantity of nP = 100,000 inrush pulses, the factor FP for a time-lag fuse is 5.2
Basic Fuse Selection

14. Summary Basic Fuse Selection
VN  Voper. max.
IN  Irms max. x FT x FI
IN  IP /40
IBC > Ifault max.
I2t melt  I2tPulse x FP
Choose the next higher available rated current determined from
the IN nf1 and IN calculations. The corresponding fuse
melting integral should exceed the I2tmelt value determined above.
The performance under fault conditions must be tested.
Basic Fuse Selection

15. Test it in your device!! Final note
Test your selected fuse in your circuit to verify that it reliably works under fault conditions.
Test it in your device!!
Basic Fuse Selection