Basic Fuse Selection

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

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

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

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

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

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

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

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 = 1.1 @ 50° for a time lag fuse Basic Fuse Selection

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

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

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

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 10.000 100.000 1.000.000 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

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

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