Guide to Fuse Selection
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Fuse Selection Guide
Purpose of Fuses
> Circuit protection is critical, and in many cases required, in electrical and electronic products. > Fuses are an inexpensive and effective way to protect your device from damage due to overcurrent conditions. > Fuses can prevent safety hazards to the end user such as fire and catastrophic failure of the product. > Fuses help design engineers comply with regulatory agencies such as UL and IEC.
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Fuse Selection Guide
Characteristics of Fuses
Schurter offers a wide variety of fuses to meet any application: > > > > > Package type (SMD, thru hole, cartridge) Current and voltage ratings (AC and DC power) Trip characteristics (quick-acting or time-lag) Breaking capacity ratings Approvals (UL, CSA, ENEC, CCC)
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Fuse Selection Guide
Size and Mounting
> > > >
Schurter offers 0402, 0603, 1206 SMD fuses Thru hole microfuses Cartridge fuses 5x20mm, 6.3x32mm and 10.3x38mm Cartridge fuses can be mounted in fuseholders, fuseblocks or fuseclips > We also offer pigtails for a low cost thru hole solution
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Fuse Selection Guide
Fuse Current Rating
> The rated current of the fuse is either designed according to IEC characteristic or UL characteristic. > A fuse, which is designed according to a IEC standard, can continuously operate at 100% of rated current of the fuse. > A fuse, which is designed according to a UL standard, can continuously operate at 75% of rated current of the fuse. > The fuse current rating should be based on the operating current in the application.
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Fuse Selection Guide
Breaking Capacity
> Breaking capacity is the maximum short circuit current a fuse can safely blow without a catastrophic failure such as a fire, breakage or explosion. > Low and high breaking capacity ratings typically range from 35A up to 10kA. > The short circuit condition in the final product determines what fuse breaking capacity is needed. > Our UMT is a compact SMD fuse with a high breaking capacity of 200A.
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Fuse Selection Guide
Trip Characteristic
> Fuses are either quick-acting or time-lag. > Time-lag fuses trip at a slower rate at high currents.
Quick-acting Time-lag
Load type
Resistive
Capacitive
Inrush Current Withstand
Low
High (10 times rated current) Power supplies Motors Circuits with capacitors Avoid nuisance tripping during inrush
Applications
Data/signal lines Electronic components Avoid damage downstream due to inrush
Advantage
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Fuse Selection Guide
Temperature Derating
> Fuse current ratings are measured at 23degC. > Fuses are temperature dependant so higher the ambient temperature the quicker the fuse will blow. > Ambient temperature of the application must be considered when choosing the current rating of the fuse.
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Fuse Selection Guide
Temperature Derating
Example UMT 250 SMD Fuse
Application example:
Fuse type: Operating current: Operating voltage: Ambient temperature: UMT 250 1.0 A @ 60 C 230 VAC < 60 C
Derating-curve UMT 250 (see data sheet)
Calculation of rated current of the fuse with the derating curve:
IN IOperating 1.0A 1.22 A DeratingFactor 0.82
Choice:
UMT 250, 1.25 A
(1 A @ 60 C)
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Fuse Selection Guide
Heat Issues
> Heat dissipated from fuses can affect other components in close proximity and vice versa. > Sufficient airflow and ventilation should be considered when designing fuses in the application. > Schurter fuseholder and fused module datasheets have power acceptance ratings which show how much heat dissipation it can withstand safely. > If a fuse dissipates more heat than the fuseholder can withstand, the fuseholder can degrade such as melt or burn.
Fused module
Fuse and fuseholder
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Fuse Selection Guide
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Power(heat)Dissipation
> Fuses dissipate heat during normal operation and this can
>
increase as ambient temperature increases. Time-lag fuses generally have lower power dissipation values than quick-acting fuses because they have a thicker fuse wire diameter. Heres our FST spec sheet where we publish the typical Power Dissipation value. When choosing a fuseholder or fused module, the power acceptance value should exceed the fuse power dissipation value.
> >
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Fuse Selection Guide
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Inrush Current
> Many applications will have inrush or peak currents at start-up and sometimes during normal operation. > The inrush current in the application should be measured and used to calculate the proper fuse I2t value. > I2t is the amount of heat energy, in terms of current and time, required to melt the fuse link
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Fuse Selection Guide
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Waveforms
Inrush Current Peak
Procedure > Step 1: Selection of the appropriate waveform of the inrush current.
Most used curve
Wave shapes Formulas Wave shapes Formulas
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Fuse Selection Guide
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I2t Calculation
Inrush Current Peak
> Step 2: Calculation of the I2t-value of the application
Application example:
Inrush current peak: Ip = 13 A, = 0.006 s Type of waveform: Typical discharge curve
Calculation of the I2t-value
1 2 1 I2 t Applicatio I * (13 A)2 * 0.006s 0.507A2s n p 2 2
After 5, the inrush current has reached operating current.
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Pulse Factor Derating
Inrush Current Peak
> Continuous exposure to pulses of high current could prematurely age the fuse. > The number of pulses the fuse would be exposed to in the application should also be considered when choosing a fuse.
Tin plating of new fuse wire Tin plating of aged fuse wire
Wire
Tin plating
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Fuse Selection Guide
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Pulse Factor Derating
Inrush Current Peak
> Step 3: Determine the minimum value of the I2t-value of the fuse.
Application example: Total number of pulses in life cycle: 10,000 UMT 250 = time-lag fuses
Calculation of time-lag T fuses
Pulse-Derating curve
I2 t Applicatio 0.507A2s n I t Fuse_T _min 1.748A2s F 0.29
2
(Calculation of quick-acting F fuses)
I2 t Applicatio 0.507A2s n I t Fuse_F_min 1.035A2s F 0.49
2
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Fuse Selection Guide
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Selecting a Fuse Part Number
> Step 4: Selection of the correct fuse rating and part number from Schurters product line. > Typical I2t-values at 10*In for Schurter 1 A time-lag T fuses > MST 250 (12 A2s), UMT 250 (2.8 A2s), FST 5x20 (3.3 A2s), SPT 5x20 (1.1 A2s)
> (Typical I2t-values at 10*In for Schurter 1 A quick-acting F fuses) > OMF 250 (0.23 A2s), MSF 250 (0.33 A2s), FSF 5x20 (1.13 A2s), SP 5x20 (0.75 A2s)
Choice:
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UMT 250, 1 A
(2.8 A2s > 1.748 A2s)
Fuse Selection Guide
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Fuse Selection
> Normal Operating Mode
Choice:
UMT 250, 1.25 A
(1 A @ 60 C)
> Inrush Current Peak
Choice:
UMT 250, 1 A
(2.8 A2s > 1.748 A2s)
> The higher value determines the selection of the rated current of the fuse. > The normal operating mode current exceeds that of the inrush current peak. Therefore, the 1.25 A fuse is the recommended fuse rating.
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Fuse Selection Guide
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Website
Selection, Datasheets, Approval Documents, CAD Drawings
Website: www.schurterinc.com
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Fuse Selection Guide
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Guide to Fuse Selection
Thank you
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