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I would like to know why I can not match Littlefuse chart with general rule of picking fuse which is 125% of peak current of the system. (I know there is more about fuse such as voltage, temp, AWG.)

When I look at the Littlefuse Average Time Current Curves chart the fuse I need is around Nom Current.

Lets take an example: VNom= 12V DC Vmax = 13V DC INom= 2.5A Irush = 3A. Fuse should blow on any current over 3.25A (20AWG cable used).

As you can see in below image Red line is close 3A ir 3.5A. But the 125% *3A = 3.75A ~ 4A.

enter image description here

Littlefuse datasheet.

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    \$\begingroup\$ If there's any doubt between a rule-of-thumb and hard data in a datasheet, go with the datasheet. \$\endgroup\$
    – John D
    May 10 at 15:27
  • \$\begingroup\$ Where you got the info that 20AWG can handle 3A. It depends if we are talking about cables, air wires, buried wires/cables. What kind, what distance, working temperature? \$\endgroup\$ May 10 at 16:45
  • \$\begingroup\$ @Marko I am using 20AWG cable to connect fuse to load (Less than 1ft) \$\endgroup\$
    – Shahreza
    May 10 at 16:55
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It takes a constant amount of energy for different fuse ratings in this size and type to start the fusing process with energy ratio and time being complex tradeoffs.

This is why they give both the cold resistance, Rc ( this R increases with temperature ) and the \$I^2t\$ value for each fuse.

The product of these two variables is \$E=I^2R_ct\$ which the amount of energy to start the thermal runaway process about the current trip value is fairly constant.

Opinion

Just as Barkhausen criteria works to stabilize a sine oscillator with gain =1 , here you have the equilibrium of self cooling and heating to stabilize the critical temperature to prevent thermal runaway which rises in speed with gain above the stable energy level at some temperature rise.

The unit you are trying to protect from fire or safety can absorb heat energy but you decide on how much energy can be absorbed from over-current in series with the fuse to start the runaway fusing process.

Given the tolerances on the weakest link of variation in wire diameter , your 125% Rule of Thumb is to prevent false fuse blowing not permit fast flowing.

It is dependent on the ambient and the ratio of overcurrent value, so you will find a wide tolerance on these values.

 % of Ampere Rating
     Opening Time
100% >=4 hours, Min
135% <=1 hour,  Max
200% >= 3 sec. Min, to <= 20 sec. Max

Faster protection requires smaller sizes with bigger tolerances or semiconductor comparator operation with a fast relay or SSR or Disable/enable to a reliable electronic switch.

For fuses; there exists “time-delay, normal and fast responding types”

In some cases , designers will detect over current to trigger an SCR to blow a fuse to reduce the time. Others may consider a PTC which tends to operate above 85’C so it too has thermal reliability issues as well as MOV’s which degrade after each surge based on energy in Joules. These choices are determine system specs on reliability, and cost of failures.

These are tradeoffs from fuse reliability for false trips and false blows with equipment failure due to inadequate other protective methods and creating a hazard if the fuse is over-rated. But just to protect a wire, we have “ampacity” specs for melting, which is trivial compared to protecting magnetics and semi’s from thermal runaway.

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Lets take an example: VNom= 12V DC Vmax = 13V DC INom= 2.5A Irush = 3A. Fuse should blow on any current over 3.25A (20AWG cable used).

There's your problem. Must carry 2.5 A, must blow on 3.25 A. That ratio is far too tight for a fusible fuse, unless you make some restrictive assumptions. Normally, fuses are used to protect against very heavy overloads like a short circuit, where the expected fusing current is waaaay above the must carry current.

You need to factor in what you are protecting, which will give you a time to blow. If you're protecting semiconductors or a loudspeaker, then you need under a second. If you're protecting wiring, then a minute should be adequate. Note that a nominally 1 A fuse takes almost two seconds to blow at 3 A. Those curves go up asymptotically to infinite time.

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  • \$\begingroup\$ Thx that make more sense now \$\endgroup\$
    – Shahreza
    May 10 at 16:26

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