How can help the i2t vs t curve in fuse selection?

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  • \$\begingroup\$ Show link and guide to ask which part you do not understand \$\endgroup\$ Jul 13 at 20:49
  • \$\begingroup\$ littelfuse.com/~/media/automotive/catalogs/… \$\endgroup\$ Jul 13 at 20:50
  • 1
    \$\begingroup\$ You cannot use it for anything before you know what just that curve set tries to present. If we look 1A curve at 1 second we get I2t value = 6 squareampereseconds. If I calculate one ampere squared multiplied by 1 second I will get one squareampere-second. Your curve set presents something which cannot be reasoned by watching the curve set. If you have taken it from a document you should check what the document tells about it. \$\endgroup\$
    – user287001
    Jul 13 at 21:22

All fuses have inverse time-current characteristics, so opening time decreases as overcurrents increase. Time-current characteristics are presented graphically on standardized “log-log” paper.\$^1\$

Your graph shown is different as a Current Squared Time v time curve (I2t vs t) on log-log scale. This curve shows power vs time on the fixed slope with energy on the curved part.

Power is \$P=I^2R~ =IV\$ and Energy is \$E=I^2R * t\$

Both curves show how to select a fuse based on the surge pulse transient over-current. After a certain thermal time value, all fuses become power limited by physical size so without using resistance they share a similar slope I^2=I^2t/t.

Normally you choose a fuse about 25% above its maximum steady current but if a load's surge current is much more, then, the short term I^2t pulse energy-related point must be less than this average fusing curve.

Keep in mind that the history of fuse temperature and ambient can determine fusing minimum time, so always plots show the fusing time at 25'C and not the max allowed operating current. Be sure to check if it refers to nominal or average and use tables for max time.

For more basic theory read here.

Plot interpretation

Any surge of current above this line at 1 ms or less on the left indicates it takes about 1ms to blow a fuse and anything above this flat part won't make much difference.

  • For a 1A fuse in 2ms I2t = 0.03 thus I = 30 A.
  • For a 1A after 1s, I2t = 5.3 so I = 2.3A the fuse is blown on average

So I expect these are not slow blow types ;) .

\$^1\$ https://www.littelfuse.com/~/media/automotive/catalogs/littelfuse_fuseology.pdf


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