Question: If a fuse is rated at 3 amps, at what value of current should it "blow?" Answer: Not at 3 amps. As this Littelfuse company advertisement from a 1954 issue of Radio & Television News magazine informs us, fuse ratings do not indicate the current value at which a fuse opens to protect a circuits, but rather the maximum current level at which it will NOT blow (open circuit). The same goes for circuit breakers. Various types of fuses are available to handle certain types of applications. For instance, a highly capacitive load will according to the well-known equation iC(t) = C * dv/dt could draw a very large amount of initial (inrush) current if there is not enough series resistance to limit it to prevent a standard fuse from blowing. The same goes for a heavy physical inertial load at startup (e.g., an air compressor motor). Highly inductive loads can cause extreme counter-emf currents to be impressed at shutdown. Both of those circuits need what is termed a slow-blow (aka slo-blo) fuse that allows a relatively high level of current to flow for a short period of time, and then settles down to the rated value of protection thereafter. At the opposite end of the spectrum are very sensitive circuits which need to be protected from overcurrents as rapidly as possible, so quick-blow (aka kwik-blo) fuses are used there. Fuses are also rated according to applied voltage, ambient temperature, vibration levels, and other operational parameters. All fuses are not the same.

Things Are Not As They Seem ... This is a perfect square. It is an optical illusion that the sides bend.

3 amps fuse will not blow at 3 amps.

Fuses are not rated by the current at which they blow. Fuses are rated by the maximum current they should carry indefinitely.

Each type of fuse blows according to the requirements of the equipment it was designed to protect.

Littelfuse has cooperated with NEC, Underwriters, Armed Forces MIL Specs Committees in establishing the characteristics of the various fuse types. Littelfuse holds more design patents on fuses than all other manufacturers combined.

3 AG "Slo-Blo"

3 AB

8 AG U/L

1 AG

4 AG Anti-Vibration

Littelfuse

Des Plaines, Illinois

Posted April 15, 2020