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
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
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"
8 AG U/L
4 AG Anti-Vibration
Des Plaines, Illinois
Posted April 15, 2020