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© 2017 Littelfuse, Inc.
Specifications are subject to change without notice.
Revised: 03/03/17
FUSE
Circuit Protection Products and Mounting Accessories
Lead-Free Soldering Parameters (most instances):
Wave Solder — 260ºC, 10 seconds max
Reflow Solder — 260ºC, 30 seconds max
TEST SAMPLING PLAN: Because compliance with certain
specifications requires destructive testing, these tests are
selected on a statistical basis for each lot manufactured.
TIME-CURRENT CURVE: The graphical presentation of
the fusing characteristic, time-current curves are generally
average curves which are presented as a design aid but
are not generally considered part of the fuse specification.
Time-current curves are extremely useful in defining
a fuse, since fuses with the same current rating can
be represented by considerably different time-current
curves. The fuse specification typically will include a life
requirement at 100% of rating and maximum opening
times at overload points (usually 135% and 200% of rating
depending on fuse standard characteristics). A time-current
curve represents average data for the design; how ever,
there may be some differences in the values for any one
given production lot. Samples should be tested to verify
performance, once the fuse has been selected.
UNDERWRITERS LABORATORIES: Reference to “Listed
by Underwriters Laboratories” signifies that the fuses
meet the requirements of UL/CSA/ANCE 248-14 “Fuses
for Supplementary Overcurrent Protection”. Some 32
volt fuses (automotive) in this catalog are listed under
UL Standard 275. Reference to “Recognized under the
Component Program of Underwriters Laboratories”
signifies that the item is recognized under the component
program of Underwriters Laboratories and application
approval is required.
VOLTAGE RATING: The voltage rating, as marked on a
fuse, indicates that the fuse can be relied upon to safely
interrupt its rated short circuit current in a circuit where the
voltage is equal to, or less than, its rated voltage.
This system of voltage rating is covered by N.E.C.
regulations and is a requirement of Underwriters
Laboratories as a protection against fire risk. The standard
voltage ratings used by fuse manufacturers for most small-
dimension and midget fuses are 32, 63, 125, 250 and 600.
In electronic equipment with relatively low output power
supplies, with circuit impedance limiting short circuit
currents to values of less than ten times the current rating
of the fuse, it is common practice to specify fuses with
125 or 250 volt ratings for secondary circuit protection of
500 volts or higher.
As mentioned previously (See RE-RATING), fuses are
sensitive to changes in current, not voltage, maintaining
their “status quo” at any voltage up to the maximum rating
of the fuse. It is not until the fuse element melts and
arcing occurs that the circuit voltage and available power
become an issue. The safe interruption of the circuit, as it
relates to circuit voltage and available power, is discussed
in the section on INTERRUPTING RATING.
To summarize, a fuse may be used at any voltage that is
less than its voltage rating without detriment to its fusing
characteristics. Please contact the factory for applications
at voltages greater than the voltage rating.
DERIVATION OF NOMINAL MELTING I
2
t: Laboratory
tests are conducted on each fuse design to determine the
amount of energy required to melt the fusing element. This
energy is described as nominal melting I
2
t and is expressed
as “Ampere Squared Seconds” (A
2
sec.).
A pulse of current is applied to the fuse, and a time
measurement is taken for melting to occur. If melting
does not occur within a short duration of about 8
milliseconds (0.008 seconds) or less, the level of pulse
current is increased. This test procedure is repeated until
melting of the fuse element is confined to within about 8
milliseconds.
The purpose of this procedure is to assure that the heat
created has insufficient time to thermally conduct away
from the fuse element. That is, all of the heat energy (I
2
t)
is used, to cause melting. Once the measurements of
current (I) and time (t) are determined, it is a simple matter
to calculate melting I
2
t. When the melting phase reaches
completion, an electrical arc occurs immediately prior to
the “opening” of the fuse element.
Clearing I
2
t = Melting I
2
t + arcing I
2
t
The nominal I
2
t values given in this publication pertain to
the melting phase portion of the “clearing” or “opening”.
Alternatively the time can be measured at 10 times of the
rated current and the I
2
t value is calculated like above.
Fuse Characteristics, Terms and Consideration Factors (continued)