Brochure

© 2017 Littelfuse, Inc.
Specifications are subject to change without notice.
Revised: 03/03/17
FUSE
Circuit Protection Products and Mounting Accessories
The purpose of this introductory section is to promote
a better understanding of both fuses and common
application details within circuit design.
The fuses to be considered are current sensitive devices
designed to serve as the intentional weak link in the
electrical circuit. Their function is to provide protection of
discrete components, or of complete circuits, by reliably
melting under current overload conditions. This section
will cover some important facts about fuses, selection
considerations and standards.
The application guidelines and product data in this guide
are intended to provide technical information that will
help with application design. The fuse parameters and
application concepts presented should be well understood
in order to properly select a fuse for a given application.
Since these are only a few of the contributing parameters,
application testing is strongly recommended and should be
used to verify performance in the circuit / application.
Littelfuse reserves the right to make changes in product
design, processes, manufacturing location and information
without notice. For current Littelfuse product infomation,
please visit our web site at www.littelfuse.com.
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AMBIENT TEMPERATURE: Refers to the temperature
of the air immediately surrounding the fuse and is not to
be confused with “room temperature.” The fuse ambient
temperature is appreciably higher in many cases, because
it is enclosed (as in a panel mount fuseholder) or mounted
near other heat producing components, such as resistors,
transformers, etc.
BREAKING CAPACITY: Also known as interrupting rating
or short circuit rating, this is the maximum approved
current which the fuse can safely break at rated voltage.
Please refer to the interrupting rating definition of this
section for additional information.
CURRENT RATING: The nominal amperage value of the
fuse. It is established by the manufacturer as a value of
current which the fuse can carry, based on a controlled set
of test conditions (See RE-RATING).
Catalog Fuse part numbers include series identification and
amperage ratings. Refer to the FUSE SELECTION GUIDE
section for guidance on making the proper choice.
RE-RATING: For 25ºC ambient temperatures, it is
recommended that fuses be operated at no more than
75% of the nominal current rating established using the
controlled test conditions. These test conditions are part of
UL/CSA/ANCE (Mexico) 248-14 “Fuses for Supplementary
Overcurrent Protection,” whose primary objective is
to specify common test standards necessary for the
continued control of manufactured items intended for
protection against fire, etc. Some common variations of
these standards include: fully enclosed fuseholders, high
contact resistances, air movement, transient spikes, and
changes in connecting cablesize (diameter and length).
Fuses are essentially temperature-sensitive devices. Even
small variations from the controlled test conditions can
greatly affect the predicted life of a fuse when it is loaded
to its nominal value, usually expressed as 100% of rating.
The circuit design engineer should clearly understand
that the purpose of these controlled test conditions is to
enable fuse manufacturers to maintain unified performance
standards for their products, and he must account for
the variable conditions of his application. To compensate
for these variables, the circuit design engineer who is
designing for trouble-free, long-life fuse protection in his
equipment generally loads his fuse not more than 75%
of the nominal rating listed by the manufacturer,keeping
in mind that overload and short circuit protection must be
adequately provided for.
The fuses under discussion are temperature-sensitive
devices whose ratings have been established in a 25ºC
ambient. The fuse temperature generated by the current
passing through the fuse increases or decreases with
ambient temperature change.
The ambient temperature chart in the FUSE SELECTION
GUIDE section illustrates the effect that ambient
temperature has on the nominal current rating of a fuse.
Most traditional Slo-Blo
®
Fuse designs use lower melting
temperature materials and are, therefore, more sensitive to
ambient temperature changes.
DIMENSIONS: Unless otherwise specified, dimensions
are in inches.
The fuses in this catalog range in size from the approx.
0402 chip size (.041”L×.020”W×.012”H) up to the 5
AG, also commonly known as a”MIDGET” fuse (13/32”
Dia.×11/2” Length). As new products were developed
throughout the years, fuse sizes evolved to fill the various
electrical circuit protection needs.
The first fuses were simple, open-wire devices, followed
in the 1890’s by Edison’s enclosure of thin wire in a lamp
base to make the first plug fuse. By 1904, Underwriters
Laboratories had established size and rating specifications
to meet safety standards. The renewable type fuses and
automotive fuses appeared in 1914, and in 1927 Littelfuse
started making very low amperage fuses for the budding
electronics industry.
The fuse sizes in following chart began with the early
“Automobile Glass” fuses, thus the term “AG”. The
numbers were applied chronologically as different
manufacturers started making a new size: “3AG,” for
example, was the third size placed on the market. Other
non-glass fuse sizes and constructions were determined
by functional requirements, but they still retained the
length or diameter dimensions of the glass fuses. Their
designation was modified to AB in place of AG, indicating
that the outer tube was constructed from Bakelite, fibre,
ceramic, or a similar material other than glass. The largest
size fuse shown in the chart is the 5AG, or “MIDGET,”
a name adopted from its use by the electrical industry
and the National Electrical Code range which normally
recognizes fuses of 9/16”×2” as the smallest standard
fuse in use.
Fuse Characteristics, Terms and Consideration Factors