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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
Fuse Selection Checklist
The application guidelines and product data in this guide
are intended to provide technical information that will help
with application design. 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.
Many of the factors involved with fuse selection are listed
below. For additional assistance with choosing fuses
appropriate to you requirements, contact your Littelfuse
products reprentative.:
Selection Factors
1. Normal operating current
2. Application voltage (AC or DC)
3. Ambient temperature
4. Overload current and length of time in which the
fuse must open
5. Maximum available fault current
6. Pulses, Surge Currents, Inrush Currents, Start-up
Currents, and Circuit Transients
7. Physical size limitations, such as length, diameter,
or height
8. Agency Approvals required, such as UL, CSA, VDE,
METI, MITI or Military
9. Fuse features (mounting type/form factor, ease of
removal, axial leads, visual indication, etc.)
10. Fuseholder features, if applicable and associated
re-rating (clips, mounting block, panel mount, PC
board mount, R.F.I. shielded, etc.)
11. Application testing and verification prior to
production
Littelfuse is at your service to help solve your electrical
protection problems. When contacting Littelfuse sales
engineers, please have all the requirements of your
applications available. Requests for quotes or assistance
in designing or selecting special types of circuit protection
components for your particular applications are also
welcome. In the absence of special requirements,
Littelfuse reserves the right to make appropriate changes
in design, process, and manufacturing location without
prior notice.
1. NORMAL OPERATING CURRENT: The current rating
of a fuse is typically derated 25% for operation at 25ºC to
avoid nuisance blowing. For example, a fuse with a current
rating of 10A is not usually recommended for operation at
more than 7.5A in a 25ºC ambient. For additional details,
see RE-RATING in the previous section and AMBIENT
TEMPERATURE below.
2. APPLICATION VOLTAGE: The voltage rating of the
fuse must be equal to, or greater than, the available circuit
voltage. For exceptions, see VOLTAGE RATING.
3. AMBIENT TEMPERATURE: The current carrying
capacity tests of fuses are performed at 25ºC and will be
affected by changes in ambient temperature. The higher
the ambient temperature, the hotter the fuse will operate,
and the shorter its life. Conversely, operating at a lower
temperature will prolong fuse life. A fuse also runs hotter
as the normal operating current approaches or exceeds the
rating of the selected fuse. Practical experience indicates
fuses at room temperature should last indefinitely, if
operated at no more than 75% of catalog fuse rating.
Ambient temperature effects are in addition to the
normal re-rating, see example. Example: Given
a normal operating current of 2.25 amperes in
an application using a 229 series fuse at room
temperature, then:
Normal Operating Current
C a t a l o g F u s e R a t i n g = ——————————————————
0.75
- or -
2.25 Amperes
——————— = 3 Amp Fuse (at 25ºC)
0.75
This charts shows typical ambient temperature effects on
current carrying capacity of Littelfuse products. For specific
re-rating information, please consult the product data sheet
www.littelfuse.com or contact a Littelfuse representative.
Curve A: Thin-Film Fuses and 313 Series (.010 to .150A)
Curve B: FLAT-PAK
®
, TeleLink
®
, Nano
2®
, PICO
®
, Blade
Terminal and other leaded and catridge fuses
Curve C: Resettable PTC’s