Brochure

© 2017 Littelfuse, Inc.
Specifications are subject to change without notice.
Revised: 03/03/17
FUSE
Circuit Protection Products and Mounting Accessories
4. OVERLOAD CURRENT CONDITION: The current level
for which protection is required. Fault conditions may be
specified, either in terms of current or, in terms of both
current and maximum time the fault can be tolerated
before damage occurs. Time-current curves should be
consulted to try to match the fuse characteristic to the
circuit needs, while keeping in mind that the curves are
based on average data.
5. MAXIMUM FAULT CURRENT: The Interrupting Rating
of a fuse must meet or exceed the Maximum Fault Current
of the circuit.
6. PULSES: The general term “pulses” is used in this
context to describe the broad category of wave shapes
referred to as “surge currents”, “start-up currents”, “inrush
currents”, and “transients”. Electrical pulse conditions can
vary considerably from one application to another. Different
fuse constructions may not react the same to a given
pulse condition. Electrical pulses produce thermal cycling
and possible mechanical fatigue that could affect the life
of the fuse. Initial or start-up pulses are normal for some
applications and require the characteristic of a Slo-Blo
®
fuse. Slo-Blo
®
fuses incorporate a thermal delay design
to enable them to survive normal start-up pulses and still
provide protection against prolonged overloads. The start-
up pulse should be defined and then compared to the time-
current curve and I
2
t rating for the fuse. Application testing
is recommended to establish the ability of the fuse design
to withstand the pulse conditions.
Nominal melting I
2
t is a measure of the energy required
to melt the fusing element and is expressed as “Ampere
Squared Seconds” (A
2
Sec.). This nominal melting I
2
t,
and the energy it represents (within a time duration of
8 milliseconds [0.008 second] or less and 1 millisecond
[0.001 second]or less for thin film fuses), is a value that is
constant for each different fusing element. Because every
fuse type and rating, as well as its corresponding part
number, has a different fusing element, it is necessary to
determine the I
2
t for each. This I
2
t value is a parameter of
the fuse itself and is controlled by the element material
and the configuration of the fuse element. In addition
to selecting fuses on the basis of “Normal Operating
Currents”, “Re-rating”, and “Ambient Temperature” as
discussed earlier, it is also necessary to apply the I
2
t
design approach. This nominal melting I
2
t is not only a
constant value for each fuse element design, but it is also
independent of temperature and voltage. Most often, the
nominal melting I
2
t method of fuse selection is applied to
those applications in which the fuse must sustain large
current pulses of a short duration. These high-energy
currents are common in many applications and are critical
to the design analysis.
The following example should assist in providing a better
understanding of the application of I
2
t.
EXAMPLE: Select a 125V, very fast-acting PICO
®
II fuse that
is capable of withstanding 100,000 pulses of current (I) of
the pulse waveform shown in Figure 1.
The normal operating current is 0.75 ampere at an ambient
temperature of 25ºC.
Step 1 — Refer to Chart 1 and select the appropriate
pulsewaveform, which is waveform (E) in this example.
Place the applicable value for peak pulse current (i
p
) and
time (t) into the corresponding formula for waveshape (E),
and calculate the result, as shown:
1 1
I
2
t= — (i
P
) = I
2
t = — (i
P
)
2
t
5 5
1
—×8
2
×.004 = 0.0512 A
2
Sec.
5
This value is referred to as the “Pulse I
2
t ”.
Step 2 — Determine the required value of Nominal Melting
I
2
t by referring to Chart 2. A figure of 22% is shown in
Chart II for 100,000 occurrences of the Pulse I
2
t calculated
in Step 1. This Pulse I
2
t is converted to its required value of
Nominal Melting I
2
t as follows:
Nom. Melt I
2
t = Pulse I
2
t/.22
0.0512/.22 = 0.2327 A
2
Sec.
Step 3 — Examine the I
2
t rating data for the PICO
®
II, 125V,
very fast-acting fuse. The part number 251001, 1 ampere
design is rated at 0.256 A
2
Sec., which is the minimum
fuse rating that will accommodate the 0.2327 A
2
Sec.
value calculated in Step 2. This 1 ampere fuse will also
accommodate the specified 0.75 ampere normal operating
current, when a 25% derating factor is applied to the 1
ampere rating, as previously described.
7. PHYSICAL SIZE LIMITATIONS: Please refer to the
product dimensions presented in current Littelfuse product
data sheets for specific information.
8. AGENCY APPROVALS: For background information
about common standards, please consult the STANDARDS
section of this guide or visit our Design Support web site
www.littelfuse.com/design-support.html. For specific
agency approval information for each Littelfuse product,
please refer to the data sheets within this catalog and
information presented on www.littelfuse.com. As agency
approvals and standards may change, please rely on the
information presented on www.littelfuse.com as current
information.
9. FUSE FEATURES: Please consult the specific product
features presented within this catalog and on our web site
www.littelfuse.com. For additional information and support
contact your Littelfuse product representative.
10. FUSEHOLDER FEATURES AND RE-RATING: For
information about the range of Littelfuse fuseholders and
specific features and characteristics, please consult with
a Littelfuse products representative or visit our web site
www.littelfuse.com.
For 25ºC ambient temperatures, it is recommended that
fuseholders be operated at no more than 60% of the
nominal current rating established using the controlled test
Fuse Selection Checklist (continued)