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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
.001 .002 .003 .004 .005 .006
Time (Seconds)
Current (Amperes)
Figure 1
2
4
6
8
10
Normal Operating Current
l t
Pulse
Energy
2
CHART 1
WAVESHAPES FORMULAS
t
tt
1
i
p
i
p
i
p
i
p
i
p
i
b
i
p
F
E
D
C
B
A
OR
t
t
t
t
i = k
I
2
t = i
p
2
t
i = i
p
-kt
I
2
t = (1/3)(i
p
2
+ i
p
i
b
+ i
b
2
) t
i = i
p
sin t
I
2
t = (1/2) i
p
2
t
I
2
t = (1/3) i
p
2
t
i = kt
2
OR i = i
p
(1-kt)
2
I
2
t = (1/5) i
p
2
t
i = i
p
e
–kt
)
I
2
t (1/2) i
p
2
t
1
CHART 2
PULSE CYCLE WITHSTAND CAPABILITY
100,000 Pulses Pulse I
2
t = 22% of Nominal Melting I
2
t
10,000 Pulses Pulse I
2
t = 29% of Nominal Melting I
2
t
1,000 Pulses Pulse I
2
t = 38% of Nominal Melting I
2
t
100 Pulses Pulse I
2
t = 48% of Nominal Melting I
2
t
100000
10000
1000
100
10
%1
00%
Number of Pulses
Pulse I
2
t / Average Melting I
2
t
Note: Adequate time (10 seconds) must exist between pulse events
to allow heat from the previous event to dissipate.
conditions specified by Underwriters Laboratories. The
primary objective of these UL test conditions is to specify
common test standards necessary for the continued
control of manufactured items intended for protection
against fire, etc. A copper dummy fuse is inserted in
the fuseholder by Underwriters Laboratories, and then
the current is increased until a certain temperature rise
occurs. The majority of the heat is produced by the contact
resistance of the fuseholder clips. This value of current
is considered to be the rated current of the fuseholder,
expressed as 100% of rating. Some of the more common,
everyday applications may differ from these UL test
conditions as follows: fully enclosed fuseholders, high
contact resistance,air movement, transient spikes, and
changes in connecting cable size (diameter and length).
Even small variations from the controlled test conditions
can greatly affect the ratings of the fuse-holder. For
this reason, it is recommended that fuseholders be
derated by 40% (operated at no more than 60% of the
nominal current rating established using the Underwriter
Laboratories test conditions, as previously stated).
11. TESTING: The factors presented here should be
considered in selecting a fuse for a given application. The
next step is to verify the selection by requesting samples
for testing in the actual circuit. Before evaluating the
samples, make sure the fuse is properly mounted with
good electrical connections, using adequately sized wires
or traces. The testing should include life tests under normal
conditions and overload tests Under fault conditions, to
ensure that the fuse will operate properly in the circuit.
FIGURE 1
Fuse Selection Checklist (cont.)