Selection Guide

Fuse Characteristics, Terms

and Consideration Factors

FUSEOLOGY          

Selection  Guide          

Fuse  Characteristics,  

Terms and Consideration 

Factors

Table of Contents

Page

Fuse Characteristics, Terms and Consideration Factors

2–4

Fuse Selection Checklist

5–7

PTC Characteristics and Terms

8–9

PTC Product Applications

10

Typical PTC Circuit Protection Designs

11

Standards

12–14

PTC Selection Worksheet

15

Fuse and PTC Products Selection Guide

16–18

Packaging and Part Numbering

19

Legal Disclaimers

20

About this guide

Fuses are current-sensitive devices that provide reliable protection for discrete components 
or circuits by melting under current overload conditions. Choosing the right fuse for your 
application can be an overwhelming, time-consuming process, even for a seasoned 
electronics design engineer. This user-friendly Fuseology Selection Guide makes the fuse 
selection process quick and easy-helping you optimize the reliability and performance of     
the application.

Specifications descriptions and illustrative material in this literature are as accurate as known at the time of publication,  

but are subject to changes without notice. Visit 

littelfuse.com

 for more information.

© 2014 Littelfuse, Inc.

Fuseology Selection Guide

© 

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

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

.

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 RERATING section).

Catalog Fuse part numbers include series identification 
and amperage ratings. Refer to the FUSE SELECTION 
CHECKLIST section for guidance on making the proper 
choice.

RERATING:

 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 
CHECKLIST 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 

Fuse Characteristics, terms and Consideration Factors

2

© 2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuseology Selection Guide

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 SIZES

SIZE

DIAMETER (Inches)

LENGTH (Inches)

1AG

1/4

.250

5/8

.625

2AG

–

.177

–

.588

3AG

1/4

.250

1¼

1.25

4AG

9/32

.281

1¼

1.25

5AG

13/32

.406

1½

1.50

7AG

1/4

.250

7⁄8

.875

8AG

1/4

.250

1

1

TOLERANCES:

 The dimensions shown in this catalog 

are nominal. Unless otherwise specified, tolerances are 
applied as follows. Tolerances do not apply to lead lengths:

 

± .010” for dimensions to 2 decimal places.

 

± .005” for dimensions to 3 decimal places.

Contact Littelfuse should you have questions regarding 
metric system and fractional tolerances.

FUSE CHARACTERISTICS:

 This characteristic of a fuse 

design refers to how rapidly it responds to various current 
overloads. Fuse characteristics can be classified into three 
general categories: very fast-acting, fast-acting, or Slo-Blo

®

 

Fuse. The distinguishing feature of Slo-Blo

®

 fuses is that 

these fuses have additional thermal inertia designed to 
tolerate normal initial or start-up overload pulses.

FUSE CONSTRUCTION:

 Internal construction may vary 

depending on ampere rating. Fuse photos in this catalog 
show typical construction of a particular ampere rating 
within the fuse series.

FUSEHOLDERS:

 In many applications, fuses are 

installed in fuseholders. These fuses and their associated 
fuseholders are not intended for operation as a “switch” 
for turning power “on” and “off “.

INTERRUPTING RATING:

 Also known as breaking 

capacity or short circuit rating, the interrupting rating is 
the maximum approved current which the fuse can safely 
interrupt at rated voltage. During a fault or short circuit 
condition, a fuse may receive an instantaneous overload 
current many times greater than its normal operating 
current. Safe operation requires that the fuse remain intact 
(no explosion or body rupture) and clear the circuit.

Interrupting ratings may vary with fuse design and range 
from 35 amperes for some 250VAC metric size (5×20mm) 

fuses up to 200,000 amperes for the 600VAC KLK series. 
Information on other fuse series can be obtained from the 
Littelfuse.

Fuses listed in accordance with UL/CSA/ANCE 248 are 
required to have an interrupting rating of 10,000 amperes 
at 125V, with some exceptions (See STANDARDS section) 
which, in many applications, provides a safety factor far in 
excess of the short circuit currents available.

NUISANCE OPENING:

 Nuisance opening is most often 

caused by an incomplete analysis of the circuit under 
consideration. 

Of all the “Selection Factors” listed in the FUSE 
SELECTION CHECKLIST, special attention must be given 
to items 1, 3, and 6, namely, normal operating current, 
ambient temperature, and pulses. 

For example, one prevalent cause of nuisance opening in 
conventional power supplies is the failure to adequately 
consider the fuse’s nominal melting I

2

t rating. The fuse 

cannot be selected solely on the basis of normal operating 
current and ambient temperature. In this application, the 
fuse’s nominal melting I

2

t rating must also meet the inrush 

current requirements created by the input capacitor of the 
power supply’s smoothing filter. 

The procedure for converting various waveforms into 
I

2

t circuit demand is given in the FUSE SELECTION 

CHECKLIST. For trouble-free, long-life fuse protection, it is 
good design practice to select a fuse such that the I

2

t of 

the waveform is no more than 20% of the nominal melting 
I

2

t rating of the fuse. Refer to the section on PULSES in the 

FUSE SELECTION CHECKLIST.

RESISTANCE:

 The resistance of a fuse is usually an 

insignificant part of the total circuit resistance. Since the 
resistance of fractional amperage fuses can be several 
ohms, this fact should be considered when using them 
in low-voltage circuits. Actual values can be obtained by 
contacting Littelfuse. 

Most fuses are manufactured from materials which have 
positive temperature coefficients, and, therefore, it is 
common to refer to cold resistance and hot resistance 
(voltage drop at rated current), with actual operation being 
somewhere in between. 

Cold resistance is the resistance obtained using a 
measuring current of no more than 10% of the fuse’s 
nominal rated current. Values shown in this publication for 
cold resistance are nominal and representative. The factory 
should be consulted if this parameter is critical to the 
design analysis. 
Hot resistance is the resistance calculated from the 

Fuse Characteristics, terms and Consideration Factors (continued)

3

Fuseology Selection Guide

© 

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

stabilized voltage drop across the fuse, with current equal 
to the nominal rated current flowing through it. Resistance 
data on all Littelfuse products are available on request. 
Fuses can be supplied to specified controlled resistance 
tolerances at additional cost.

SOLDERING RECOMMENDATIONS:

 Since most fuse 

constructions incorporate soldered connections, caution 
should be used when installing those fuses intended to 
be soldered in place. The application of excessive heat can 
reflow the solder within the fuse and change its rating. 
Fuses are heat-sensitive components similar to semi-
conductors, and the use of heat sinks during soldering is 
often recommended.

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 RERATING section), 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)

4

© 2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuseology Selection Guide

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 

rerating (clips, mounting block, panel mount, PC 
board mount, R.F.I. shielded, etc.)

11. Application testing and verification prior to 

production  

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 RERATING 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 1.5 amperes in an application using 
a traditional Slo-Blo

®

 fuse at room temperature, then:

  

 

Normal Operating Current

 

Catalog Fuse Rating = ——————————————————

  

0.75

 

 

 

          - or -

 

 1.5 Amperes

 

———————  = 2.0 Amp Fuse (at 25ºC)

 

        0.75

Similarly, if that same fuse were operated at a very high 

ambient temperature of 70°C, additional derating would 

be necessary.  Curve "A" (Traditional Slo-Blo

®

 Fuse) of 

that ambient temperature chart shows the maximum 

operating "Percent of Rating" at 70°C to be 80%, in 

which case;

 

                                      

  

 

Normal Operating Current

 

Catalog Fuse Rating = ——————————————————

 

 

0.75 x Percent of Rating

 

 

 

          - or -

 

 1.5 Amperes

 

———————  = 2.5 Amp Fuse (at 70ºC)

 

   0.75 x 0.80 

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 
at 

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

5

Fuseology Selection Guide

© 

2015 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

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”, “Rerating”, 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                    

  I

2

t = —  (i

P

) 

2

t

           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 
at 

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 at 

www.littelfuse.com

. For additional information and 

support contact your Littelfuse product representative.

Fuse Selection Checklist (continued)

6

© 2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuseology Selection Guide

.001

.002

.003

.004 .005

.006

Time (Seconds)

Cur

rent (Amperes)

Figure 1

2

4

6

8

10

Normal Operating Current

l  t

Pulse

Energy

2

CHART 1

WAVESHAPES

FORMULAS

t

t

t

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%

100%

Number of P

ulses

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.

10. FUSEHOLDER FEATURES AND RERATING: 

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 

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 (continued)

7

Fuseology Selection Guide

© 

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Overcurrent circuit protection can be accomplished 
with the use of either a traditional fuse or PTC (positive 
temperature coefficient) device. 

PTCs are typically used in a wide variety of telecom, 
computer, consumer electronics, battery and medical 
electronics product applications where overcurrent events 
are common and automatic resettability desired. 

Littelfuse offers PTCs with the following general forms and 
features, and come in a variety of sizes and capacities: 

Surface Mount Devices:

 

 

•  A full range of compact footprints

 

•  Low hold current

 

•  Very fast trip time

 

•  Low resistance

Radial Leaded Series: 

 

•  Protection devices up to 600Vdc

 

•  A very high hold current

 

•  Low trip-to-hold current ratio

 

•  Low resistance.  

Battery Strap Devices:

 

 

•  A narrow low profile design

 

•  A weldable band Nickel terminal

 

•  Low resistance–for extended battery run time

If your application requirements fall outside of our product 
range, in certain instances we can offer customized 
solutions. Please contact Littelfuse for more information.

Traditional Fuses Vs. PTCs

Fuses and PTCs are both overcurrent protection 
devices, though each offer their own unique operating 
characteristics and benefits. Understanding the differences 
between the two technologies should make the choice in 
selection easier, depending on the application. 

The most obvious difference is that PTCs are automatically 
resettable whereas traditional Fuses need to be replaced 
after they they are tripped. Whereas a fuse will completely 
stop the flow of current (which may be desired in critical 
applications) after most similar overcurrent event, PTCs 
continue to enable the equiment to function, except in 
extreme cases.

Because they reset automatically, many circuit designers 
choose PTCs in instances where overcurrent events are 
expected to occur often, and where maintaining low 
warranty and service costs, constant system uptime, and/
or user transparency are at a premium. They are also often 
chosen in circuits that are difficult to access or in remote 
locations, were fuse replacement would be difficult.

There are several other operating characteristics to be 
considered that distinguish PTCs and fuses, and it is also 
best to test and verify device performance before use 
within the end application.

Littelfuse PTC Characteristics

Both Polymeric (Positive Temperature Coefficient) PTC 
and traditional Fuse devices react to heat generated by 
the excessive current flow in a circuit. A fuse melts open, 
interrupting the current flow whereas a PTC limits current 
flow as it rises in temperature, changing from low to high 
resistance state. In both 
cases this condition is 
called "tripping." The 
graph at right shows the 
typical response of a 
PTC to temperature. 

Littelfuse Polymer PTCs 
are made chiefly of high 
density polyethylene 
mixed with graphite. 
During an overcurrent 
event, a Polymer PTC 
will heat and expand, which in turn causes the conducting 
particles to break contact and stop the current. 

The general procedure for resetting the device after an 
overload has occurred is to remove power and allow the 
device to cool down. 

Log resistance (ohms)

Temperature (C)

Trip Point

PtC Characteristics and terms

To view a short online tutorial on using the Littelfuse 
iDesign tool, visit: http://vimeo.com/90118164

Littelfuse iDesign

TM

 Online Fuse Design 

and Selection Tool

8

© 2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuseology Selection Guide

Leakage Current:

  When a PTC is in a "tripped state" it 

protects the circuitry by limiting the current flow to a low 
leakage level. Leakage current can range from less than 
a hundred milliamps (mA) at rated voltage up to a few 
hundred milliamps (mA) at lower voltages. Fuses on the 
other hand completely interrupt the current flow when 
tripped, and this open circuit results in no leakage current 
when subjected to an overload current.

Interrupting Rating:

 PTCs are rated for a maximum 

short circuit current at rated voltage also known as 
"breaking capacity" or Imax. This fault current level is the 
maximum current that the device can withstand safely, 
keeping in mind that the PTC will not actually interrupt 
the current flow (see Leakage Current above). A typical 
Littelfuse PTC short circuit rating is 40A; or for the battery 
strap PTCs, this value can reach 100A. Fuses do in fact 
interrupt the current flow in response to the overload 
and the range of interrupting ratings, vary from tens of 
amperes (A) up to 10,000 (A) amperes at rated 
voltage.

Operating Voltage  Rating:

 

General use 

Littelfuse PTCs are not rated above 60V while 
fuses are rated up to 600V.

Hold Current Rating:

 

The hold (operating) 

current rating for PTCs can be up to 14A, while 
the maximum level for fuses can exceed 30A.

Resistance

:

 Reviewing product 

specifications indicates that similarly rated 
PTCs have about twice (sometimes more) the 
resistance of fuses.

Agency Approvals:

 

Littelfuse PTCs 

are Recognized under the Component Program of 
Underwriters Laboratories to UL Standard 1434 for 
Thermistors. The devices have also been certified under 
the CSA Component Acceptance Program. 

Time-Current Characteristic

:

 Comparing the time-

current curves of PTCs to time-current curves of fuses 
show that the speed of response for a PTC is similar to the 
time delay of a Littelfuse Slo-Blo® fuse.

Temperature Rerating:

 

The useful upper limit for a 

PTC is generally 85ºC, while the maximum operating 
temperature for fuses is 125ºC.

Ambient temperature effects are in addition to the normal 
rerating. PTCs hold and trip rating must be rerated when 
applied at conditions other than room ambient. For 
example, any rise in ambient temperature will decrease the 
hold current rating as well as the trip current. A reduction in 
ambient temperature will increase the trip current as well 
as the hold current. 

The temperature rerating curves in the table below 
compare PTCs to fuses and illustrate that more rerating is 
required for a PTC at a given temperature.

 Temperature Rerating Curves Comparing PTCs to Fuses

Chart Key

Curve A

  

Thin-Film Fuses and 313 

Series (.010 to .150A)

Curve B

  

FLAT-PAK

®

, Nano

2®

, 

PICO

®

, Blade Terminal, 

Special Purpose and 

other leaded and 

cartridge fuses 

(except 313.010 – .150A)

Curve C

  

Resettable PTCs

-60°C

-76°F

20

40

60

80

100

120

140

-40°C

-40°F

-20°C

-4°F

25°C

C

C

A
B

AMBIENT TEMPERATURE

PERCENT OF RA

TING

A

B

0°C

32°F

20°C

68°F

40°C

104°F

60°C

140°F

80°C

176°F

100°C

212°F

120°C

248°F

PtC Characteristics and terms (continued)

9

Fuseology Selection Guide

© 

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Surface Mount

raDIaL LeaDeD

Battery Strap

Series name

LoRho 0603L 0805L 1206L 1210L 1812L 2016L 2920L 250s usbR

16R

30R

60R

72R

250R 600R

LR

LR

Lt

st

VL

Vt

Telecom

application

ul60950 ,tiA-968-A, gR-1089 Req’s

X

X

X

itu-t Recoomendations

X

X

X

CPe (Customer Premises equipment)

X

X

X

Analog Line Card

X

X

X

t1/e1/J1 And hdsL

  

X

X

X

isdN 

X

X

X

AdsL

X

X

X

Cable telephony

X

X

X

PbX/Kts And Key telephone system

X

X

X

Comput

er

CPu

X

X

X

usb

X

X

X

X

X

X

X

X

ieee1284 Parallel data bus

X

X

X

X

X

ieee 802.3

X

X

X

X

ieee 1394

X

X

X

i/o Ports

X

X

X

X

X

X

X

PC Card

X

X

X

X

X

X

X

X

X

sCsi

X

X

X

X

X

X

Video Port

X

X

X

X

X

X

LCd Monitor

X

X

X

X

X

X

X

X

Consumer

  

Electr

onics

set top box

X

X

X

X

Loudspeaker

X

smart Card Reader

X

Mobile Phone

X

X

X

X

X

Linear AC/dC Adapter

X

X

X

X

X

X

X

X

X

X

Portable electronic input Port

X

X

X

X

X

X

X

electromagnetic Loads, Motor

X

X

X

X

X

solenoid Protection

X

X

X

X

X

Bat

tery

Lithium Cell

X

X

X

X

X

X

battery Pack

X

X

X

X

X

X

Medical

 

electr

onic

Voltage / Current input terminal

X

X

Note:  the application summary is for reference only. determination of suitability for a specific application is the responsibility of the customer.

PTCs are typically used as circuit protection in applications 
where sensitive components are at constant risk of 
damage from overcurrent conditions. The ability of PTCs to 
reset themselves after exposure to a fault current makes 
them ideal within circuits that are not easily accessible to a 
user or technician or where constant uptime is required. 

Typical applications include port protection on personal 
computers (USB, Firewire, keyboard/mouse, and serial 
ports), peripherals (hard drives, video cards, and hubs), cell 
phone, battery packs, industrial controls, lighting ballast 
and motor controls.

The chart below is meant as a quick guide in narrowing to 
a Littelfuse PTC device that may be appropriate to certain 
end applications.

For detailed application assistance please contact a 
Littelfuse product specialist or visit our new reference 
design center at 

www.littelfuse.com/designcenter

.

For detailed product specifications, please consult the 
Littelfuse PTC datasheets within this catalog or visit

www.littelfuse.com/PTCs.

PtC Product Applications 

Littelfuse Electronics eCatalogs includes the latest standards, products, diagrams and videos in a new interactive 
format. View the eBook on your mobile device, tablet or desktop for easy access to Littelfuse products, technologies 
and technical resources. Quickly order free product samples, request more information and download datasheets all 
within the new user-friendly design.

10

© 2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuseology Selection Guide

Outside 

Wo

rl

d

D+
D -

VBU

Signal

Ground

Shield

Ground

(2) V0402MHS03

1206L150

USB Port

USB Controller

V5.5MLA0603

PTC

Cell Phone

Battery -
Control

Battery +

Signal

Ground

Shield

Ground

Battery Connector

Battery Controller IC

(3) V5.5MLA0402
(Multilayer
Varistor)

PTC

Outside 

Wo

rl

d

D+
D -

VBUS

Signal

Ground

Shield

Ground

1206L150

USB Port

USB Controller

V5.5MLA063

(2) PGB1010603
(PulseGuard

®

ESD Suppressor)

PTC

Telco

Tip

Signal

Ground

RJ11 Connector

Tx/Rx Circuits

SMD/leaded
SIDACtor

®

Device

Ring

PTC

600R Series
250R Series

Outside W

orld

TPA+
TPA-

VBU

Signal

Ground

Shield

Ground

1812L110/33

1394 Port

1394 Controller

V33MLA1206

(4) PGB1010603
(PulseGuard

®

ESD Suppressor)

TPB+
TPB-

PTC

Data Pair

Data Pair

Data Pair

Data Pair

Power Source

Equipment

(PSE)

Powered 

Device

(PD)

Switch/Hub

Powered End Station

PTC

POWER OVER ETHERNET 

 

LI-ION BATTERY PACK

The following are examples of typical circuits using 
Littelfuse PTCs in combination with other Littelfuse circuit 
protection devices to provide a comprehensive protection 
solution. Contact a Littelfuse application expert for design 

assistance or visit 

www.littelfuse.com/designcenter

 

or 

www.littelfuse.com/PTCs

 for additional information. 

Be sure to to veryify specifications and test device 
performance before use in the end application.

USB 1.1 

 

USB 2.0

IEEE 1394 - FIREWIRE   

TIP/RING CIRCUIT - METALLIC

typical PtC Circuit Protection designs

11

Fuseology Selection Guide

© 

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

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.

Fuse ratings and other performance criteria are evaluated 
under laboratory conditions and accptance criteria, as 
defined in one or more of the various fuse standards. It is 
important to understand these standards so that the fuse 
can be properly applied to circuit protection applications.

UL/CSA/ANCE (Mexico) 248-14 FUSES FOR 

SUPPLEMENTARY OVERCURRENT PROTECTION (600 Volts, 

Maximum) (Previously UL 198G and CSA C22.2, No. 59)

 

UL LISTED

A UL Listed fuse meets all the requirements of the UL/
CSA/ANCE 248-14 Standard. Following are some of the 
requirements. UL ampere rating tests are conducted at 
100%, 135%, and 200% of rated current. The fuse must 
carry 100% of its ampere rating and must stabilize at a 
temperature that does not exceed a 75ºC rise.

The fuse must open at 135% of rated current within one 
hour. It also must open at 200% of rated current within 2 
minutes for 0-30 ampere ratings and 4 minutes for 35-60 
ampere ratings.

The interrupting rating of a UL Listed fuse is 10,000 
amperes AC minimum at 125 volts. Fuses rated at 250 
volts may be listed as interrupting 10,000 amperes at 125 
volts and, at least, the minimum values shown below at 
250 volts.

Ampere Rating of 

Fuse

Interrupting Rating 

In Amperes

Voltage Rating

0 to 1

35

250 VAC

1.1 to 3.5

100

250 VAC

3.6 to 10

200

250 VAC

0.1 to 15

750

250 VAC

15.1 to 30

1500

250 VAC

  

Recognized Under the Component Program of 

       Underwriters Laboratories

The Recognized Components Program of UL is different 
from UL Listing. UL will test a fuse to a specification 
requested by the manufacturer. The test points can be 
different from the UL Listed requirements if the fuse 
has been designed for a specific application. Application 
approval is required by UL for fuses recognized under the 
Component Program.

standards

UL 275 AUTOMOTIVE GLASS TUBE FUSES (32 Volts)

 

UL LISTED

UL ampere ratings tests are conducted at 110%, 135%, 
and 200%. Interrupting rating tests are not required.

 CSA Certification

CSA Certification in Canada is equivalent to UL Listing in 
the United States.

 The Component Acceptance Program of CSA is 

equivalent to the Recognition Program at UL.

METI (Japan Ministry of Economy, Trade and Industry)

PS

E

  

METI APPROVAL

METI  approval in Japan is similar to UL Recognition in the 
United States.

METI

B

 has its own design standard and characteristics.

INTERNATIONAL ELECTROTECHNICAL 
COMMISSION (IEC)

Publication 60127, Parts 1, 2, 3, 4, 6

The IEC organization is different from UL and CSA, since 
IEC only writes specifications and does not certify. UL 
and CSA write the specifications, and are responsible for 
testing and certification.

Certification to IEC specifications are given by such 
organizations as SEMKO (Swedish Institute of Testing 
and Approvals of Electrical Equipment) , BSI (British 
Standards Institute)   and VDE (German Standard Insitute) 

E

D

V

, as well as UL and CSA.

IEC Publication 60127 defines three breaking capacity 
levels (interrupting rating). Low breaking capacity fuses 
must pass a test of 35 amperes or ten times rated current, 
whichever is greater, while enhanced breaking capacity 
fuses must pass a test of 150 amperes and high breaking 
capacity fuses must pass a test of 1500 amperes.

60127 Part 2

Sheet 1

   — Type F Quick Acting, High Breaking Capacity

Sheet 2

  — Type F Quick Acting, Low Breaking Capacity

Sheet 3

  — Type T Time Lag, Low Breaking Capacity

Sheet 4

  — Style Fuses 1/4×1 1/4

Sheet 5

  — Type T Time Lag, High Breaking Capacity

Sheet 6

  — Type T Time Lag, Enhanced Breaking Capacity

The letters ‘F’ and ‘T’ represent the time-current 

characteristic

 of the fast-acting and time delay fuses. One 

of these letters will be marked on the end cap of the fuse.

12

© 2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuseology Selection Guide

UL/CSA/ANCE (Mexico) 248-14 vs. IEC 60127 Part 2 FUSE 
OPENING TIMES vs. METI/MITI

Percent of 

Rating

uL & CsA 

std 248-14

ieC tYPe F 
sheet 1 (*)

ieC tYPe F 
sheet 2 (*)

ieC tYPe t 

sheet 3 (*)

ieC tYPe t 

sheet 5 (*)

Meti/Miti

100

4Hr.Min.

—

—

—

—

130

—

—

—

—

—

1Hr.Min.

135

60 Minutes 

Max.

—

—

—

—

150

—

60 Minutes 

Min.

60 Minutes 

Min.

60 Minutes 

Min.

60 Minutes 

Min.

160

—

—

—

—

—

1Hr.Max.

200

2 Minutes 

Max.

—

—

—

—

2 Minutes 

Max.

210

—

30 Minutes 

Max.

30 Minutes 

Max.

2 Minutes 

Max.

30 Minutes 

Max.

(*)  Note: The IEC Specification is written up to 

10.0A. Any components above these ratings are 
not recognized by the IEC (although the fuses 
may have similar opening characteristics).

IEC also has opening time requirements at 275%, 400% 
and 1000%; however, the chart is used to show that 
fuses with the same ampere rating made to different 
specifications are not interchangeable. According to the 
IEC 60127 Standard, a one ampere-rated fuse can be 
operated at one ampere. A one ampere-rated fuse made to 
UL/CSA/ANCE 248-14 should not be operated at more than 
.75 ampere (25% derated — See RERATING section).

METI

B

 does not differentiate between fast acting and time 

delay characteristics. 

Publication IEC 60127-4 (Universal Modular Fuse-Links 
[UMF])

This part of IEC 60127-4 covers both PCB through-hole 
and surface mount fuses. This standard covers fuses rated 
32, 63, 125, and 250 volts. This standard will be accepted 
by UL/CSA making it the first global fuse standard. This 
specification uses different fusing gates than IEC 60127-2; 
the gates used here are 125%, 200%, and 1000%.

The fuses must not open in less than one hour at 125% 
of rated current and open within two minutes at 200% of 
rated current. The 1000% overload is used to determine 
the fuse characteristic. The opening time for each rating is 
listed below.

Type FF  : Less than 0.001 sec.
Type F  : From 0.001 - 0.01 sec.
Type T  : From 0.01 - 0.1 sec.
Type TT  : From 0.1 - 1.00 sec.

These characteristics correlate to the terminology used in 
IEC 60127-1.

Breaking capacity (interrupting rating) varies based on 

voltage rating. Parts rated at 32 & 63 volts must pass a 
test of 35 amperes or ten times rated current, whichever 
is greater. Parts rated at 125 volts must pass a test of 50 
amperes or ten times rated current, whichever is greater. 
Parts rated at 250 volts are further defined as either low, 
intermediate or high breaking. The low breaking capacity 
fuses must pass a test of 100 amperes rated current, while 
intermediate breaking capacity fuses must pass a test of 
500 amperes and high breaking capacity fuses must pass a 
test of 1500 amperes.

MILITARY/FEDERAL STANDARDS

MIL-PRF-15160 and MIL-PRF-23419

These specifications govern the construction and 
performance of fuses suitable primarily for military 
electronic applications.

MIL-PRF-19207

This specification governs the construction and 
performance of fuseholders suitable for military 
applications.

DSSC Drawing #87108

This drawing governs the construction and performance 
of .177” × .570” (2AG size) cartridge fuses and axial lead 
versions suitable for military applications. DSSC #87108 
designation is included in the fuse end cap marking.

standards (continued)

A robust web-based tool to help circuit designers 

identify the optimal electronic fuses for their products.

To use the Littelfuse iDesign tool, simply register to 

create a free online account at the iDesign Login.

13

Fuseology Selection Guide

© 

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

MILITARY/FEDERAL STANDARDS

FEDERAL  SPECIFICATION W-F-1814

This specification governs the construction and 
performance of fuses with high interrupting ratings that are 
approved for federal applications. Fuses approved to these 
specifications are on the Federal Qualified Products List.

Write to the following agencies for additional information 

on standards, approvals, or copies of the specifications.

 

Underwriters Laboratories Inc. (UL)

 

333 Pfingsten Road 

 

Northbrook, Illinois, USA 60062-2096 

 

Canadian Standards Association (CSA)

 

 

5060 Spectrum Way, Suite 100 

 

Mississauga, Ontario, Canada  L4W 5N6

 

International Electrotechnical Commission (IEC)

 

3, Rue de Varembe 

 

1211 Geneva 20, Switzerland

 

Naval Publications and Military StandardsForm  

 

Center (for Military and Federal Standards)

 

5801 Tabor Avenue 

 

Philadelphia, Pennsylvania, USA 19120

 

Defense Supply Center Columbus (DSCC)

 

3990 East Broad Street 

 

Columbus, Ohio, USA 43218-3990

 

Ministry of Economy Trade and Industry (METI)

 

1-3-1 Kasumigaseki 

 

Chiyouda-ku, Tokyo 100-8901, Japan

standards (continued)

14

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Fuseology Selection Guide

1.  Define the circuit operating parameters.

 

Normal operating current in amperes:   

....................................................................................................

 

Normal operating voltage in volts: 

 

....................................................................................................

 

Maximum interrupt current: 

 

 

....................................................................................................

 

Ambient 

temperature:   ....................................................................................................

 

Typical overload current: 

 

 

....................................................................................................

 

Required opening time at specified overload: 

....................................................................................................

 

Transient pulses expected: 

 

 

....................................................................................................

 

Agency 

approvals: 

   ....................................................................................................

 

Mounting type/form factor: 

 

 

....................................................................................................

 

Typical resistance (in circuit):    

 

....................................................................................................

2.  Select the proper circuit protection component.

 (

Refer to Table on Page 3 and specifications with Data Sheets

)

3.  Determine the opening time at fault.

  

 

Consult the Time-Current (T-C) Curve of each PTC series to determine if the 
selected part will operate within the constraints of your application. 

 

If the device opens too soon, the application may experience nuisance 
operation. If the device does not open soon enough, the overcurrent may 
damage downstream components.

 

To determine the opening time for the chosen device, locate the overload 
current on the X-axis of the appropriate T-C Curve and follow its line up to its 
intersection with the curve. At this point read the time tested on the Y-axis. 
This is the average opening time for that device. 

 

If your overload current falls to the right of the curve the device will open.     
If the overload current is to the left of the curve, the device will not operate.

4.  Verify ambient operating parameters.

 

Ensure that the application voltage is less than or equal to the device’s rated 
voltage and that the operating temperature limits are within those specified 
by the device.

5.  Verify the device’s dimensions.

 

Compare the maximum dimensions of the device to the space available in 
the application. The dimension of each product is included within each data 
sheets on the following pages.

6.  Test the selected product in an actual application.

PtC selection Worksheet

A robust web-based tool to help circuit designers 

identify the optimal electronic fuses for their products.

Littelfuse iDesign

TM

 Online Fuse Design and Selection Tool

15

Fuseology Selection Guide

© 

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuse and PtC Products selection guide

Fuse PRoduCts

Fuses provide protection by completely stopping the flow of energy to sensitive circuits. If current exceeds the fuse’s operating range, the metal 
wire or strip melts safely within an enclosure. Littelfuse offers the most extensive range of fuses available, and for easy replacement of cartridge 
fuses Littelfuse offers a wide selection of fuseholders including panel mount, in-line, and surface and thru-hole circuit board mount devices. 

Series

Name

1

Vie

w Dat

asheet

Order Samples

Size

2

Time Lag

Fast A

cting

Ver

y 

Fast A

cting

Device Range

 3

(Operating 

Current Options 

in Amps)

Max. Voltage 

Rating

 3

(Volts)

Interrupting 

Rating at 

Max. Voltage 

Rating

 3

(Amps)

 

Operating

Temperature

Range

Agency 

Approvals

 3

Halogen F

ree

R

oHS Compliant

Lead F

ree

UL

UR

C

SA

PSE

UMF

Surface Mount:

Ceramic Chip

437

•

•

1206

•

0.25 - 8

125 / 63 / 32

50

-55°C to +150°C

•

• • •

438

•

•

0603

•

0.25 - 6

32 / 24

50

•

• • •

440

•

•

1206

•

1.75 - 8

32

50

• •

• • •

441

•

•

0603

•

2 - 6

32

50

• •

• • •

469

•

•

1206

•

1 - 8

24 / 32

24 - 63

• •

• • •

501

•

•

1206

•

10, 12, 15, 20

32

150

•

• • •

Thin Film

466

•

•

1206

•

0.125 - 5

125 / 63 / 32

50

-55°C to +90°C

• •

• •

429

•

•

1206

•

7

24

35

• •

• •

468

•

•

1206

•

0.5 - 3

63 / 32

35 - 50

• •

• • •

467

•

•

0603

•

0.25 - 5

32

35 - 50

• •

• • •

494

•

•

0603

•

0.25 - 5

32

35 - 50

• •

• • •

435

•

•

0402

•

0.25 - 5

32

35

• •

• • •

Nano

2®

 Fuse

 

448

•

•

2410

•

0.062 - 15

125 / 65

35 - 50

-55°C to +125°C

• • •

• •

449

•

•

2410

•

0.375 - 5

125

50

• • •

• •

451 / 453

•

2410

•

0.062 - 15

125 / 65

35 - 50

• • • •

• •

452 / 454

•

2410

•

0.375 - 12

125 / 72

50

• • •

• •

456

•

•

4012

•

20, 25, 30, 40

125

100

•

• •

458

•

•

1206

•

1.0 - 10

75 / 63

50

•

• •

443

•

•

4012

•

0.5 - 5

250

50

•

• •

464

•

•

4818

•

0.5 - 6.3

250

100

• • • •

465

•

•

4818

•

1 - 6.3

250

100

• • • •

462

•

•

4118

•

0.500 - 5

350

100

-40°C to +80°C

•

•

•

485

• 

•

4818

•

0.500 - 3.15

600

100

-55°C to +125°C

•

• •

Telelink

®

 Fuse

461

•

•

4012

0.5 - 2.0

600

60

-55°C to +125°C

• •

• •

461E

4012

1.25

600

60

•

• •

OMNI-BLOK

®

  

Fuseholder

154

•

•

*

•

0.062 - 10.0

125

35 - 50

-55°C to +125°C

•

• •

154T

*

•

0.375 - 5

125

50

•

• •

Fuse and Clip 

Assemblies

157

*

•

0.062 - 10

125

35 - 50

-55°C to +125°C

•

• •

157T

*

•

0.375 - 5

125

50

•

• •

159

•

•

*

0.5 - 2

600

60

•

• •

160

*

•

0.5 - 5

250

50

•

• •

PICO

® 

SMF

 

Fuse

459

•

•

*

•

0.062 - 5

125

50 - 300

-55°C to +125°C

• •

460

•

•

*

•

0.5 - 5

125

50

• •

Flat Pak

202

•

•

*

•

0.062 - 5

250

50

-55°C to +125°C

• •

203

•

•

*

•

0.25 - 5

250

50

• •

EBF

446

•

•

*

•

2.0 - 10.0

350

100

-40°C to +125°C

• •

447

•

•

*

•

2.0 - 10.0

350

100

• •

tAbLe CoNtiNues oN NeXt PAge

(1) Detailed information about most product series listed here can be found on our web site.
(2) Size for these surface mount items refers to common industry length and width dimensions of the device surface area. Example: 0402 = .04” x .02”
(3) In some cases for these categories the ratings, agency approvals and specifications vary by part number and are presented here as ranges representing the whole series.
     Please refer to product data on www.littelfuse.com and in our data sheets for detailed information by part number.
 *  Please refer to data sheet for detailed specifications.

16

Series

Name

1

View Datasheet

Order Samples

Time Lag

Medium Acting

Fast Acting

Very Fast Acting

Device 

Range

 3

(Operating 

Current 

Options in 

Amps)

Max. Voltage 

Rating

 3

(Volts)

Interrupting

 

Rating at 

Max Voltage 

Rating

 3

(Amps)

 

Operating

Temperature 

 

Range

Agency Approvals

 3

RoHS Compliant

Lead Free

Americas

Europe

Asia

UL

UR

CSA

QPL

UMF

CE

VDE

TUV

BSI

Semko

PSE

K

CCC

CQC

Radial Leaded / Socket:

Micro

™ 

Fuse /

 

TR3 Fuse

262/268

• ••

•

0.002 - 5

125

10,000

-55°C to +125°C

• •

•

269

•

•

0.002 - 5

125

10,000

-55°C to +125°C

• • •

•

272/278

• ••

•

0.002 - 5

125

10,000

-55°C to +125°C

• •

•

273/279

• ••

•

0.002 - 5

125

10,000

-55°C to +85°C

• •

•

274

•

•

0.002 - 5

125

10,000

-55°C to +85°C

•

•

303

• •

•

0.5 - 5

125

50

-55°C to +70°C •

•

•

• •

TR5

® 

Fuse

370

• •

•

0.4 - 6.3

250

35 - 50

-40°C to +85°C

•

• •

• •

•

• •

372

• •

•

0.4 - 6.3

250

35 - 50

•

• •

• • • • • • •

373

• •

•

0.5 - 10

250

50

•

•

•

• •

374

• •

•

0.5 - 10

250

50

•

•

•

• •

382

• •

•

1 - 10

250

100

•

• •

• • • • • • •

383

• •

•

1 - 10

300

50 - 100

•

• •

•

• •

TE5

369

• •

•

1 - 6.3

300

50

-40°C to +85°C

•

•

•

• •

385

• •

•

0.35 - 1.5

125

50

•

•

• •

391

• •

•

0.125 - 4

65

50

•

•

• •

392

• •

•

0.8 - 6.3

250

25 - 63

•

• •

• • •

• • •

395

• •

•

0.05 - 6.3

125

100

•

•

•

•

396

• •

•

0.05 - 6.3

125

100

•

•

•

• •

397

• •

•

0.35 - 1.5

125

50

•

•

• •

398

• •

•

0.125 - 4

65

50

•

•

• •

399

• •

•

0.125 - 4

65

50

•

•

• •

400

• •

•

0.5 - 6.3

250

130

•

• •

•

• •

804

• •

•

0.8 - 6.3

250

150

-40°C to +125°C

• • •

• •

• • •

808

• •

•

2 - 5

250

100

-40°C to +85°C

•

•

• •

TE7

807

• •

•

0.8 - 6.3

300

100

-40°C to +125°C • • •

•

• •

• •

Axial Leaded / Cartridge:

PICO

®

 

Fuse /

 

PICO

®

 II 

Fuse Axial

251

• •

•

0.062 - 15

125

300DC / 50AC

-55°C to +125°C

• •

•

•

•

•

253

• •

•

0.062 - 15

125

300DC / 50AC

•

•

•

•

•

275

• •

•

20 - 30

32

300DC / 50AC

• •

•

263

• •

•

0.062 - 5

250

50

• •

•

•

•

471

• •

•

0.5 - 5

125

50

• •

•

•

•

472

• •

•

0.5 - 5

125

50

•

•

•

473

• •

•

0.375 - 7

125

50

• •

•

•

•

265/266/267

• •••

•

0.062 - 15

125

300DC / 50AC

• • •

•

•

3.6x10 mm

874

• •

•

0.1 - 10

250

50

-55°C to +125°C

•

•

• •

875

• •

•

0.1 - 10

250

50

•

•

• •

876

• •

•

0.125 - 5

250

35 - 50

•

• •

• •

877

• •

•

2 - 6.3

250

35 - 63

•

• •

• •

4.5x14.5 

mm 

(2AG)

208

• •

•

0.125 - 10

350

100

-55°C to +125°C

•

•

•

•

• •

209

• •

•

0.25 - 7

350

100

•

•

•

•

• •

220

• •

Special Fuse

0.3 - 7

250 / 300 / 350

35 - 100

• • •

•

•

• •

2205

• •

•

0.25 - 2.5

250

35

• •

•

• •

224/225

• ••

•

0.375 - 10

250 / 125

35 - 500

• • •

•

•

• •

229/230

• ••

•

0.25 - 7

250 / 125

35 - 400

• • •

•

•

• •

5x20 

mm

217

• •

•

0.032 - 15

250

35 - 150

-55°C to +125°C

• •

• •

• • • • •

• •

218

• •

•

0.032 - 16

250

35 - 100

• •

• •

• • • • •

• •

213

• •

•

0.2 - 6.3

250

35 - 63

• •

• •

• • •

•

• •

219XA

• •

•

0.04 - 6.3

250

150

• •

• •

• • •

•

• •

216

• •

•

0.05 - 16

250

750 - 1500

• •

• •

• • • • •

• •

215

• •

•

0.125 - 20

250

400 / 1500

• •

• •

• • • • •

• •

232

• •

•

1 - 10

250 / 125

300 / 10,000

•

• •

• •

235

• •

•

0.1 - 7

250 / 125

35 - 10,000

•

•

•

• •

• •

233

• •

•

1 - 10

125

10,000

•

•

•

• •

• •

234

• •

•

1 - 10

250

100 - 200

•

•

•

• •

• •

239

• •

•

0.08 - 7

250 / 125

35 - 10,000

•

•

•

• •

• •

285

• •

•

0.125 - 20

250

400 - 1500

•

•

• •

477

• •

•

0.5 - 16

400DC / 500AC

100 - 1500

• •

•

• •

• •

977

• •

•

0.5 - 16

450DC / 500AC

200 / 100

•

• •

• •

6.3x32 mm 

(3AG/3AB)

312/318

• ••

•

0.062 - 35

250 / 32

35 - 300

-55°C to +125°C

• • •

•

• •

• •

313/315

• ••

•

0.01 - 30

250 / 125 / 32

35 - 300

• • •

•

• •

• •

314/324

• ••

•

0.375 - 40

250

35 - 1000

• • •

•

• •

• •

322

• •

•

12 - 30

65

200 - 1000

•

•

•

•

332

• •

•

1- 10

250

100 / 200

• •

•

•

• •

325/326

• ••

•

0.01 - 30

250

100 - 600

• • •

•

• •

• •

505

• •

•

10 - 30

450 / 500

20,000 - 50,000

• •

•

• •

506

• •

•

15 - 20

600DC

10,000

• •

•

• •

508

• •

1000VAC / DC 

High Voltage 

Fuse

0.315 - 1

1000

10,000

• •

•

• •

Fuseology Selection Guide

© 

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuse and PtC Products selection guide (continued)

PtC PRoduCts

PTCs (positive temperature coefficient) increase resistance as temperature increases. They are designed to prevent unsafe levels of current 
while allowing constant safe current levels, and their resistance will “reset” automatically when the current and temperature returns to a 
safe level. PTCs are typically used in applications where automatic reset is desired.

Series Name

1

Vie

w Dat

asheet

Order Samples

Size

2

Hold Current (I

HOLD

)

Max Voltage 

(V

MAX

)

Max Fault 

current (I

MAX

)

Operating 

Temperature

 

Range

Agency Approvals

Halogen Free

RoHS

Lead Free

cUR

UR

TUV

Surface Mount:

LoRho PTC

 

(Low Resistance)

• •

0402

0.1 - 0.5A

6 / 12 V

50 A

•

•

•

•

•

•

0603

0.5 - 2.0A

0805

0.75 - 3.0A

-40°C to 85°C

1206

0.75 - 4.5A

1812

1.9 - 3.7A

2920

7.0A

0603L

• •

0603 

(1608)

0.04 - 0.5A

6 - 15 V

40 A

•

•

•

•

•

•

0805L

• •

0805

(2012)

0.10 - 1.10 A

6 - 15 V

100 / 40 A

•

•

•

•

•

•

1206L

• •

1206

(3216)

0.10 - 2.0A

6 - 30 V

100 A

•

•

•

•

•

•

1210L

• •

1210

(3225)

0.05 - 2.0A

6 - 30 V

10 / 100 A

•

•

•

•

•

•

1812L

• •

1812

(4532)

0.1 - 3.0A

6 - 60 V

10 / 20 / 100 A

•

•

•

•

•

•

2016L

• •

2016 

(5041)

0.30 - 2.00 A

6 - 60 V

20 / 40 A

•

•

•

•

•

•

2920L

• •

2920

(7351)

0.30 - 3.00 A

6 - 60 V

10 / 40 A

•

•

•

•

•

•

250S

• •

see data sheet

0.13 A

60 V

3 A

•

•

•

•

Radial Leaded:

USBR

• •

see data sheet

0.75 - 2.50 A

6 / 16 V

40 A

-40°C to 85°C

•

•

•

•

•

16R

• •

2.50 - 14.00 A

16 V

100 A

•

•

•

•

•

30R

• •

0.90 - 9.00 A

30 V

40 A

•

•

•

•

•

60R

• •

0.10 - 3.75 A

60 V

40 A

•

•

•

•

•

72R

• •

0.20 - 3.75 A

72 V

40 A

•

•

•

•

•

250R

• •

0.08 - 0.18 A

250 V

3 / 10 A

•

•

•

•

•

600R

• •

0.15 - 0.16 A

600 V

3 A

•

•

•

•

•

Battery Strap:

SL

 

(Low Resistance)

• •

see data sheet

1.9 - 3.7 A

6 V

50 A

-40°C to 85°C

•

•

•

•

•

•

(1) detailed information about most product series listed here can be found on our web site.
(2) 

Size for these surface mount items refers to common industry length and width dimensions of the device surface area. Example: 0402 = .04” x .02”

 

18

© 2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuseology Selection Guide

Packaging and Part Numbering

1.-3.

4.

5.-7.

8.

9.

10.

Stelle

3xx

X

XXX

X

X

X

digit

0

4

3

TR3

0

303

370

1

2

372

3

382

4

385

5

391

6

950

Y

373

374

 Variant

0

Standard, long leads 18,8 mm

1

long leads 18,8 mm, TR3

392

2

395

4

short leads 4,3 mm

396

5

T²CP

Version

397

0

Standard

1

MP

S

PIP Surface Mount (TR5 blister tape 2x500 pcs.)

398

Rated Current Specification

IP

3-digit

399

062

 = 62mA

example 4.-7. digit

100

 = 100mA / 1A / 10A

Pico

125

 = 125A

0062 =   62mA

275

0100 = 100mA

0

< 1A

1100 =     1A

1

≥ 1   - < 10A

2100 =   10A

2

≥ 10 - < 100A

3125 = 125A

3

≥  100A

tape in bulk 100 pcs., only Picofuse 275

bulk 2.500 pcs., only Picofuse 275

Tape, Ammopack 
Tape, Ammopack
Tap, Rolle/ Reel

varying production

customized

short leads 3,3 / 3,5 mm (special model)

bulk 1.400 pcs., only TE5

®

 / T²CP / MP / IP

bulk, 200 pcs., TR3 long leads / TR

® 

Packaging

Tape, Ammopack 1.000 pcs. TR5

®                                              

bulk, 1.000 pcs. TR5

®

bulk, 300 pcs., TR3 short leads

Explanation

TR5

® 

TE5

® 

Littelfuse Wickmann Products Part Numbering System

0 X X X   X X X X   X    X   X X X X X X 

Packaging Type *

A

  Ammo packed

B

  Bubble packed

C

  Chip packed

R

   Reeled

X

   Filler

A/X

  1

V

  5

T

  10

S

  20

L

  50

H

  100

F

   200

G

   250

U

   500

M

  1000

D

  1500

P

  2000

E

  2500

W

  3000

Y

  4000

N

  5000

K

  10000

J

  12000

Z

  Misc.

Options Codes *

RT1

  Reel and Tape, 2.062in (52.4mm) lead spacing

RT2

  Reel and Tape, 2.50 in (63.5mm) lead spacing

RT3

  Reel and Tape, 2.874 in (73mm) lead spacing

E

   Pigtail lead type fuse

  

ID

  Indicating fuse

L

 

RoHS compliant

P

 

Lead-free

Product Series Code

* Not all options and codes listed here are available for all products. 

   For information about the specific options available for any Littelfuse product,

   please refer to the packaging details information within each product data sheet

   or contact your Littelfuse products representative.

Ampere Rating Code 

Decimal is to far right for whole number 

amp ratings, to far left for ratings less than

one, and within center for fractional amp ratings.
Examples: 

   10A fuse is “010.”

   1/4A or 0.25A fuse is “.250”

   1 1/2A or 1.5A fuse is “01.5”

   1 1/4A or 1.25A fuse is “1.25”
Refer to the Electrical Characteristics tables

presented in each product data sheet 

for specific amp rating codes

Packaging Quantity Code *

y

x

Tape and Reel packaging per EIA-296:

Tape width is defined as the width of the tape and reeled fuse (x) as measured from inside tape to inside tape.

Pitch is defined as the space between two tape and reeled fuses (y) as measured from lead to lead. 

Example: 

    437 series fuse is “0437”

Littelfuse Fuse Products Traditional Part Numbering System

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Fuseology Selection Guide

© 

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Liability

Littelfuse, Inc. its affiliates, agents, and employees,
and all persons acting on its or their behalf (collectively,
“Littelfuse”), disclaim any and all liability for any errors,
inaccuracies or incompleteness contained here or in any
other disclosure relating to any product. Littelfuse disclaims
any and all liability arising out of the use or application
of any product described herein or of any information
provided herein to the maximum extent permitted by law.
The product specifications do not expand or otherwise
modify Littelfuse terms and conditions of purchase,
including but not limited to the warranty expressed therein,
which apply to these products.

Right to Make Changes

Littelfuse reserves the right to make any and all changes to
the products described herein without notice.

Not Intended for Use in Life Support or Life Saving
Applications 

The products shown herein are not designed for use
in life sustaining or life saving applications unless
otherwise expressly indicated. Customers using or selling
Littelfuse products not expressly indicated for use in such
applications do so entirely at their own risk and agree
to fully indemnify Littelfuse for any damages arising or
resulting from such use or sale. Please contact authorized
Littelfuse personnel to obtain terms and conditions
regarding products designed for such applications.

Intellectual Property 

No license, express or implied, by estoppel or otherwise,
to any intellectual property rights is granted by this
document or by any conduct of Littelfuse. Product
names and markings noted herein may be registered
trademarks of their respective owners. Littelfuse makes
no representations or warranties of non-infringement or
misappropriation of any third party intellectual property
rights unless specifically provided for herein.

Legal disclaimers

Disclaimer 

Specifications, descriptions and data contained in this 
document are believed to be accurate.  However, users 
should independently evaluate each product for the 
particular application. Littelfuse reserves the right to 
change any information contained herein without notice 
and may, at its sole discretion, change the design, 
manufacture or construction of any product.  Visit        

www.littelfuse.com

 for the most up-to-date information.  

Littelfuse’s only obligations for any of its products are 
specified in its Standard Terms and Conditions and 
Littelfuse shall not be liable for any indirect, consequential 
or incidental damages from any sale or use of any of its 
products.

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