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.

Fuseology Selection Guide

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