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          Fuse Choosing Guide

  Fuse Ology


Fuse Ology lets us increase and better master the particular knowledge about fuse and aspects of fuse application. Fuse is a weak tache, which is purposely set in the circuit and is sensitive to current. Its main function is to melt safely and reliably under various conditions of current loading so as to provide protection for each independent component or the whole circuit. This course of Fuse Ology contains some important features of fuse, various conditions while choosing fuses and various standards.


   Fuse Features


In order to choose a suitable fuse for some special occasions, subscribers should have a good mastery of the following features and some concepts of application.

Environmental Temperature: It means the air temperature directly around the fuse. Do not interblend with the room temperature. On many practical occasions, the fuse temperature is rather high as the fuse is closed such as the fixed rack installed in the electrical panel or the fuse is fixed on other heating components, such as the neighborhood of the impedance and the transformer.

Interruption Capability:Please see the blowout rated value.

Current Rated Value: Nominal amperage noted on the fuse. The value is the current that the fuse can carry and the value is set by the manufacturer according to a series of already-controlled testing conditions. (Please see the decrease of rated value.)

The content code of the fuse products contains serial sign and ampere parameter. Please see those contents about conducting correct choosing guide in the Fuse Choosing Guide and Time Current Curve List.

Decrease of Rated Value:When at the environmental temperature of 25℃, the working current of fuse that we present should not be over 75% of the nominal current value because that nominal current value is set according to a series of already-controlled testing conditions. These testing conditions are one part of the Fuse Used for Assisting Overcurrent Protection 198G Standard of Underwriters Laboratories, whose main aim is to set the common testing standards required for the serial control of the components which are used for guard against fire. The commonly seen alteration factors of these testing standards contain fully-closed fuse holder, high-touch resistance, air flowage, instantaneous peak value and changes in the aspect of connecting cable (diameter and length). Fuse Is actually temperature-sensitive component. Although the controlled testing conditions change a little, the expected service life of the fuse would be largely affected, especially when the load is its nominal value. The nominal value is commonly expressed as 100% rated value.

Circuit design engineer should clearly know that the aim of setting these controlled testing conditions is to make the fuse manufactures be able to keep the performance standard of the products unified so he must know various change conditions while using the fuse. In order to compensate for these changes, the load added to the fuse often should not be over 75% of the nominal rated value listed by the manufacturers while the circuit design engineer is design the equipment with protection equipment which is both safe and has long service life. Meanwhile, he must also provide enough overload and shortcut protection.

Fuse Is actually temperature-sensitive component. Each parameter of the fuse is set at the environmental temperature of 25℃. The fuse temperature produced when the fuse passes current increases or decreases along with the change of the environmental temperature.

The Environmental Temperature in the section of Fuse Choosing shows that the environmental temperature has effect on the nominal rated current value of a certain fuse. The material adopted for designing the majority of traditional Slo-Blo fuses has comparatively low melting temperature thus it is quite sensitive to the changes of the environmental temperature.

Dimension: The dimension has millimeter as its unit if without other regulations. The fuse dimension scope of this product content ranges from the minimum dimension of 0603 circuit wafer (1.60mm length*0.79mm width*0.46mm height) to the maximum of 5AG. 5AG is usually called mini-type fuse (10.3mm diameter*38.1mm length). During the past years many kinds of new products were developed out, which boosted the continuous evolution of fuse dimension to satisfy the requirements of various circuit protection. The earliest fuse is very simple easy-disconnection component. Later in the 19th century, there occurred the first plug-in fuse with the fine lead closed in the reading light holder by Edison. In 1904, the Underwriters Laboratories set the various items specifications of the dimension and rated value to suit the safety standards. In 1914, there occurred reusable fuse and automobile fuse. In 1927, some companies began to manufacture fuse wire of very low ampere for the budding electronic industry.

The fuse dimension in the blank below begins with the early glass fuse which was used for automobiles, among them there are the sign of AG and as A represents automobile and G represents glass, namely Automobile Glass. Figures used before AG is set according to the chronological order. Because fuses of every new specification and dimension are made by different manufacturers, for example, 3AG is the third fuse dimension that is put into the market. Although dimension and structure of other non-glass fuses are set by the function requirement, the fuses still preserve the length or diameter of the glass fuse. Their signs are changed to AB to replace AG, which means that the fuse tube is made of bakelite, glass fiber, ceramics or other similar non-glass material. The fuse with the largest dimension in the blank below is 5AG, namely mini-type. This name is adopted by American electric industry and the State Electric Code scope. American State Electric Code Scope always views fuse of 14.3mm*50.8mm as the smallest standard fuse which is in use.

Fuse Dimension
1 1/4
1 1/4
1 1/2

Fuse Performance:Performance in fuse design means the degree that the fuse conducts speedy reaction to various current loads. Fuse performance is generally separated into 3 types, namely super quick blowout, quick blowout and slow blowout. The features of the slow-blowout fuses are that this sort of fuses is designed with additional thermal inertia to undertake the overcurrent increase during the normal start-up.

Fuse Structure:The internal structure will change along with the different changes of the rated ampere value. Fuse pictures in this product content give the model structure of the fuses that have special rated ampere value in the fuse series.

Fuse Holder:On many practical occasions, fuses are fixed in fuse holders. The fuse together with the assisting fuse holder is not used only as a shape to directly connect or cut the power supply.

Blowout Rated Value:Also called disconnection capability or shortcut rated capacity. Blowout rated value is the biggest allowable current the fuse can carry under the rated voltage. When there is shortcut, the fuse will pass instantaneous overcurrent many times, which is bigger than the normal working current. Safe operation requires the fuse to keep complete state (without detonation or rupture) and to eliminate shortcut.

Along with different designs of the fuse, the blowout rated value ranges from fuse of 250V, 5*20mm in the metric system and 35-ampere AC to fuse of 600V and 200,000-ampere AC in the series of KLK. Subscribers can get data of other series of fuses from the manufacturers.

The blowout rated value of fuses classified according to the UL Standard 198G must be 10,000 ampere. There are also some exceptions. Please see the section of Standard. Fuses of these exceptional cases have safety guidelines which are largely over the guidelines of those possible shortcut current on many practical occasions.

Interference Open Circuit:Interference open circuit is often caused by imperfect circuit analysis. Among all of the factors listed in the Fuse Choosing Guide, we should pay special attention to the first, the third and the sixth item, namely the normal working current, the environmental temperature and the overcurrent increase. For example, a commonly-seen cause of conventional power supply interference open circuit is that we are not able to fully consider the nominal fuse I2t rated value of the fuse. We couldn’t choose fuses only according to the normal working current and the environmental temperature. Under the application condition, the nominal fuse I2t rated value of the fuse must also meet the requirements that various inrush currents put forward to the fuse, which are caused by the power supply smoothing filter’s inputting capacity to the electric device. There are steps of transferring various wave shapes into wave shapes required by I2t circuit in the Fuse Choosing Guide. For those safe and long-service life fuse protections, a good designing method is to keep the I2t of the inrush current wave shape not higher than 20% of the nominal fuse I2t rated value of the fuse while choosing fuses. Please refer to the Overcurrent Increase of the Fuse Choosing Guide.

Impedance:Fuse impedance isn’t important in the total impedance of the circuit. Because impedance of the fuse, whose amperage is lower than 1 is only several ohms, we should take this point into consideration while adopting fuses in low-voltage circuit. Subscribers can gain the practical impedance value of the fuses from the manufacturers. Most fuses are made of positive temperature coefficient. Therefore, we will often mention cold impedance and hot impedance (voltage drop under rated current). The practical working impedance is between the two. Cold impedance can be obtained through testing current, which is not higher than 10% of the nominal rated value of the fuse. The cold impedance value given in this publication is both nominal and model. If this parameter is a limit value as far as the design analysis is concerned, subscribers should consult with the manufacturers. Hot impedance is gained when the current value passing the fuse is equal to the nominal rated current. We can send subscribers with fuse impedance data on getting requirements. And we can also provide fuses according to the impedance control tolerance regulated by the subscribers and we will collect additional cost fee for this.

Shortcut Capacity: Please refer to Blowout Rated Value.

Soldering Notes:Because most fuses have soldered joints we should pay special attention while planning to fix these fuses through soldering method. Too much quantity of heat during the soldering process will make the solder inside the fuse reflux and accordingly change its rated value. Fuse is a thermal-sensitive component which is similar to semi-conductor so we recommend that subscribers use decalescence facilities while soldering.

Sampling Notes:As it needs destructive inspection when confirming whether a certain specification is qualified or not, sampling inspection method should be conducted to each passel of finished products according to the principle of statistics.

Time-Current Features Curve List:As a picture indication of blowout features, Time-Current Features Curve List is commonly middle curve. The reason why we give this curve list is to make it as an assisting method of design but not as a part of fuse specifications. Time-Current Features Curve List is very helpful while choosing fuses because fuses of the same rated current value may well have rather different features curve. The fuse specification often contains the service life requirement when the working current is 100% or 110% of the rated value or contains the longest open circuit time when overcurrent passes (often 135% or 200% of the rated value). Destructive inspection is needed when confirming whether a certain specification is qualified or not. Destructive inspection should be conducted to each passel of finished products according to the principle of statistics. Time-Current Features Curve List gives the average data required by the design. However, to any passel of special products, this average may have some difference. So we should inspect some samples to identify their performance immediately after choosing a fuse.

Underwriters Laboratories:Please refer to Underwriter Laboratory Classification. The fuse must satisfy each item of requirements set by the underwriter laboratory standard, namely the regulations of No198G of Fuse Assisting Overcurrent Protection. Some 32-voltage fuses in this product content are classified according to the UL Standard 275. On the other hand, Component Precis Acquisition Recognition of Underwriters Laboratories means that this item of product gains the application Appraisal document approved by the Underwriters Laboratories component précis.

Voltage Rated Value: Voltage rated value labeled on the fuse that the fuse can certainly safely interrupt the rated shortcut circuit current when the voltage is equal to or smaller than the rated voltage of the circuit. The voltage rated value series is contained in the regulation of American N.E.C and is also a requirement of the Underwriters Laboratories used for guarding against fire dangers. For most small-dimension fuses and minitype fuses, the standard voltage rated value adopted by fuse manufacturers are 32V, 125V, 250V and 600V.

In the electronic equipment which has rather low output power supply or whose shortcut current value is lower than 10 times of the fuse current rated value, the commonly-used method is to regulate that fuses with its voltage rated value being 125V or 250V can be used for secondary circuit protection of 500V or circuit of even higher voltage.

Just as what has been mentioned above (Please refer to Rated Value Decrease.), the fuse is sensitive to current changes but not to voltage changes. The fuse will keep its original state under any voltage between zero and its maximum rated value. Circuit voltage and active power become problem only after the fuse has melted and caused electric arc. Safe melt of the circuit is relative to circuit voltage and active power, which has been discussed in Blowout Value.

Generally speaking, fuses can be used without any damage to its melting features under any voltage which is smaller than its rated voltage. If the maximum power electrical level, which occurs in the fuse under the condition of complete shortcut circuit, can only low-energy and non-destructive electric arc, the fuse can be used under various voltages which are higher than its already-tested rated voltage value.

Nominal Melting Performance I2t Derivation:Conduct laboratory test to every fuse to confirm the energy needed to melt the part. This energy is called Nominal Melting Performance I2t. The testing method is like this, inflict a current increase to the fuse and measure the time of occurring melting. If there is no melting phenomenon within about 0.008 seconds or even shorter time, we should increase the intensity of impulse current. Repeat this testing procedure until the melting of the fuse part is limited to about 0.008. The aim of conducting this testing procedure is to ensure that the procreant heat energy has not enough time to dissipate by heat conducting through the fuse part. That is to say, all of the I2t is used for melting. Once the testing result of the current (I)and the time (t) have been confirmed, it is very easy to calculate melting heat energy I2t. When the melting process finishes, there occurs electric arc first and then the fuse is disconnected. The nominal I2t value given in this publication belongs to elimination, namely the section of the melting state of disconnection.


   Fuse Choosing Guide


Various factors involved in choosing fuse are listed as follows:

1. Normal working current.

2. Application voltage(AC or DC).

3. Environmental temperature.

4. Overcurrent and time the fuse must need to melt.

5. Maximum fault current.

6. Impulse current, impact current, inrush current, startup current and circuit transient.

7. Structure dimension limit, such as length, diameter or height.

8. Necessary institution appraisal documents, such as UL, CSA, VDE and other military affairs department and institutions.

9. Items that should be considered, such as easiness to replace, axial lead, eyeballing indication and so on.

10. Fuse holder parts: fuse clip, mount box, panel fixing, P.C. board fixing, RF interference screen and so on.

Normal Working Current:When operating at 25℃, the rated current value of the fuse should be decreased by 25% to avoid interfering the melting. For example, a fuse with its rated current value being 10A is usually not recommended to operate under the environmental temperature of 25℃and under the current larger than 7.5A. Please refer to the mentioned-above Rated Value Decrease and the following Environmental Temperature for details.

Voltage:Rated voltage value of the fuse must be equal to or bigger than efficient circuit voltage. Please see Voltage Rated Value for exceptions.

Environmental Temperature:Current carrying capability test of the fuse is conducted under the environmental temperature of 25℃ and this kind of test is affected by the environmental temperature changes. The higher the environmental temperature, the higher the fuse working temperature and accordingly the shorter the service life. On the contrary, fuse service life can be prolonged if operated under low temperature. When the normal working current reaches or surpasses the rated current of the fuse that we have chose, the fuse will gradually become hotter and hotter. The experiment shows that when under room temperature, if operate under the condition of no more than 75% of the fuse rated current, the fuse can be operated without expiration.

The following is the model curve list of environmental temperature’s effect on the current carrying capability.


  In this picture:

Curve A:Curve of traditional slow-blowout fuse.

Curve B:Curve of super quick blowout, quick blowout and spiral winding.

Curve C:Curve of resettable polymer protector.

*Apart from the decrease of the rated value, there is still the effect of the environmental temperature. Please see the example for details.

Eg: On a certain condition, the given normal current is 1.5A. If use a traditional slow blowout fuse to work under room temperature, then

Listed Fuse Rated Value=Normal Working Main Current/0.75

Namely 1.5A/0.75=2.0A fuse (at 25℃)

Similarly, if the same fuse works at the high environmental temperature of 70℃, working current should be decreased repeatedly. Traditional slow blowout fuse indicates that when curve A in the environmental temperature curve list is 70℃, the biggest operation rated value percentage is 80%. Under this condition,

Listed Fuse Rated Value =Normal Working Main Current/0.75*rated value percentage

Namely 1.5A/0.75*0.80=2.5A fuse (at 70℃)

Overcurrent Circumstance:This is the current intensity when the circuit needs protection. The fault circumstance can be indicated through the carrying current before the destruction occurs or the longest time that it can bear. In order to try matching fuse features and circuit requirements, we should consider Time-Current Features Curve, meanwhile we should also consider the case at times that the Time- Current Features Curve is based on the average value.

Maximum Fault Current:Melting rated value of the fuse must satisfy or surpass the maximum fault current amount in the circuit.

Impulse:The ordinary term of impulse is used to describe various wave shapes in this article and these wave shapes can be called impact current, startup current, inrush current and transient. The circumstances of electric impulse differ greatly because of different application occasions. For a special impulse circumstance, different fuse structures take different reactions. Electrical impulse produces heat circulation and produces mechanic fatigue which can affect service life of the fuse. Startup impulse is normal for some practical occasions, in which the slow blowout fuse must be used. Slow blowout fuse has heat lag design, which can make the fuse be kept in good condition under normal startup impulse as well as be able to provide protection for long-time overloading. We should confirm the startup impulse and make a comparison between it and the Time-Current Features Curve and I2t rated value of the fuse. We recommend to undertaking utility test to ensure that the fuse can bear the design capability of the impulse circumstances.

Nominal melting heat energy I2t is the measure to the energy required by the would-melting component and is read as A2Sec. This nominal melting heat energy I2t together with the energy it represents (within 8 milliseconds, namely 0.008s, or even shorter time) is a constant value to every kind of different fuse parts. Because there are different fuse components to every kind of fuse type, rated value and corresponding part code, I2t value of each kind of fuse should be confirmed. I2t value is a parameter of the fuse itself and its determinant is the component material and shape. Apart from choosing fuse according to Normal Working Current, Rated Value Decrease and Environmental Temperature which have already been mentioned above, we must also use I2t design method. For each fuse component design, the nominal melting heat energy I2t is not only a constant but also isn’t relative to temperature nor voltage. The method of nominal melting heat energy I2t in fuse choosing is most frequently seen on some practical occasions where the fuse must bear big current impulse in short time. These high-energy currents are commonly seen on many practical occasions and there are many terms used for describing these high-energy currents, such as impact current, startup current, inrush current and other similar circuit transient which can be put into the common type of impulse. Each kind of fuse design is conducted with laboratory test to confirm its rated value of nominal melting heat energy I2t. The I2t value given in this publication is nominal and representative. If this parameter is a limit value in the design analysis, subscribers should consult with the manufacturers. The following case is helpful for better understanding the application of I2t.

Eg. Choose a kind of 125V, super quick blowout and Pico-typed fuse, which can bear 100,000 times of impact given by the impulse wave shape in Picture A with its normal working current being 0.75A and the environmental temperature being 25℃.

Step 1. Refer to Picture 1 and choose suitable wave shape. The wave in this example is E wave shape. Introduce the suitable value of the maximum impulse current (ip) and the time into the corresponding formula of E wave shape and then calculate as follows:


This value is called Impulse I2t.

Step 1. Refer to Picture II and confirm the required I2t value of the nominal melting heat energy. Picture II shows what has been calculated in step 1, namely the I2t value is 22% when there occurs 100,000 times of impulse. The way of transferring this impulse I2t value into the required nominal melting heat energy I2t value is as follows:

Nominal Melting Heat Energy I2t=impulse I2t/0.22=0.0512/0.22=0.2327A2Sec

Step 3. Inspect the rated I2t value of Pico II type, 125V and super quick blowout fuse. The part code is 251001, the rated value of 1ampere design is regulated as 0.281A2Sec, which can be able to suit the minimum fuse rated value of the peak value of 0.2327A2Sec calculated in step 2. Just like what has been mentioned above, when the decrease coefficient of 1 ampere rated value is 25%, this 1-ampere fuse will also suit the regulated normal working current of 0.75A.

Test: The above-mentioned factors should be considered when choosing fuses for given practical occasions. The next step needs to conduct tests on some samples in practical circuits to validate the choice. Before assessing the samples, we should ensure that the fuse is correctly fixed with high quality connection wire and enough-dimension lead. This test contains service life test under normal condition and overloading test under fault condition so as to make sure that the fuse can operate normally in the circuit.




Various items of rated value and other performance guidelines of the fuse are tested according to the experimental conditions and inspection criterion. Inspection criterion is set as per one or several kinds of fuse standards. It is very important to master these standards in order to correctly choose fuses for circuit protection.

American UL and Canadian CSA248.14 Assisting Overcurrent Protection Fuse (maximum 600V)(Originally called UL198G and CSA C22.2, No. 59.)

UL Classification

Fuse of UL classification meets each item of requirement of UL/CSA248.14 standard. The following is some requirements.

The test of UL ampere rated value is conducted when the current is 110%, 135% and 200% of the rated current value. The fuse must bear current that is 110% of the rated value and must be stable when the temperature not higher than 75℃.

The fuse must be disconnected within one hour under the current that is 135% of the rated value. When the current is 200% of the rated value and the rated value is lower than 30A, the fuse must be disconnected within 2 minutes. And while the rated value is between 35A and 60A, the fuse must be disconnected within 4minutes.

The melting capability of UL classification has the minimum of 10,000 Ampere AC when under the rated voltage of 125V. For those fuses whose rated voltage are 250, they can be classified as having the 10,000 Ampere of melting capability when under the voltage of 125V. And they at least have the minimum melting rated capacity listed below when under the voltage of 250V.

Fuse Ampere Rated Value
Fuse Rated Capability (Ampere)
Voltage Rated Value

      According to Underwriters Laboratories UL Component Precis Recognition (UR)

UL certification précis is different from UL classification. UL tests fuses according to the specifications required by the manufacturers. If the fuse is designed for an exclusive function, the testing point may be different from the requirements of the UL classification. For those fuses approved by component précis, UL requires application Appraisal documents.

UL275 Automobile Glass Tube Fuse (32V)


The test of UL ampere rated value is conducted when the current is 110%, 135% and 200% of the rated current value.It requires no melting capability tests.

CSA Certificate

Canadian CSA certificate is similar to American UL Classification Appraisal Document and its component précis is also similar to that of American UR. This component précis is very convenient for the manufacturers’ setting specifications and then the CSA’s testing the results.

MITI Appraisal Document

Japanese MITI Appraisal Document is similar to the UL Classification Appraisal Document of America.

International Electric Committee (IEC)

Publication 127, page 1, 2, 3 and 5. (250V)

IEC organization is different from American UL and Canadian CSA because IEC only sets regulations without issuing the appraisal documents.

UL and CSA set regulations and take charge of testing and issuing appraisal documents. IEC regulations appraisal documents are issued by SEMKO, (Sweden Electric Equipment Testing and Appraisal Institution), BSI (British Standard Institution), UL and CSA.

IEC Publication 127 defines two kinds of disconnection capability grades (melting rated capability). Fuse of low disconnection capability must pass current test of 35A or 9 times higher than the rated current or even higher. Fuse of high disconnection capability must pass current test of 1500A

Page 1-F-modeled quick operating and high disconnection capability.

Page 2-F-modeled quick operating and low disconnection capability.

Page 3 -T-modeled prolonging and low disconnection capability.

Page 4-T modeled prolonging and high disconnection capability.

The letter of F and T represents the Time-Current Features of F-modeled fuse and T-modeled fuse. F or T will be labeled on the end cap of the fuse.

Disconnection time of IEC127 and UL/CSA248-14 fuses.

Rated Value Percentage UL Standard 248-14 IEC F Page 1 and Page 2 IEC T Page 3 IEC T Page 5
4Hours Minimum
60 Minutes Maximum
60 Minutes Minimum
60 Minutes Minimum
60 Minutes Minimum
2 Minutes Maximum
30 Minutes Maximum
2 Minutes Maximum
30 Minutes Maximum

  IEC also has the testing requirements of 275%, 400% and 1000% of the rated value. However, this blank is used for indicating that the fuses, which are made as per different specifications and has the same ampere rated value, can not be interchanged. According to the standard of IEC127, fuse with its ampere rated value being 1can operate under the condition of 1 ampere. According to the standard regulated by UL Standard 198G, fuse with its ampere rated value being 1can not operate under the condition of more than 0.75 ampere. (Decrease by 25%-Please see the Rated Value Decrease of the Fuse Ology. )

Military Use/Federal Standard (Please refer to the Military Articles-Content)

Mil-F-15160 and Mil-F-23419

This regulation determines the structure and performance of the fuse which is mainly suitable for military electric use.


This regulation determines the structure and performance of the fuse clip which is suitable for military electric use.


This regulation determines the structure and performance of the canister lamp, camera lens and lamp rack, which is suitable for military electric use.

DESC Picture#87108

This picture sample determines the structure and performance of 4.50mm*0.588mm(2AG dimension) canister fuse and axial lead model which is suitable for military use. The symbol of DESC#87108si concluded in the mark of the fuse end cap.

Federal SpecificationsW-F-1814

This regulation determines the structure and performance of the canister fuse, which has high melting rated capacity and which is appraised for federal use.

Please write to the following institutions if claiming for other data or specification duplicate copy about various standard appraisal documents.

Underwriters Laboratories Inc. (UL)

Underwriters Laboratories Inc. (UL)

333 Pfingsten Road

Northbrook,IL 60062

Att:Publications Stock

Canadian Standard Association (CSA)

Canadian Standards Association (CSA)

178 Rexdale Boulevard

Rexdale, Ontario, Canada M9W 1R3

Att: Standard Sales

International Electric Committee (IEC)

3,Rue de Varembe

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