Fuses - types of fuses (2023)

Definition and specifications of automotive fuse links

Automotive Use Links are automatic disconnect devices to protect electrical equipment from improper current loads. The flow of current is interrupted by the melting of the fuse wire in which the current is flowing.

The following international regulations and recommendations apply to fuse links in their currently valid version:

  • DIN 72581
  • DIN 43560
  • ISO 8820
  • UL275
  • SAE

(Furthermore, the state of the art, the details of the currently valid implementation regulations, the safety principle "People, animals and property must be protected from danger" and the qualification of the installed components must be taken into account - personal responsibility of the manufacturer of electrical devices.)

Selection statements and recommendations

The nominal voltage (UN) of a fuse link must be at least equal to or greater than the operating voltage of the device or assembly to be protected with the fuse link. In the case of very low operating voltages, the intrinsic resistance (voltage drop) of the fuse link may have to be taken into account.

The voltage drop (UN) is measured according to standards, e.g. DIN, ISO, JASO, sometimes Littelfuse standard maximum values ​​are also given.

The rated current (IGears) of a fuse link should roughly correspond to the operating current of the device or assembly to be protected (according to the ambient temperature and rated current definition, i.e. the permissible continuous currents).

Higher ambient temperatures (Ta G) mean an additional load for the fuse links. Especially with high rated currents of the fuses and strong heat radiation from neighboring components, the heating conditions of the maximum occurring ambient temperature must be checked. For such applications, the fuse should be reduced according to the following diagram or table (see factor FT):

Due to different rated current specifications, the recommended continuous current of the fuse links is max. 80% of their rated current (at an ambient temperature of 23 °C), see also fuse-specific ampacity (F) on the individual catalog pages.

The limits for the pre-ignition time indicate the relationship between melting time and current. (They are shown as an envelope for all rated currents mentioned.)

The melting integral (I2t) results from the square of the melting current and the associated melting time. In the case of overcurrent with melting times < 5 ms, the melting integral remains constant. The data in this catalog are based on 6 or 10 x Irat. The melting integral is a measure of the time-current characteristic and provides information about the impulse constancy of a fuse link. The melting integrals mentioned are typical values.

The breaking capacity (IB) should be sufficient for all operating and fault conditions. The short-circuit current (maximum fault current) which the fuse-links are intended to interrupt at rated voltage under standard conditions must not be higher than the current corresponding to the breaking capacity of the fuse-link.

The maximum power loss (pv) is determined when loaded with rated current after temperature equilibrium has been reached. During operation, these values ​​can occur for some time.

Typical values ​​are specified as well as the guideline values ​​for standard-compliant fuses.

Selection of automotive fuse links

With regard to the product safety of the device and the service life/reliability of the fuse links, it is important to make the right choice. A clear function of the fuse links as a protective component (rated breaking point) The responsibility of the manufacturers of electrical equipment applies here:

"Any person who is involved in the manufacture of electrical systems or the manufacture of electrical equipment, including those who are involved in the operation of such systems or equipment, is responsible for compliance with the recognized rules and procedures in every respect according to the present legal opinion of electrical engineering."

  1. The required nominal voltage of a fuse link results from its required operating voltage (taking into account the voltage drop of the fuse link).
  2. The ampacity of a fuse link (IN fuse) is determined by the max. effective current load (Ioperating max) taking into account the ambient temperature (factor FT) and the different nominal current definitions (definition "constant current") (see factor FI). The following applies: iN fuse3IBetrieb max. xFIxFT
  3. t-value (current-time integral).2In the case of impulse loads and for semiconductor protection, a suitable current carrying capacity can also be determined using the I
  4. The above two points will help you determine the most suitable amperage of a fuse link and its melting time limits (check experimentally if necessary).
  5. The required breaking capacity of the fuse link is determined by the maximum possible residual current that can occur.
  6. In addition to the points mentioned above, the type of installation is also important for the correct selection of the fuse link (taking into account possible approvals).

With regard to the specific conditions of the respective application (product safety), it is generally necessary to check the fuse link and/or the thermal circuit breaker or holder in the device to be protected under normal and fault conditions!

temperature rating curve

Fuses - types of fuses (1)

Fuse link derating

Ta G/ °C%FTTa G/ °C%FT

Fuse selection for electronics applications

Many of the factors to consider when selecting fuses for electronic applications are listed below. You can find more information in ourBackup Technology Reference GuideorContactyour local Littelfuse product representative:

selection factors

  1. Normal operating current
  2. Application voltage (AC or DC)
  3. ambient temperature
  4. Overload current and length of time the fuse must open
  5. Maximum available leakage current
  6. Impulses, surge currents, inrush currents, starting 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 characteristics (mounting style/form factor, ease of disassembly, axial ports, visual indication, etc.)
  10. Fuse holder features, if applicable, and associated re-evaluation (clips, mounting block, panel mount, PCB mount, R.F.I. shield, etc.)
  11. Application testing and verification prior to production

Littelfuse fuse packaging and part numbering systems

Fuses - types of fuses (2)Fuses - types of fuses (3)

definitions and terms

Ambient temperature:

Refers to the temperature of the air immediately around the fuse and is not to be confused with "room temperature". The ambient temperature of the fuse is in many cases significantly higher because it is enclosed (as in a panel mount fuse holder) or mounted close to other heat generating components such as resistors, transformers, etc.

breaking capacity:

Also known as breaking capacity or short circuit capacity, this is the maximum current rating that the fuse can safely interrupt at rated voltage. See the break score definition in this section for more information.

Current rating:

The current rating of the fuse. It is specified by the manufacturer as the current value that the fuse can carry, based on a controlled set of test conditions (see EVALUATION).

Catalog fuse part numbers include serial designation and amperage rating. See the FUSE SELECTION GUIDE section for guidance on making the right choice.


At ambient temperatures of 25 °C, it is recommended that fuses be operated with no more than 75% of the rated current determined under controlled test conditions. These test conditions are part of UL/CSA/ANCE (Mexico) 248-14 “Fuses for Supplementary Overcurrent Protection”, the main objective of which is to establish common test standards required for continuous control of manufactured articles for protection against fire etc. Some common variations on these standards include: fully enclosed fuse holders, high contact resistances, air movement, transient voltage spikes, and changes in interconnect wire size (diameter and length). Fuses are essentially temperature sensitive devices. Even small deviations from the controlled test conditions can greatly affect the predicted life of a fuse when charged to its rating, which is 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 consistent performance standards for their products and must take into account the variable conditions of their application. To compensate for these variables, the circuit designer designing glitch-free, long-life fuse protection in their devices will generally load their fuse no more than 75% of the manufacturer's specified rating, bearing in mind that overload and short-circuit protection must be adequately provided.

The fuses discussed are temperature sensitive devices and are rated in a 25°C environment. The fuse temperature generated by the current flowing through the fuse increases or decreases as the ambient temperature changes.

The Ambient Temperature Chart in the FUSE SELECTION GUIDE section illustrates the effect of ambient temperature on a fuse's current rating. Most traditional Slo-Blo® fuse designs use lower melting temperature materials and are therefore more sensitive to changes in ambient temperature.


Unless otherwise noted, dimensions are in inches.

The fuses in this catalog range in size from approximately 0402 chip size (0.041"L x 0.020"W x 0.012"H) up to 5 AG, also commonly known as a "MIDGET" fuse (13/32" diameter x 11 /2 inch length). As new products were developed over the years, fuse sizes have evolved to meet various electrical circuit protection requirements.

The first fuses were simple open wire devices, followed in the 1890s by Edison's thin wire housing in a lamp base to produce the first plug-in fuse. By 1904, Underwriters Laboratories had established size and rating specifications to meet safety standards. Renewable fuses and automotive fuses appeared in 1914, and in 1927 Littelfuse began making very low amperage fuses for the burgeoning electronics industry.

The fuse sizes in the table below started with the early "automotive glass" fuses, hence the term "AG". The numbers were applied chronologically as different manufacturers started making a new size: "3AG" for example was the third size to be launched. Other non-glass fuse sizes and designs were dictated by functional requirements, but still retained the length or diameter dimensions of glass fuses. Their designation was changed to AB instead of AG, indicating that the outer tube was made of bakelite, fiber, ceramic, or some similar material other than glass. The largest fuse shown in the table is the 5AG or "MIDGET", a name adopted by the electrical industry and the National Electrical Code range, which normally recognizes 9/16 inch x 2 inch fuses as the smallest standard fuse in use .

Industrial fuses and how they work

Look theLittelfuse POWR-GARD-Katalogfor complete information on fuse selection

An important part of designing quality overcurrent protection is understanding the system requirements and basics of overcurrent protection devices. This section covers these topics with special attention to the application of fuses. If you have additional questions, please contact our Technical Support and Engineering Services Group at 1-800-TEC-FUSE (1-800-832-3873).

Why overcurrent protection?

All electrical systems eventually experience overcurrents. If not removed in time, even moderate overcurrents will quickly overheat system components and damage insulation, conductors, and equipment. Large overcurrents can melt conductors and vaporize insulation. Very high currents create magnetic forces that bend and twist busbars. These high currents can pull cables from their connectors and damage insulators and spacers.

Too often, uncontrolled overcurrents are accompanied by fires, explosions, toxic fumes and panic. Not only does this damage electrical systems and equipment, but it can also injure or kill people in the vicinity.

To reduce these hazards, the National Electrical Code® (NEC®), OSHA codes, and other applicable design and installation standards require overcurrent protection that disconnects overloaded or faulty equipment.

Industry and government organizations have developed performance standards for overcurrent protective devices and test methods that demonstrate compliance with the standards and the NEC. These organizations include: the American National Standards Institute (ANSI), the National Electrical Manufacturers Association (NEMA), and the National Fire Protection Association (NFPA), all associated with Nationally Recognized Testing Laboratories (NRTL) such as Underwriters Laboratories (UL).

Electrical systems must meet applicable regulatory requirements, including those for overcurrent protection, before electric utilities can supply power to a facility.

What is high quality overcurrent protection?

A system with high-quality overcurrent protection has the following properties:

  • Meets all legal requirements such as NEC, OSHA, local regulations, etc.
  • Provides maximum security for staff and exceeds minimum regulations when required.
  • Minimizes overcurrent damage to property, equipment and electrical systems.
  • Provides coordinated protection. Only the protection device immediately on the line side of an overcurrent opens to protect the system and minimize unnecessary downtime.
  • Is cost effective while providing reserve interrupting capacity for future growth.
  • Consists of equipment and components that are not subject to obsolescence and require minimal maintenance that can be performed by regular maintenance personnel using readily available tools and equipment.

Overcurrent Types and Effects

An overcurrent is any current that exceeds the amperage rating of conductors, equipment, or devices under conditions of use. The term "overcurrent" includes both overloads and short circuits.


An overload is an overcurrent that is limited to normal current paths where there is no insulation breakdown.

Persistent overloads are often caused by the installation of excessive equipment such as additional lighting fixtures or too many motors. Persistent overloads are also caused by mechanical equipment overload and equipment failures such as B. caused failed bearings. If not disconnected within specified time limits, sustained overloads will eventually overheat circuit components and cause thermal damage to insulation and other system components.

Overcurrent protection devices must disconnect circuits and devices subject to continuous or sustained overload before overheating occurs. Even moderate overheating of the insulation can significantly reduce the life of the components and/or equipment involved. For example, motors overloaded by only 15% may experience less than 50% of the normal insulation life.

Temporary overloads are common. Common causes are temporary overloads of devices, e.g. when a machine tool makes a cut that is too deep, or simply starting an inductive load, e.g. B. an engine. Because transient overloads are harmless by definition, overcurrent protection devices should not open or disconnect the circuit.

It is important to know that the fuses selected must have a sufficient time delay to allow the motors to start and momentary overloads to subside. However, should the overcurrent persist, fuses must blow before system components are damaged. Littelfuse POWR-PRO® and POWR-GARD® time delayed fuses are designed to meet these types of protection requirements. In general, time delayed fuses will hold 500% of rated current for at least 10 seconds, but will still open quickly at higher current ratings.

Although government mandated high efficiency motors and NEMA Design E motors have much higher locked rotor currents, POWR-PRO® time delayed fuses such as the FLSR_ID, LLSRK_ID or IDSR series have sufficient time delay to allow motors to start , if the fuses are correctly selected according to NEC®.

short circuits

A short circuit is an overcurrent that flows outside of its normal path. Types of short circuits are generally classified into three categories: bolt faults, arcing faults, and ground faults. Each type of short circuit is defined in the Terms and Definitions section.

A short circuit is caused by an insulation breakdown or a faulty connection. During normal operation of a circuit, the connected load determines the current. When a short circuit occurs, the current bypasses the normal load and takes a 'shorter path', hence the term 'short circuit'. Because there is no load impedance, the only factor limiting current flow is the impedance of the entire distribution system from the utility generators to the fault point.

A typical electrical system might have a normal load impedance of 10 ohms. But in a single-phase situation, the same system can have a load impedance of 0.005 ohms or less. The best way to compare the two scenarios is to apply Ohm's law (I = E/R for AC systems). A single phase 480 volt circuit with a 10 ohm load impedance would draw 48 amps (480/10 = 48). If the same circuit has a system impedance of 0.005 ohms when the load is shorted, the available fault current would increase significantly to 96,000 amps (480/0.005 = 96,000).

As said, short circuits are currents that flow outside of their normal path. Regardless of the magnitude of the overcurrent, the excess current must be removed quickly. If not removed immediately, the large currents associated with short circuits can have three profound effects on an electrical system: heating, magnetic stress, and arcing.

Heating occurs in every part of an electrical system when current flows through the system. If the overcurrents are large enough, the heating is practically instantaneous. The energy at such overcurrents is measured in ampere-square-seconds (I2t). An overcurrent of 10,000 amps lasting 0.01 seconds has an I2t of 1,000,000 A2s. If the current could be reduced from 10,000 amps to 1,000 amps for the same time, the corresponding I2t would be reduced to 10,000 A2s, or just one percent of the original value.

If the current in a conductor increases 10 times, I2t increases 100 times. A current of just 7,500 amps can melt #8 AWG copper wire in 0.1 second. In eight milliseconds (0.008 seconds or half a cycle), a current of 6,500 amperes can raise the temperature of #12 AWG THHN thermoplastic insulated copper wire from its operating temperature of 75°C to its maximum short circuit temperature of 150°C. Larger currents can immediately vaporize organic insulation. Arcing at the fault location or from mechanical switching operations such as automatic transfer switches or circuit breakers can ignite the fumes and cause violent explosions and electrical flashes.

Magnetic stress (or force) is a function of peak current squared. Fault currents of 100,000 amps can exert forces in excess of 7,000 pounds per foot of bus bar. Stresses of this magnitude can damage insulation, pull conductors from terminals, and stress equipment terminals sufficiently to cause significant damage.

Arcing at the fault location melts and vaporizes all conductors and components involved in the fault location. The arcs often burn through ducts and equipment enclosures, showering the area with molten metal that can quickly start fires and/or injure people in the area. Additional shorts often occur as vaporized material deposits on insulators and other surfaces. Persistent arcing faults will vaporize the organic insulation and the fumes can explode or burn.

Whether it is heating, magnetic loading and/or arcing, the potential damage to electrical systems as a result of short circuits occurring can be significant.

II. Selection Considerations

Fuse Selection Considerations (600 volts and less)

Because overcurrent protection is critical to the reliable operation and safety of electrical systems, careful consideration should be given to the selection and application of overcurrent devices. When selecting fuses, the following parameters or considerations must be evaluated:

  • Current rating
  • voltage value
  • pause assessment
  • Degree of protection and fuse ratings
  • current limit
  • physical size
  • Indication

General recommendations for industrial fuses

Based on the above selection considerations, the following is recommended:

Fuses with ampere ratings from 1/10 to 600 amps

  • When available fault currents are less than 100,000 amps and when equipment does not require the higher current-limiting characteristics of UL Class RK1 fuses, the FLNR and FLSR_ID Series RK5 Class current-limiting fuses offer superior time delay and cycling characteristics at a lower cost than RK1 fuses. If the available fault currents exceed 100,000 amps, the equipment may require the additional current limiting capabilities of the LLNRK, LLSRK, and LLSRK_ID series Class RK1 fuses.
  • JLLN and JLLS series Class T fast acting fuses have space saving characteristics that make them particularly suitable for protecting molded case circuit breakers, meter banks and similar applications where space is limited.
  • The JTD_ID and JTD series Class J time-delayed fuses are used in OEM motor control center applications, as well as other MRO motor and transformer applications requiring space-saving IEC Type 2 protection.
  • Class CC and Class CD series fuses are used in control circuits and switchboards where space is at a premium. Littelfuse POWR-PRO CCMR series fuses are best suited for protecting small motors, while Littelfuse KLDR series fuses offer optimal protection for control transformers and similar equipment.

For product application questions, call our technical support group at 800-TEC-FUSE.

Fuses with ampere ratings from 601 to 6,000 amps

For excellent protection of most general purpose and motor circuits, the use of POWR-PRO® KLPC Series Class L fuses is recommended. The Class L fuses are the only time delayed fuse series available in these higher amperage ratings.

For information on all of the above Littelfuse fuse series, please refer to the UL/CSA Fuse Ratings and Application Tables in the Technical Application Guide at the end of the POWR-GARD Product Catalog.

Industrial circuit protection checklist

To select the correct overcurrent protective device for an electrical system, circuit and system designers should ask themselves the following questions before designing a system:

  • What is the expected normal or average current?
  • What is the maximum expected continuous current (three hours or more)?
  • What inrush currents or transient surge currents are to be expected?
  • Are the overcurrent protective devices able to distinguish between expected inrush and surge currents and will they open during sustained overload and fault conditions?
  • Which environmental extremes are possible? Dust, humidity, extreme temperatures and other factors must be taken into account.
  • What is the maximum available fault current that the protection device is allowed to interrupt?
  • Is the overcurrent protection device designed for the system voltage?
  • Does the overcurrent protection device provide the safest and most reliable protection for the specific device?
  • Under short circuit conditions, does the overcurrent protection device minimize the possibility of a fire or explosion?
  • Does the overcurrent protection device meet all applicable safety standards and installation requirements?

The answers to these questions and other criteria will help determine the type of overcurrent protection device that should be used for optimal safety, reliability, and performance.

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