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Fuse Definitions

When specifying the use and application of any safety or protective device, it is obviously essential, particulary from a liability standpoint, that the terminology used is both correct and fully understood. This section briefly explains the most commonly used terms associated with electronic fuses.

Ambient Temperature

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The temperature of the surrounding medium which normally comes in contact with the fuse. The medium is usually air. The current carrying capacity tests of fuses are performed at 25C and will be affected by changes in ambient temperature. A fuse runs hotter as the normal operating current approaches or exceeds the current rating of the selected fuse. Practical experience indicates fuses at room temperature (25C) should last indefinitely if operated at no more than 75% of fuse ampere rating. The fuse ambient temperature is significantly higher in many cases, because it is enclosed or mounted near other heat producing components, such as resistors, transformers, etc.

Interrupting Rating (Abbreviated I.R.)

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Same as breaking capacity or short circuit rating. The maximum current a fuse can safely interrupt at rated voltage. Some special purpose fuses may also have a "Minimum Interrupting Rating". This defines the minimum current that a fuse can safely interrupt. Safe operation requires that the fuse remain intact. Interrupting ratings may vary with fuse design and range from 35 amperes AC for some 250V metric size (5 x 20mm) fuses up to 200,000 amperes AC for the 600V industrial fuses (for example, ATDR series).

Ampere Rating

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Same as Current Rating. The continuous current carrying capability of a fuse under defined laboratory conditions. The ampere rating is marked on each fuse. Continuous load current should not exceed 75% of fuse ampere rating (at 25C ambient) except Class L fuses and E rated fuses that may be loaded to 100% of their ampere rating.

Ampere Squared Seconds, I2t

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A measure of thermal energy associated with current flow. I2t is equal to IRMS2 x t, where t is the duration of current flow in seconds. It can be expressed as melting I2t, arcing I2t or the sum of them as Clearing I2t. Clearing I2t is the total I2t passed by a fuse as the fuse clears a fault, with t being equal to the time elapsed from the initiation of the fault to the instant the fault has been cleared. Melting I2t is the minimum I2t required to melt the fuse element.

Current-Limiting Range

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The available fault currents a fuse will clear in less than 1/2 cycle, thus limiting the actual magnitude of current flow.


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The fuses in this catalog range in size from the chip size .24"L x .01"W x .01"H (6.2L x 2.6W x 2.6H mm.) up to the 5AG, also commonly known as a "MIDGET" fuse 13/32" dia. x 11/2" length (10 x 38 mm.).


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A calibrated conductor inside a fuse which melts when subjected to excessive current. The element is enclosed by the fuse body and may be surrounded by an arc-quenching medium such as silica sand. The element is sometimes referred to as a link.

Fast-Acting Fuses

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Fast-acting fuses have no intentional built in slow-blow and are used in circuits without transient inrush currents. Fast-acting fuse opens on overload and short-circuits very quickly. This type of fuse is not designed to withstand temporaryoverload currents associated with some electrical loads.


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An overcurrent protective device containing a calibrated current carrying member which melts and opens a circuit under specified overcurrent conditions.

Fuse Selection Guide

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The fuse must carry the normal load current of the circuit without nuisance openings. However, when an overcurrent occurs the fuse must interrupt the overcurrent, limit the energy let-through, and withstand the voltage across the fuse during arcing. To properly select a fuse the followings must be considered:

  • Normal operating current (The current rating of a fuse is typically derated 25% for operation at 25C 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 25C ambient.)
  • Overload current and time interval in which the fuse must open.
  • Application voltage (AC or DC Voltage).
  • Inrush currents, surge currents, pulses, start-up currents characteristics.
  • Ambient temperature.
  • Applicable standards agency required, such as UL, CSA, VDE.
  • Considerations: Reduce installation cost, ease of removal, mounting type/form factor, etc.

Fuse Type
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There are three basic types of fuses:
(1) Slow Blow/Time Lag/ Time Delay fuses
(2) Fast acting fuses
(3) Very fast acting fuses
A major type of Time Delay fuse is the dual-element fuse. This fuse consists of a short circuit strip, soldered joint and spring connection. During overload conditions, the soldered joint gets hot enough to melt and the spring shears the junction loose. Under short circuit conditions, the short circuit element operates to open the circuit. Slow-blow fuse allows temporary and harmless inrush currents to pass without opening, but is so designed to open on sustained overloads and short circuits. Slow-blow fuses are ideal for circuits with a transient surge or power-on inrush. These circuits include: motors, transformers, incandescent lamps and capacitate loads. This inrush may be many times the circuit's full load amperes. Slow-blow fuses allow close rating of the fuse without nuisance opening. Typically, Slow Blow fuses are rated between 125% to 150% of the circuit's full load amperes.


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Any current in excess of conductor ampacity or equipment continuous current rating. Overcurrents take on two separate characteristics - overloads and short circuits.


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The operation of conductors or equipment at a current level that will cause damage if allowed to persist. Typical for this type of overcurrent is that it does not leave the normal current carrying path of the circuit - that is, it flows from the source, through the conductors, through the load, back through the conductors, to the source again.

Peak Let-Thru Current (Ip)

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The maximum instantaneous current passed by a current- limiting fuse when clearing a fault current of specified magnitude.

Power Factor

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Microfuses built to the UL/CSA standard and subminiature fuses built to the IEC 60127-3 standard have their breaking capacity tests conducted at a power factor of 0.95 to 1.0. Test set-ups on UL/CSA and IEC 60127-2 miniature fuses use a power factor of 0.7-0.8 with an exception for IEC 60127-2 glass fuses. Tests on these low breaking capacity types use a power factor of 1.0. Required power factors in IEC 60127-4 vary by breaking capacity category.


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A main fuse and a branch fuse are said to be selective if the branch fuse will clear all overcurrent conditions before the main fuse opens. Selectivity is desirable because it limits outage to that portion of the circuit which has been overloaded or faulted. Also called selective coordination.

Short Circuit

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Excessive current flow caused by insulation breakdown or wiring error. What happens is the current flow is shorted, so, it doesn't run through the load overcoming thus load resistance that limits the current value acording to Ohm's Law.

Soldering Recommendations

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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.

Threshold Current

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The minimum available fault current at which a fuse is current limiting.

Time-Current Curve

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A time-current characteristic curve for a specific fuse is shown as a continuous line and represents the opening time in seconds for that fuse for a range of overcurrents. The opening time is considered nominal unless noted otherwise. Several curves are traditionally shown on one sheet to represent a family of fuses.

Time Delay Fuse

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A fuse which will carry an overcurrent of a specified magnitude for a minimum specified time without opening. The specified current and time requirements are defined in the UL/CSA fuse standards.

Very Fast-Acting Fuses

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Very fast-acting (Current-Limiting) fuses will limit both the magnitude and duration of current flow under short circuit conditions. Because of their high current limiting ability, these fuses are frequently used to protect semiconductor circuits.

Voltage Rating

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The maximum voltage at which a fuse is designed to operate. Exceeding the voltage rating of a fuse impairs its ability to clear an overload or short circuit safely. Fuse can be used at any voltage below the fuse voltage rating; a 250V fuse can be used in 125V circuits. Voltage ratings are assumed to be for AC unless specifically labeled as DC.


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