FUSE SELECT

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.

Fuse Construction And Operation

The typical fuse consists of an element which is surrounded by a filler and enclosed by the fuse body. The element is welded or soldered to the fuse contacts (blades or ferrules). The element is a calibrated conductor. Its configuration, its mass, and the materials employed are selected to achieve the desired electrical and thermal characteristics. The element provides the current path through the fuse. It generates heat at a rate that is dependent upon its resistance and the load current. The heat generated by the element is absorbed by the filler and passed through the fuse body to the surrounding air. A filler such as quartz sand provides effective heat transfer and allows for the small element cross-section typical in modern fuses. The effective heat transfer allows the fuse to carry harmless overloads. The small element cross section melts quickly under short circuit conditions. The filler also aids fuse performance by absorbing arc energy when the fuse clears an overload or short circuit. When a sustained overload occurs, the element will generate heat at a faster rate than the heat can be passed to the filler. If the overload persists, the element will reach its melting point and open.

Increasing the applied current will heat the element faster and cause the fuse to open sooner. Thus fuses have an inverse time current characteristic, i.e. the greater the overcurrent the less time required for the fuse to open the circuit. This characteristic is desirable because it parallels the characteristics of conductors, motors, transformers and other electrical apparatus. These components can carry low level overloads for relatively long times without damage. However, under high current conditions damage can occur quickly. Because of its inverse time current characteristic, a properly applied fuse can provide effective protection over a broad current range, from low level overloads to high level short circuits.

Ambient Temperature  

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.

CHART SHOWING EFFECT OF AMBIENT TEMPERATURE ON CURRENT-CARRYING CAPACITY

Interrupting Rating (Abbreviated I.R.)  

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   

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  

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  

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

Dimensions  

The fuses in this catalog range in size from the chip size .24"L x .1"W x .1"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.).

Element  

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 

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.

Fuse 

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

Fuse Selection Guide  

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  

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.

Overcurrent  

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

Overload  

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)  

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

Power Factor  

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.

Selectivity  

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  

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  

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  

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

Time-Current Curve  

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  

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  

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  

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.

Polymeric Positive Temperature Coefficient (PPTC) Devices

Resettable fuses are designed and made of patented novel polymeric PTC material in thin chip form. With electrodes and leads attached on both sides, it is placed in series to protect a circuit. At 搉ormal operating condition?the device remains at an extremely low resistance (mini-ohms) and allows the electrical current to flow through it without any restriction. When overcurrent conditions occur, the polymeric PTC material heats up and its resistance increases sharply. Such a sharp resistance increase (to an insulated status) cuts off the current in the circuit, and consequently protects the element and device in the circuit. Upon fault condition being removed, the resettable fuse cools and its resistance drops to the original extremely low value. The resettable fuse is 搑esetted?and allows the current through the circuit again.

Standards.

UL Listed

This symbol, granted by the U.S. agency, guarantees that a fuse has been manufactured in full compliance with the UL UL/CSA/ANCE (Mexico) 248-14 standard FUSES FOR SUPPLEMENTARY OVERCURRENT PROTECTION (600 Volts, Maximum) (Former UL 198G and CSA C22.2, No. 59). Some of the requirements are as follows:

       UL ampere rating tests are conducted at 100%, 135%, and 200% of rated current. The fuse must carry 110% of its ampere rating and must stabilize at a temperature that does not exceed a 75C rise at 100%. 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.

Fuse Amperage,
Amp

Fuse I.R.,
Amp

Fuse Rated Voltage,
Volt

0 to 1

35

250VAC

1.1 to 3.5

100

250VAC

3.6 to 10

200

250VAC

10.1 to 15

750

250VAC

15.1 to 30

1500

250VAC

CSA Certification

This symbol, granted by the Canadian agency, guarantees that a fuse or holder has been manufactured in full compliance with the CSA C22.2 No. 248.14 or CSA C22.2 No. 39 standard respectively. It's equivalent to UL Listing in the U.S.A.

SEMKO Approval

This symbol, granted by the Swedish agency, guarantees that a fuse or holder has been manufactured in full compliance with the appropriate section of the IEC 60127 standard.

VDE Approval

This symbol, granted by the German agency, guarantees that a fuse or holder has been manufactured in full compliance with the appropriate section of the IEC 60127 standard.

BSI Kitemark License

This symbol, granted by the British agency, guarantees that a fuse has been manufactured in full compliance with the appropriate section of the IEC 60127 (BS 4265) standard.

UL Recognition

UL抯 Component Recognition Program allows the testing of components (including fuses and holders) for which no UL standard exists or where only certain sections of a particular UL standard are referenced. A fuse or holder may be submitted to UL for testing according to criteria defined by the manufacturer. If basic safety requirements are met during testing and the component performs as predicted, it can be UL Recognized. Normally, parts bearing these marks are incorporated into equipment manufactured by an OEM and determined based on the electrical characterisrics of the product designed to provide protection for the components in the equipment. Fuses built to the European IEC 60127 fuse standard (with SEMKO, VDE, and/or BSI approvals) are technically qualified to apply for UL Recognition.

MITI Approval (same as Dentori Approval)

This symbol, granted by the Japan Electrical Testing Laboratory, guarantees that a fuse has been manufactured in full compliance with the Japanese MITI standard. This document is similar to UL 248-14 with subtle differences in voltage ratings and breaking capacity criteria.

International Electrotechnical Commission (IEC)

The IEC organization is different from UL and CSA, since IEC only writes specifications and does not certify. UL and CSA write the specifications, are responsible for testing, and give certification. Certification to IEC specifications are given by such organizations as SEMKO (Swedish Institute of Testing and Approvals of Electrical Equipment) and BSI (British Standards Institute), as well as UL and CSA. IEC Publication 127 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 finally high breaking capacity fuses must pass a test of 1500 amperes.
 
60127 Part 2

Sheet I -

Type F Quick Acting, High Breaking Capacity

Sheet II -

Type F Quick Acting, Low Breaking Capacity

Sheet III -

Type T Time Lag, Low Breaking Capacity

Sheet V -

Type T Time Lag, High Breaking Capacity

Sheet VI -

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.
 
UL/CSA/ANCE (Mexico) 248-14 vs. IEC 127 FUSE OPENING TIMES (UL/CSA/ANCE (Mexico) 248-14 Was Previously UL 198G and CSA 2.2, No. 59)
 

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 5
Sheet 5(*)

110

4 Hr.

135

60 Min.
Max.

150

60 Min.
Min.

60 Min.
Min.

60 Min.
Min.

60 Min.
Min.

200

2 Min.
Max.

210

30 Min.
Max.

30 Min.
Max.

30 Min.
Max.

30 Min.
Max.

(*) Note: The IEC Specification is only written up to 6.3A, any components above these ratings are not recognized by the IEC (although the fuses may have those opening characteristics).
 
IEC also has 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 127 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.

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