Fuse Final

7/31/2019 Fuse Final

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S Ghosh

R S Rajput

Somenath Kundu

Pawan Kumar


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Fuse : An Introduction

A fuse is a short length of wire (link), connectedin series with the component to be protectedfrom overcurrent, designed to melt and

separate in the event of excessive current dueto the heat produced . Fuse blows, opening theentire circuit and stopping current through thecomponent.

Fuse wire, contained within a safety sheath,minimizes hazard of arc blast if the wire burnsopen with violent force, in the case of severeovercurrents.


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Cont.

Fuse serve two main purposes:

To protect components and equipmentfrom damage caused by overcurrents.

To isolate sub-systems from the mainsystem once a fault has occurred.

Fuses are rated in terms of their voltagecapacity as well as the current level at which

they will blow.


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Fuse : Basics

Increasing the applied current will heat the elementfaster and cause the fuse to open sooner. Thus fuseshave an inverse time current characteristic, i.e. thegreater the overcurrent the less time required for the

fuse to open the circuit. This characteristic is desirablebecause conductors, motors, transformers and otherelectrical apparatus can carry low level overloads forrelatively long times without damage. However, underhigh current conditions damage can occur quickly.

Because of its inverse time current characteristic, a

properly applied fuse can provide effective protectionover a broad current range, from low level overloads tohigh level short circuits.


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Time-Current Curve:


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Terminology

Ambient Temperature

The temperature of the surrounding medium.The medium is usually air. The current carrying

capacity tests of fuses are performed at 25Cand will be affected by changes in ambienttemperature. A fuse runs hotter as the normaloperating current approaches or exceeds thecurrent rating of the selected fuse. Practicalexperience indicates fuses at room temperature(25C) should last indefinitely if operated at nomore than 75% of fuse ampere rating.


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Cont.

Effect of

ambienttemp. onfuse rating


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Cont.

Interrupting Rating (I.R.)

Same as breaking capacity or short

circuit rating. The maximum current afuse can safely interrupt at ratedvoltage.

Ampere Rating or Current Rating

The continuous current carryingcapability of a fuse under definedlaboratory conditions.


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Cont.

Ampere Squared Seconds, I2t

A measure of thermal energy associated withcurrent flow.

I2

t = IRMS2

x t, t is the duration of current flow inseconds.

Clearing I2t = melting I2t + arcing I2t

Clearing I2t is the total I2t passed by a fuse asthe fuse clears a fault, with tbeing equal to the

time elapsed from the initiation of the fault tothe instant the fault has been cleared.

Melting I2t is the minimum I2t required to meltthe fuse element.


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Cont.Calculation of I2t rating For different waveforms


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Cont.

ElementA calibrated conductor inside a fusewhich melts when subjected to

excessive current. The element isenclosed by the fuse body and maybe surrounded by an arc-quenchingmedium such as silica sand. Theelement is sometimes referred to asa link.


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Cont. Overcurrent

Any current in excess of conductor ampere capacity orequipment continuous current rating. Overcurrents takeon two separate characteristics - overloads and shortcircuits.

OverloadThe operation of conductors or equipment at acurrent level that will cause damage if allowed topersist.

Short Circuit

Excessive current flow caused by insulationbreakdown or wiring error. What happens is thecurrent flow path is shorted, so, it doesn't runthrough the load thus overcoming load resistancethat limits the current value according to Ohm's Law.


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Cont. Time-Current Curve

A time-current characteristic curve for a specific fuse isshown as a continuous line and represents the openingtime in seconds for that fuse for a range of overcurrents.

Time Delay FuseA fuse which will carry an overcurrent of a specifiedmagnitude for a minimum specified time withoutopening.

Very Fast-Acting FusesVery fast-acting (Current-Limiting) fuses will limit boththe magnitude and duration of current flow under shortcircuit conditions. Because of their high currentlimitingability, these fuses are frequently used to protectsemiconductor circuits.


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Cont.

Voltage RatingThe maximum voltage at which a fuse isdesigned to operate. Exceeding thevoltage rating of a fuse impairs its abilityto clear an overload or short circuit

safely. Voltage ratings are assumed tobe for AC unless specifically labeled asDC.


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Cont.

Plug or screw-type fuses are typically usedin smaller sizes up to about 30 amps butare made in larger sizes, too. Cartridge

fuses are normally found in very smallelectrical devices and are also used on highcurrent-carrying circuits over 30 amps ormore in electric wiring applications. Both

types are made in non-delay and time-delay -- also called "slow blow" -- designs.


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TYPES OF FUSES Based on Time Response there are three basic types of

fuses: Slow Blow/ Time Lag/ Time Delay fuses Fast acting fuses

Very fast acting fuses Based on Technology there are two basic types of fuses: Cartridge type fuses, Glass fuses & HRC Fuses Semiconductor Fuses

Based on Physical Design:

plug fuses, sometimes called screw-base fuses; cartridge fuses.


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A major type of Time Delay fuse is the dual-element fuse. This fuse consists of a shortcircuit strip, soldered joint and springconnection. During overload conditions, the

soldered joint gets hot enough to melt and thespring shears the junction loose. Under shortcircuit conditions, the short circuit elementoperates to open the circuit.

Slow Blow Fuses


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Time-Delay Fuses The connector is held in place by low melting point

solder. With momentary overloads of 100 to 200percent of the fuse rating, neither element

is affected. But with a continuousoverload, heat builds up in thefuse to a point where the soldermelts, releasing the spring whichopens the contact. If there is a shortcircuit, the "burn out link meltsjust as in an ordinary fuse.


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Time-Delay Fuses (Cont.) The time-delay, or "slow blow", fuses were developed

for situations where acceptable momentary overloadsare encountered, such as starting a motor.As the name implies, thistype of fuse will carry an

overload several timesthe normal load for ashort period of time without blowing.


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Time-Delay Fuses (cont.) Slow-blow fuse allows temporary and harmless

inrush currents to pass without opening, but isso designed to open on sustained overloads andshort circuits. Slow-blow fuses are ideal for

circuits with a transient surge or power-oninrush. These circuits include: motors,transformers, incandescent lamps andcapacitate loads. This inrush may be manytimes the circuit's full load amperes. Slow-blowfuses allow close rating of the fuse without

nuisance opening.


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Fast-acting fuses have no intentionalbuilt in slow-blow and are used incircuits without transient inrushcurrents. Fast-acting fuse opens onoverload and short-circuits veryquickly. This type of fuse is not

designed to withstand temporaryoverload currents associated withsome electrical loads.

Fast Acting Fuses


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Cartridge Type Fuses Application: Automotive applications & industrial applications

Designed to be replaced easily in a circuit, inserted into some typeof holder rather than being directly soldered or bolted to thecircuit.

Enclosed Fuse Holderfor cartridge-type fuses is commonlyused for installation in equipment control panels, this type of fuse

holder completely encloses the fuse in an insulating housing:


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Cartridge Type Fuses

NH fuse : This is the tried and tested LowVoltage fuse type for general industrialapplications and it comes in a wide range ofsizes and ratings.

Chip fuses : These are made by depositingfusible elements on to a ceramic substrate.Using this procedure, fuses as small as3mm X 1.5mm can be made. These are

ideal for automated direct mounting on toPCBs for the protection of electronicequipment.


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CARTRIDGE TYPE FUSES


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HRC Fuse

The HRC (High Rupture Capacity)Fuse is used in high currentindustrial applications.

Fuse links of rating upto 200 Amps canbe used in HRC type porcelain fuseholders fuse fittings & also in

Switchfuse units.


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HRC Fuse Construction andOperation

The typical fuse consists of an element which issurrounded by a filler and enclosed by the fuse body.

The element is welded or soldered to the fusecontacts (blades or ferrules). The element is acalibrated conductor. Its configuration, its mass, and

the materials employed are selected to achieve thedesired electrical and thermal characteristics. Theelement provides the current path through the fuse.It generates heat at a rate that is dependent upon itsresistance and the load current.


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Cont. The heat generated by the element is absorbed by

the filler and passed through the fuse body to thesurrounding air. A filler such as quartz sandprovides effective heat transfer and allows for thesmall element cross-section typical in modern

fuses. The effective heat transfer allows the fuse tocarry harmless overloads.

The small element cross section melts quicklyunder short circuit conditions. The filler also aidsfuse performance by absorbing arc energy whenthe fuse clears an overload or short circuit. When asustained overload occurs, the element willgenerate heat at a faster rate than the heat can bepassed to the filler. If the overload persists, theelement will reach its melting point and open.


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Semiconductor Fuses These fuses are used to protect against over-current

conditions in semiconductors devices. Because of theirfast action, semiconductor fuses help to limit the shortcircuit current significantly.

Semiconductor fuses are available in several differentenclosures and in two different ranges: gR-type: full range protection. Overload and short

circuit protection. aR-Type: partial range protection. Short circuit

protection only. Faster acting than full range fuses.


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Contd. Semiconductor fuses are very compact and have

extremely fast acting trip characteristics. They haveexceptionally low I2t, low power losses and arc voltagesand provide short circuit and overload protection fordiodes, SCRs, IGBTs and many other types of power

semiconductors. These devices are commonly used in power equipment

including variable speed drives, power rectifiers, UPSsystems, DC power supplies and in a wide range ofelectronic equipment. By their very nature they aresensitive to overheating, overloads, voltage spikes,

surge and peak currents.


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FUSE SELECTION GUIDE Normal operating current (The current rating of

a fuse is typically derated 25% for operation at25 C to avoid nuisance blowing. For example, afuse with a current rating of 10A is not usuallyrecommended for operation at more than 7.5Ain a 25 C ambient.)

Overload current and time interval in which thefuse must open.

Application voltage (AC or DC Voltage).

Inrush currents, surge currents, pulses, start-upcurrents characteristics. Ambient temperature. Applicable standards agency required.


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Contd. Important performance ratings to consider when

specifying fuses include voltage rating, current ratingand interrupt rating.

The rated breaking capacity of the fuse (interruptingrating) is the short circuit current at which the fuse can

blow (at the rated voltage) without destruction or arcingbeing maintained. Important performance characteristics to consider

include fast acting and time lag. Quick-acting fuses are typically used in circuits with

small inrush currents such as resistive loads or

where overcurrent or short circuit must be quicklyinterrupted,. Time-delay fuses are used where high starting

inrush currents will occur and decay gradually, suchas inductive or capacitive loads.


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Contd.

Feature include resetting and indicating. Resettablefuses are unlike traditional fuses that have to bereplaced after a fault condition, this may not benecessary when using resettable fuses. The internalresistance of the component increases in response toan overcurrent or a short-circuit condition, limitingsubsequent disturbances. After the fault condition isremoved, the device cools and the internal resistancedrops to nearly its initial value, permitting normalequipment operation to resume. Indicating fuseshave visual indication and / or alarm activation. Usedfor telecommunication, computer and control circuitapplications.


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FUSE SELECTION FLOWCHART


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Fuse selection For UPS

Maximum Feeder fuse rating will depend on thefact that UPS has sufficient I2t capacity to clearslow acting HRC fuse having continuous ratingat least 20% of full load current of inverter,

while feeding 100% rated load.UPS AC distribution board fuses are HRC slowacting type.

Selectivity ratios of the fuses shall be such that

there is sufficient margin between the totalelectric energy of the downstream fuse and thetotal melting energy of the upstream fuse. Theselectivity ratios shall not be less than 1:2.


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SAFETY

Fuses are always supposedto be placed on the "hot"side of the load in systemsthat are grounded. Theintent of this is for the loadto be completely de-energized in all respectsafter the fuse opens. To seethe difference betweenfusing the "hot" side versusthe "neutral" side of a load,

compare these two circuits:

The first circuit design is much safer.


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Fuse Final