what is fuses || Characteristics of Fuse Element|| Current rating||Types of Fuses

What is Fuses?

 Fuses A fuse is a short piece of metal , inserted in the circuit , which melts when excessive current flows through it and thus breaks the circuit . The fuse element is generally made of mate rials having low melting point , high conductivity and least deterioration due to oxidation e.g. , silver , copper etc.

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Advantages

 ( i ) It is the cheapest form of protection available . 

( ii ) It requires no maintenance .

 ( iii ) Its operation is inherently completely automatic unlike a circuit breaker which requires an elaborate equipment for automatic action . 

( iv ) It can break heavy short - circuit currents without noise or smoke . 

( v ) The smaller sizes of fuse element impose a current limiting effect under short - circuit condi tions . 

( vi ) The inverse time - current characteristic of a fuse makes it suitable for overcurrent protection . 

( vii ) The minimum time of operation can be made much shorter than with the circuit breakers 

 Disadvantages

 ( i ) Considerable time is lost in rewiring or replacing a fuse after operation . 

( ii ) On heavy short - circuits , * discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned . 

( iii ) The current - time characteristic of a fuse cannot always be co - related with that of the pro tected apparatus . 

Characteristics of Fuse Element 

The function of a fuse is to carry the normal current without overheating but when the current excee its normal value , it rapidly heats up to melting point and disconnects the circuit protected by it . order that it may perform this function satisfactorily , the fuse element should have the followin desirable characteristics : 48 ( i ) low melting point e.g. , tin , lead . ( ii ) high conductivity e.g. , silver , copper . ( iii ) free from deterioration due to oxidation e.g. , silver . ( iv ) low cost e.g. , lead , tin , copper . The above discussion reveals that no material possesses all the characteristics . For instance , lead has low melting point but it has high specific resistance and is liable to oxidation . Similarly , copper has high conductivity and low cost but oxidises rapidly . Therefore , a compromise is made in the selection of material for a fuse .

Types of Fuses 

Fuse is the simplest current interrupting device for protection against excessive currents . Since the invention of first fuse by Edison , several improvements have been made and now - a - days , a variety of fuses are available . Some fuses also incorporate means for extinguishing the arc that appears when the fuse element melts . In general , fuses may be classified into : ( i ) Low voltages fuses ( ii ) High voltage fuses It is a usual practice to provide isolating switches in series with fuses where it is necessary to permit fuses to be replaced or rewired with safety . If such means of isolation are not available , the fuses must be so shielded as to protect the user against accidental contact with the live metal when the fuse carrier is being inserted or removed .

 Important Terms The following terms are much used in the analysis of fuses :

 Current rating of fuse element: It is the current which the fuse element can normally carry without overheating or melting . It depends upon the temperature rise of the contacts of the fuse holder , fuse material and the surroundings of the fuse . ) Fusing current . It is the minimum current at which the fuse element melts and thus disconnects the circuit protected by it . Obviously , its value will be more than the current rating of the fuse element . For a round wire , the approximate relationship between fusing current I and diameter d of the wire is

Prospective Current : shows how a.c. current is cut off by a fuse . The fault current would normally have a very large first loop , but it actually generates sufficient en energy to melt the fuse able element well before the peak of this first loop is reached . The r.m.s. value of the first loop of fault current is known as prospective current . Therefore , prospective current can be defined as under : It is the r.m.s. value of the first loop of the fault current obtained if the fuse is replaced by an ordinary conductor of negligible resistance .

 Cut - off current . It is the maximum value of fault current actually reached before the fuse melts . On the occurrence of a fault , the fault current has a very large first loop due to a fair degree of asymmetry . The heat generated is sufficient to melt the fuse element well before the peak of first loop is reached The current corresponding to point ' a ' is the cut off current . The cut off value depends upon : ( a ) current rating of fuse ( b ) value of prospective current ( c ) asymmetry of short - circuit current It may be mentioned here that outstanding feature of fuse action is the breaking of circuit before the fault current reaches its first peak . This gives the fuse a great advantage over a circuit breaker since the most severe thermal and elector - magnetic effects of short - circuit currents ( which occur at the peak value of prospective current ) are not experienced with fuses . Therefore , the circuits pro tected by fuses can be designed to withstand maximum current equal to the cut - off value . This consideration together with the relative cheapness of fuses allows much saving in cost .

 Pre - arcing time :. It is the time between the commencement of fault and the instant when cut off occurs . When a fault occurs , the fault current rises rapidly and generates heat in the fuse element . As the fault current reaches the cut off value , the fuse element melts and an arc in initiated . The time from the start of the fault to the instant the arc is initiated is known as pre - arcing time . The pre - arcing time is generally small : a typical value being 0.001second ( vii ) Arcing time . This is the time between the end of pre - arcing time and the instant when the arc is extinguished . ( viii ) Total operating time . It is the sum of pre - arcing and arcing times . It may be noted that operating time of a fuse is generally quite low ( say 0-002 sec . ) as compared to a circuit breaker ( say 0-2 sec or so ) . This is an added advantage of a fuse over a circuit breaker . A fuse in series with a circuit breaker of low - breaking capacity is a useful and economical arrangement to provide adequate short - circuit protection . It is because the fuse will blow under fault conditions before the circuit breaker has the time to operate . ( ix ) Breaking capacity . It is the r.m.s. value of a.c. component of maximum prospective current that a fuse can deal with at rated service voltage .