What is Fuse?Important terms

what is fuse

What is fuse?It is a piece of metal which is inserted in the circuit in series and it will melt when excessive current flows through it and thus breaks the circuit.

The fuse clement is generally made of materials having low melting point, high conductivity and least deterioration due to oxidation e.g. silver, copper etc,. It is connected in series with the circuit which will be protected. Under normal operating conditions, the temperatures of fuse element remain below the melting point Therefore, it carries the normal current without overheating.

However, in the circuit when overload or short circuited occur, the current increase beyond the rated current which flow through the fuse wire. This raises the temperature and element melts for blows out disconnecting the circuit protected by it. Due to the melting of wire the equipment or other circuit protect from damages.


The time required to melt fuse wire which will depends upon the magnitude of excessive current. If the current value is greater then it will take less time to blow out. Thus ,a fuse has inverse time current characteristics. Such a characteristic permits use for over-current protection.


  • It is the cheapest form of protection available,
  • It requires no maintenance.
  • Its operation is inherently completely automatic unlike a circuit breaker which require elaborate equipment for automatic action,
  •  It can break heavy shirt-circuit currents Without not or smoke.
  • The smaller sizes of fuse element impose a current limiting effect under short-circuit conditions.
  •  The inverse time-current characteristic of a fuse makes it suitable for over-current protection.
  • The minimum time of operation can be made much shorter than the circuit breakers.


  • Considerable time is lost rewiring or replacing a fuse after operation.
  • On heavy short-circuits, *discrimination between fuses in series cannot, be obtained unless there is sufficient difference in the sizes of the fuses concerned.
  • The current-time characteristic of a fuse cannot always be co-related with that of the protected apparatus.

Symbol of fuse

Different standard symbol of fuse are give below

symbol of fuse


The fuse wire carry the normal current without overheating but when the current exceeds beyond its normal value, it rapidly heats up to melting point and disconnects the circuit protected by it. To perform this function accurately and satisfactorily, the fuse element should have the following desirable characteristics:

  • low melting point e.g. tin lead.
  • high conductivity e.g. silver, copper.
  • It should free from deterioration due to oxidation e.g. silver.
  • 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 it is liable to oxidation.

Similarly copper has high conductivity and low cost but oxides rapidly. Therefore a compromise is made in the selection of fuse material


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

1-Current rating of fuse element:

 It is the current which the fuse element on normally carry without overheating or melting. It depends upon the temysature rise of the contacts of the fuse holder, fuse material and the surroundings of the fuse.

2-Fusing current:

It is the minimum current at which the F element melts and thus disconnects the circuit protected by it. Obviously, its value will be more than she current rating of the fuse element.

For a round wire, the approximate relationship between fusing current I and diameter d of the wire is.

                     I = kd3/2  where k is a constant, called the fuse constant. Its value depends upon the metal at which the element is made. Sir W.H. Preece found the value of k for different matenals given in the table below:

S.nomaterials  Value of K
d in cmd in mm

The fusing current which will depends upon the following factor the various factors such as

  • material of fuse element.
  • If the lengt of fuse wire is small then current will be greater because a short fuse can easily conduct away all the heat.
  • diameter.
  • size and location of terminals
  • previous history.
  • type of enclosure used.

 3-Fusing Factor:

It is the ratio of minimum fusing current to the current rating or fuse element i.e.

Fusing factor = Minimum fusing current/Current rating of fuse

Its value is always more than one. The smaller the fusing factor, the greater is the difficultly in avoiding deterioration due to overheating and oxidation at rated carrying current. For a a fuse which is  semi enclosed or rewirable ,copper wire are used as the fuse element, the fusing factor is usually 2.

fuse characteristic curve

4-Prospective Current:

 Shows how a.c. current is cut off by a fuse. The fault current have very large loop, but it actually generates  much energy to melt the fuseable element well before the peak of this first loop is reached. The  rms value  of the first loop of fault current is known as prospective current. Therefore, e “went can be defined as under:

Prospective current is the r.m.s value of the current obtained when the fuse wire s replaced by an ordinary conductor of negligible resistance.

5-Cut-off Current:

Cut off current 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 fair degree of asymmetry. The heat generated is sufficient to melt the fuse element well before the peak of first loop is reached (point ‘a’). The cut off value depends upon:

  • current rating fuse
  • value of prospective current
  • asymmetry of short-circuit current.

It may be mentioned here that outstanding feature of fuse action is the breaking circuit before the fault current reaches its first peak.

6-Pre-arcing time:

It is the time between the commencement of fault and till instant when cut off occurs. When a fault occursin the circuit the current increase very quickly and the fuse element start heat up . As the fault current reaches the cut off value, the fuse element melts and an as is initiated. The time from the start of the fault to the instant the arc is initiated is knot as pre-arcing time. Generally the pre-arcing time is small ,usually have value of 0.001 second.

7-Arcing time:

 This is the time between the end of pre-arcing time and the instant when the arc is extinguished.

8-Total operating time:

 Total operating time is the combination of pre-arcing and arcing times.

Generally operating time of fuse is very low (say 0.002 sec.) as compared to a circuit breaker (say 0.2 sec. or so). This is the main  advantage over a circuit breaker. A fuse is series with a circuit breaker of low-breaking capacity is a useful and economical arrangement to provide adequate short protection. This arrangement is useful due to the F wire  blow out  under fault conditions before the circuit breaker operated .

9-Breaking capacity;

 It is the r.m.s. value of a component of maximum prospective current that a fuse can deal with at rated service voltage.


 In order to ensure that the fuses will correctly and reliably protect any section or element of an electrical circuit, their characteristics and ratings must be  selected in accordance with the following requirements.

1-Rated Voltage:

Fuses and their fusible elements must be selected so that voltage rating is the same as the service voltage of the circuit they are to be inserted

2-Maximum Current Rupturing Capacity:

 To extinguish the arc resulting blowing of the element in a fuse safely, the later and its element must have a cu interrupting capacity at least equal to or greater than the largest short-circuit cu which can flow in the circuit to be protected.

 3-Rated Current:

The fuse element to be used in a it must be selected I rated current large enough to ensure that the element will not blow when the circuit carrying its sustained maximum load current and when the circuit is carrying a short-1 overload current.

To satisfy the first of these requirements the following condition must observed to continue normal operation when practically a continuous maximum I current is being carried.

 Rated current = K times sustained maximum load current

where K is a factor of safety with respect to the load current and depends upon the nature of the load. For practically constant loads such as lighting load K = 1.1 — 1.2. If the load Current varies considerably as in the case of motors which, during starting periods draw large currents greatly in excess of the normal operating current, the current rating of fuse element must be selected so as to satisfy the second requirement. This consists fleeting the current rating so that the element will not blow during short-time overload(of 2—10 second duration). It is. therefore, necessary to select an element with such a time current characteristic that the duration of the over load current will be shorter than the period in which the element will blow at the same value of current.

 To select the fuse element as mentioned above. its time-current characteristic must be available. As this is not convenient in practice. the element current rating is determine  from the following simple expression

Continuous current rating=   short –time overload current/K

Where K  is the factor of safety with respect to the load current

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