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FUSES

                                                                                                                                    (By R.K.Sinha)

Though fuses are the most commonly used electrical protection elements in household and industry,the technical details of the same are not very clear to most of the people using them.This article tries to cover some facts related to fuses for common understanding.

What is a fuse ?

A fuse,in its simplest form,is a piece of thin metal wire,which opens the circuit in which it is inserted,by melting when the current through it exceeds a certain value for a sufficient period of time.Melting or blowing of the fuse is caused by the heat generated by the current.The part which actually melts and opens the circuit is known as the fuse-element.

Fuses are basically meant for providing short circuit protection.

SOME IMPORTANT DEFINITIONS

  1. Rated Carrying Current : It is the maximum current that a fuse can carry without any undue heating and melting.

2.   Fusing Current : It is the minimum current at which a fuse element shall melt in such a time interval as shall be necessary for the fuse-element to attain steady temperature.

  1.  Fusing Factor : It is defined as the ratio of Fusing Current and the Rated Carrying Current.

 

Fusing Factor = Fusing Current / Rated Carrying Current

            The value of fusing factor is always greater than 1.

  1. Prospective current and cut-off current : The above figure shows how AC current is cut-off by a   fuse.The fault current normally has a very large first cycle amplitude,but since it generates sufficient energy,the fuse melts well before the peak of the cycle is reached.

The RMS value of the first loop of the fault current is called the prospective current which is defined as the current which would flow under fault conditions when the fuse is replaced by a link of negligible impedance.

The cut-off current is the maximum value actually reached .Since the cut-off current is largely determined by the amount of energy needed to melt the element,the cut-off will vary according to the prospective current.

Cut-off is an important feature when fuses are used to provide back-up protection to circuit breakers as considerable reduction in electromagnetic forces (proportional to the square of the current) is achieved.

  1. Pre-arcing time (Melting time) : This is the time between the commencement of the current which  tends

      to melt the fuse-element and and the instant that the arc is initiated. The pre-arcing phenomenon is mainly dependent on the fault current passing through the fuse element              .

  1. Arcing time : This is the time between the end of the pre-arcing time and the instant when the circuit is broken and the current becomes zero.The arcing phenomenon is dependent on various factors like voltage,power factor,design of fuse etc.
  2. Total operating time : This is the sum of the pre-arcing and arcing time.
  3. Breaking capacity : It is the RMS value of the AC component of the maximum prospective current and the system voltages.A fuse never actually passes a current equivalent to its rupturing or breaking capacity.However,this value is assigned to a fuse because it prevents the passing of this high fault current through the circuit by its cut-off action.Higher the value of the rated breaking capacity,higher is the safety provided to the operating personnel and downstream equipments.

The area under the curve gives the thermal energy ( I square t) generated by the current.It is also divided into pre-arcing I square t (mainly responsible for the thermal stressing of the switchgear system),arcing I square t ;total of which gives total operating I square t.

 

CHARACTERISTICS OF FUSES

A fuse,being a thermal device,possesses inverse current-time characteristic,ie the operating time decreases as the fault current increases.

FUSE ELEMENT MATERIAL

Experience has shown that the most generally suitable material for the fuse element is a low melting point material such as tin,lead or zinc.A low melting point is,however,available with a high specific resistance metal.

Oxidation of the fuse element tends to produce an outer layer which may be strong enough to support the metallic core of the fuse element when in a molten state.

The present trend is to use silver despite its higher cost because :

 

TYPES OF FUSES

Fuses can generally be divided into two categories :

  1. Rewirable type
  2. Cartridge type

 

Rewirable fuse wire may be of lead,tinned copper,aluminium or an alloy of tin-lead.They are not permitted for higher ratings.Rewirable fuses of 16 A and below are permitted to be used for breaking capacity upto 2 KA only and those upto 100 A rating for breaking capacity upto 4 KA only.Where the breaking capacity exceeds 4 KA,cartridge fuse must be used.

Rewirable type fuse suffers from the following disadvantages :

 

Cartridge type fuses are of two types :

 

Types of HRC Fuses

  1. Ultra Fast Type : Used for protection of diodes and thyristors.Here,the current square multiplied by time ( I square t) rating is of prime importance as the thermal withstand capacity of semiconductor devices is much less in comparision to other switchgear components.
  2. Quick Response Type : Used for steady loads eg heating loads.
  3. Delayed Action Type : Used for motor loads where heavy starting currents are involved.

 

Advantages of HRC Fuses :

  1. Simple and easy installation.
  2. No Maintenance required.
  3. High breaking capacity.
  4. Low initial cost compared to circuit breakers.
  5. Extreme reliability.
  6. No aging effect.
  7. Co-ordination and discrimination can be achieved to a high degree.
  8. Current limitation by ‘cut-off’ property,thus protects the equipment against maximum thermal and electromagnetic stresses.

 

Disadvantages of HRC Fuses :

  1. In motor circuits MCCBs are preferred because a blown fuse may cause single phasing and motor failure.
  2. Fuses need replacement after every fault clearing.

 

FUSES IN MOTOR CIRCUITS

In motor circuits,fuses are meant primarily for short circuit protection.Protection against overloading is provided by the overload relay.The characteristics of the fuses and overload relay are so coordinated that the overload relay operates for currents within the contactor breaking capacity,and the fuses operate for larger fault currents.

 

FUSES AS BACK-UP PROTECTION TO CIRCUIT-BREAKERS

HRC fuses are sometimes used as back-up protection for circuit breakers.The characteristics of the fuse and circuit breaker are so coordinated that all faults within the range of the breaker are cleared by it and the faults beyond its range are cleared by the back-up fuses.This helps in reducing installation costs.

Thus,we see that the ‘Simple Simons’ of the electrical world ie. fuses pack a lot of punch in fighting the evil forces of fault currents and are the saviour of switchgears and human beings if chosen correctly for the application.