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
SOME IMPORTANT DEFINITIONS
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.
- Fusing Factor : It is defined as the ratio
of Fusing Current and the Rated Carrying Current.
Fusing Factor = Fusing
Current / Rated Carrying Current
value of fusing factor is always greater than 1.
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.
time (Melting time) : This is the time between the commencement of the current
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
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.
operating time : This is the sum of the pre-arcing and arcing time.
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
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
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 :
coefficient of expansion of silver is so small that no aging effect occurs
and thus the continuous full capacity of normal current rating is assured
for a long time.
conductivity of silver is unimpaired by continuous operation and by surges
of current that produce temperatures just near the melting point.
conductivity of silver does not deteriorate with oxidation.
fuse elements can be raised from normal operating temperature to vaporization
much quicker than other fuse element materials because of its comparatively
low specific heat.Also,silver has an abrupt increase in its resistance as
melting point is reached,which makes the transition from melting to vaporization
nearly spontaneous.As a result,fuse blowing is very much faster at higher
temperature of silver is very much below the one at which the vapor will readily
ionize.This means that when an arc is formed through the vaporized portion
of the element,the arc path hascomparatively high resistance and the short
circuit current is quickly reduced.
TYPES OF FUSES
Fuses can generally be divided into two categories
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
operation because of oxidation of the fuse wire and its consequent thinning
to small time lag these fuses have a tendency to blow while handling motor
starting currents and switching on of transformers,capacitors etc.
is a general tendency of misuse as any available piece of wire is inserted
as fuse element.
to be used in hazardous areas as the arc generated while blowing may be very
to be used in circuits with fault current exceeding 4 KA because of its low
Cartridge type fuses are of two types :
: These are available upto 63 A.The breaking capacity is of the order of
4 KA for 2 and 4 amp fuses and 16 KA for 6 to 63 A fuses.
(High Rupturing Capacity) Fuses : Fuses with rupturing capacity exceeding
16 KA are covered in this category.In an HRC fuse the fuse element is surrounded
by an inert arc quenching medium like chemically treated sand and is enclosed
in an outer body of ceramic or some other non-inflammable material.The fuse
element is made of one or more wires of silver which are connected to two
end caps of brass or copper.
Types of HRC Fuses
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.
Response Type : Used for steady loads eg heating loads.
Action Type : Used for motor loads where heavy starting currents are involved.
Advantages of HRC Fuses :
and easy installation.
initial cost compared to circuit breakers.
and discrimination can be achieved to a high degree.
limitation by ‘cut-off’ property,thus protects the equipment against maximum
thermal and electromagnetic stresses.
Disadvantages of HRC Fuses :
motor circuits MCCBs are preferred because a blown fuse may cause single phasing
and motor failure.
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