INTRODUCTION
Electrical
supply system has different stages like Generation, Transmission, Distribution
and Utilization. The system is very complex. The systems are often to be made
ON or OFF for different purposes. These are also to be
protected from faults by automatically making off and resuming after clearing
the faults. For these purposes circuit interrupting devices are needed to be
introduced in the system at proper places.
Circuit interrupting Devices
1.
Fuses 2.
Drop-out-fuse link
3.
Expulsion fuse 4.
Isolators
5.
Air break switches 6.
Circuit breakers.
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 and protects the
equipment.
Materials of Fuse: Most commonly used
materials for fuse elements are lead, tin, copper, zinc, silver. Silver is used
no-a-days. It is best fuse material.
Important Terms Related to Fuses
Current rating of fuse element: It is the (r.m.s. value
of) current which the fuse element can carry without over heating or melting.
Fusing current: It is the minimum current
at which the fuse element melts and disconnects the circuit protected by it.
This value is obviously more than current rating.
Fusing
current I – kd3/2 ….. d = diameter, k=
constant
Fusing
current depends upon material of fuse element, length, diameter and type of
enclosure.
Fusing factor: It is the ratio of
minimum fusing current to current rating of fuse.
Minimum fusing current
= ----------------------------------
Current rating of fuse
The value is always greater than one.
The smaller the fusing factor the greater the difficulty in avoiding
deterioration due to overheating and oxidation at rated current.
Prospective current; 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.
The
cut-off value depends upon:
1.
Normal
current rating of fuse
2.
Prospective
current.
3.
The
asymmetry of circuit waveform
Thus,
the fuse breaks the circuit before the short circuit current reaches to its
peak. This is an advantage of fuse over CB because peak fault current produces
most severe electro-dynamic stresses and thermal effects. And because the fuse
breaks the fault current (S.C. current) before its peak value, the circuit to
be protected can be designed according to cut-off value. And cut-off value is
less than peak value of S.C. current. The system is relatively cheaper and thus
saving in cost is achieved.
Pre-arcing time: It is the time between
commencement of fault and instant when cut-off occurs. Typical value 0.001 sec.
Arcing time: It is the time between
the end of pre-arcing time and instant when arc is extinguished.
Total operating time; It is the sum of pre-arcing
time and arcing time.
The
operating time of fuse is quite low (0.002 sec) as compared to C.B. (0.2 sec).
This is an added advantage.
Breaking capacity: It is the r.m.s. value of
AC component of maximum prospective current that a fuse can deal with a rated
service voltage.
Desirable Characteristic of a Fuse
Element:
1.
Low
melting point
2.
High
conductivity
3.
Free
from oxidation
4.
Low
cost.
Advantages and Disadvantages of Fuse
Advantages:
1.
Very
cheap in cost
2.
Maintenance
is minimum
3.
Automatic
operation
4.
Breaks
the circuit without smoke and noise
5.
Inverse
time current characteristic therefore suitable in over current protection
6.
Minimum
time of operation.
Disadvantages:
1.
Refusing
requires considerable time
2.
Discrimination
between fuses in series cannot be obtained in short circuits.
3.
Current-time
characteristic of a fuse cannot always be co-related with that of the protected
apparatus.
Time Current Characteristic of Fuse
The
time required to blow out the fuse depends on the strength of excessive
current. The greater the current, smaller is the time taken by the fuse to blow
off i.,e. fuse has time-current inverse characteristic. Study of this
characteristic facilitates the selection of fuse for over current protection.
The characteristic of different fuses are shown below in figure.
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