26-02-2013, 02:40 PM
HRC Fuse Links
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Introduction
HRC fuse links are important and
indispensable part of an electrical
circuit. Proper selection of a fuse
and its co-ordination with other
protective devices in the circuit is
very vital for foolproof protection of
the circuit and its uninterrupted
performance.
The basic functioning of a fuse calls
for:
- continuously carrying the rated
current without getting
overhead
- instantaneously interrupting
the circuit case of a fault.
These functions make fuse
element design very critical.
Low Power-loss
One of the most important factors
while selecting a HRC fuse-link is
its Power-loss. The rated powerloss
of the fuse-link is the powerloss
value as stated by the
manufacturer when the fuse-link
is carrying its current under
specified conditions. The fuselink
selected should preferably
have low power-loss.
(i) Saving in power/energy
(ii) Lower temperature rise at
fuse-switch terminals and
fuse-knives.
Table 1 gives the power-loss
value limits as recommended in
the relevant
National/International standards.
It is evident that over a period of
time the Standards recognised
the importance of power-loss and
revised to reduce the limiting
values.
Guidelines for selecting a
suitable fuse for a given motor
Since various manufacturers offer
different characteristics, they specify
different fuses for the same rating of
motor. It leaves the choice to the
user. In the forthcoming discussion,
we bring you the guidelines to be
followed while selecting a fuse
suitable for a given rating of motor.
Let us consider DOL starting of a 3
phase Induction Motor rated
50HP/37kW, 415V AC. Here two
typical cases are considered where
two different ratings of fuse-links are
recommended for the same
application. Our objective is to
determine the appropriate rating of
fuse-link suitable for this application
meeting the requirements
mentioned above. In both the
cases, fuse-links recommended are
DIN type, commercially available
in the market (see fig. 2).
In case A, a 125A fuse-link is
recommended whereas in case B
100A fuse-link is recommended.
Power-loss in fuse-links
It can be seen that in Case A, the
power loss in fuse is 9.8 watts at
125A i.e. 2.73 watts at 66A (full load
current of motor). As against that, in
Case B, the power loss in fuse is
10.3 watts at 100A i.e. 4.48 watts at
66A. Thus, in Case B, the power
loss is almost 64% more as
compared to that in Case A.
Since, the power loss in fuses
results in higher temperature in
cubicle and thereby affects the life
of the equipment, it is recommended
that the user should verify power
loss figures indicated by the
manufacturer before finalising a
particular fuse rating. Reduction in
power loss in fuses also results in
direct saving of energy
consumption.