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Fuses
General questions / Current carrying
capability
Why does a fuse link dissipate
power?
When a current passes through a fuselink, a small amount of
energy is dissipated due to the fuselinks resistance. The
maximum power dissipation for each type/rating is specified
in the IEC Standards.
Is the peak
current (cut-off) important?
Short circuits are dangerous for several reasons:
-The destructive electrodynamics effects increase as the square
of the peak current value, during the short circuit.
-The destructive thermal effects increase in proportion with
the I2t reached during this short circuit.
Fuselinks are designed to drastically limit these effects.
For the prospective short circuit current of 10,000 Amp rms
with a totally asymmetrical current, the maximum value could
reach 25,000 Amp peak. A 100A gG fuselink limits the first
peak to 8,000 Amp, less than the third part of the prospective
maximum value. The destructive electrodynamic effects are
reduced by 90% (8,000/25,000)2.
(add a link to the page “HRC fuses”, section 3.3)
How does one select the
voltage rating?
Fuse links are voltage sensitive devices and it is important
to note that the satisfactory operation of a fuse link under
fault conditions is dependent on the system voltage. They
must not therefore be installed in circuits above their voltage
rating. They can however be used satisfactorily in circuits
at lower voltage levels. For example, to protect a 10 kV system
a 12 kV or 24 kV MV-fuse link can be used, but not a 7,2 kV
MV-fuse link.
How does one select the current
rating?
Every fuse link has a specific ampere rating. In selecting
the ampere rating of an MV-fuse link consideration must be
given to the type of load and circuit requirements.
How do high ambient
temperatures affect the current rating of a fuse link?
A fuse link is a thermal device and as such may require some
de-rating when used at elevated ambient temperatures. Fuse
links can carry rated current up to an ambient of 40 °C.
When the ambient temperature is higher than 40 °C de-rating
may be required (a simple rule is to de-rate by 0,5 % per
degree centigrade). The voltage rating is not dependent upon
ambient temperature.
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