A fuse is a device usually used as a protection method that prevents the damage of these circuits connected to a power source. That is why the fuse must be always placed between the power source and the protected load. An accurate election of the fuse is a very important decision that affects not only the security, but also the reliability and the efficiency of an
electric system. Conventional fuses are activated by the temperature that the fuse reaches in an excessive current situation and can be subdivided in two groups: blow-out fuses and those constructed with materials with a positive temperature coefficient. In both cases, the fusion of the device is caused by the calorific power generated by the power dissipation.
A fuse is designed to correctly dissipate the heat generated by a current under normal conditions, reaching a stationary temperature quite below the melting temperature; when the equilibrium disappears and the temperature exceeds the fuse melting temperature, the fuse melts.
It is well known that the heat generated in the fuse needs time to be transferred to the environment and that the dissipated power is I2Rfuse. It can be deduced from this that the activation energy needed by a concrete fuse at a particular room-temperature is always the product I2t.
According to this, it is normal that the I-t curves look like the ones depicted in Figure 1.6, where curve (II) represents a slow blow-out fuse and curve (III) represents a faster one.
Conventional fuses are destroyed when any working point of the I-t curve is reached.
To come back to a normal operational situation, after the disappearance of the problem that caused the overcurrent, a new fuse must be put in place. This requirement represents a big problem that new electric systems try to overcome.
At the moment, two different circuit-protection devices that offer reusability are being developed: the Polymeric Positive Temperature Coefficient (PPTC) fuse and the Smart Power Switch (SPS).
Handbook of
Automotive Power
Electronics
and Motor Drives
Edited by
Ali Emadi
Illinois Institute of Technology
Chicago, Illinois, U.S.A.
Boca Raton London New York Singapore
A CRC title, part of the Taylor & Francis imprint, a member of the
Taylor & Francis Group, the academic division of T&F Informa plc.
© 2005 by Taylor & Francis Group, LLC
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