Switchgear-Working criteria of fuses


Working criteria of fuses


A fuselink which is to protect a piece of equipment and or a circuit should ideally satisfy several criteria. This may be illustrated by considering an example based on the simple circuit shown in figure 1.17.


The criteria are:

1. The minimum fusing current of the fuse should be slightly less than the current which can be carried continuously by the source, the supply, and connecting cables and the piece of equipment.



2. The item of equipment should be able to carry currents in excess of its rated current for a Limited period and the fuel should operate when carrying this overload current in times slightly shorter than those which the equipment can withstand.

3. The cables should also be able to cope with the above over currents without being damaged.

4.  The high current may flow due to faults within the item of equipment and in these circumstances, consequential damage should not occur to the remainder of the circuit. The extreme case would occur if the input terminals of the piece of equipment become short-circuited. In this situation, clearance should be affected quickly enough to prevent damage to the cables.

5.  A further possibility is a short circuit between the conductors of the connecting cables. The most serious situation would arise if the fault was at the input end that is between points A and B. For such faults the fuse should operate rapidly enough to prevent damage to the source and the supply cables.

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