switchgear-Time-current characteristics of fuse wire

Time-current characteristics of fuse wire

To achieve the above criteria fuse links should have time/current characteristics which lie close to the withstand curves of their associated circuit shown in figure 1.18. Obviously, the operating time of fuse should always be less at any current level then the period for which its associated circuit can withstand the condition of a very important factor which must be taken into account is that fuse operating times include the arcing period and change of state occurs before clearance is affected. Once arcing has commenced it is clearly impossible 

for an element to return to its original form and even when melting begins distortion main result and an element may not return to its original shape on cooling. Fuselinks with low melting point materials on their elements which will be subjected to acceptable and Irreversible changes if overcurrents pass through them long enough to initiate the M effect diffusion process.

There is called a band below its time-current characteristics in which a fuse should if possible not be called upon to perform. while in practice it is not likely that a high current will flow through a fuse link for a period just less than its operating time the possibility nevertheless exists. if such a condition should arise then the probable result which must be accepted is that the fuse May operate more quickly than expected on a future occasion.

Difficulties would certainly arise if a fuse link Carried a current just below its minimum fusing value for prolonged periods. To ensure that the situation is avoided fuse links are assigned rated current somewhat below their minimum fusing levels. The ratio of the minimum fusing current to the rated value which is defined as the fusing factor usually has a value in the range of 1.2 to 2. The significance of this factor is that circuits must be able to operate continuously at levels significantly above the current of the fuse protecting them to satisfy Criterion one above namely that the continuous rating of the circuits should exceed the minimum fusing current. This situation arises with other protective equipment in which current settings about the full load level are used. clearly, circuits and equipment are not normally designed without some overload capacity, in providing factors of safety are usual and difficulties are not encountered in providing adequate fuse protection. clearly, relatively low fusing factors are nevertheless desirable on economic grounds.

A further factor which must be recognized is that not all fuselinks are capable of operating satisfactorily at all current levels between the maximum breaking capacities and minimum fusing values. Satisfactory arc extinction may not take place at certain current levels usually those just above the minimum fusing values. care must be taken to ensure that users are only used in circuits where currents of these levels will not be encountered or if this cannot be guaranteed then some other protective scheme must be provided to open the circuit before the fuse attempts to operate. In the USA the term 'full range' has been introduced to designate those fuses which can clear all levels of current satisfactorily.