The fuses have been produced for about a hundred years and today they are being used all around the world. They play a vital role in the protection of equipment and electrical networks, assuring that the failures that happen inevitably, don't cause too much damage and that the continuity of the supply of electrical power to the customers is maintained at a high level. Besides, the cost of a fuse is incomparably lower than the cost of the equipment it protects (e. G. A transformer), so the use of fuses considerably reduces the final cost of energy.
there are various types of fuse links, depending on the characteristics of their design and nominal and fault values they handle:
type k fuses are called fast element fuse links. They have velocity ratio* which is 6 for 6 amperes systems and 8 for the 200 amperes ones;
type t fuses are fuse links with a slow element. Their velocity ratio, for the same systems, is 10 and 13, respectively;
type h fuses are called extra-fast element fuse links. Their velocity ratio is 4 and 6.
dual type fuses are extra-slow, their velocity ratio is 13 and 20 (for 0. 4 and 21 amperes, respectively).
*velocity ratio is the relation between the melting current at 0. 1 seconds and at 300 seconds. (for fuses whose capacity exceeds 100 amperes, the value of 600 seconds is used. )
the k and t type fuses have been preferred by the electric sector during more than 20 yeas due to their mechanic and electric interchangeability. This longevity has been contrasted with the introduction of the dual slow-rapid class fuse link. This kind of fuses results advantageous not only in the protection of the network but also in the maximum usage of the capacity of the distribution transformers. (to see the characteristics of the dual fuse link, click here)
the k and t fuse links comply with the ansic 3742 standard, while dual and h types - with nema sg2 – 1986 standard.