5.6 Co-ordination with Fuses

When co-ordinating overcurrent relays it may be necessary to consider the time/current characteristics of fuses which are used to protect MV/LV substation transformers. When a fuse operates, the circuit is left in an open-circuit condition until the fuse is replaced. It is therefore necessary to consider the case of preventing fuse operation because of the problems of replacing them after they operate, which is called fuse saving. In these cases it may be preferable to forgo the selectivity of the protection system by not taking account of the fuse characteristic curve, so that the fuse will then act as a back-up.