Here is guide iI found in the net. It is entitled Fuse Selection Guide. The purpose of using a fuse is for protection and safety of life and equipments in case of overcurrent conditions.
it is a general rule that fuses should be used both for primary winding protection and secondary winding protection of a PT. This is to protection the PT from high currents due to circuit faults and to protect the circuit from faults that may occur in the PT or the secondary circuit attached to a PT.
Primary fuses must be selected so they will interrupt the max short circuit current and not blow for inrush magnetizing current to the transformer which occurs during switching. Ask the manufacturer for short circuit current info on PT primary and secondary terminals.
Secondary fuses must also be used because faults on the secondary side my not always blow the fuse on the primary circuit… so a secondary fuse must be used.
