In the operation condition suddenly one sever earth fault occurred between one of the three phases and transformer case in secondary side. Because of fault location, the secondary electrical protection couldn't detect any fault current. (Differential or REF protection were not used)Therefore it seemed that there was no electrical protection system to prevent damaging of transformer ; because the primary zero phase sequence current due to secondary short circuit was trapped in delta connection and couldn’t appear in primary feeding line of transformer. However the incoming fuses saved transformer in last-ditch successfully.
How can you explain the fuses function against fault condition?