I am looking for guidelines for protecting distribution systtem networks, especially between 11kV (possibly lower) and 66kV. In particular explicit information on when to use fuses or a combination of fuse and circuit breaker.
In MV system like 13.8kv and below, we normally used fused-contactor line-up in the outgoing feeder instead of circuit breakers for economical reason, but this is valid only for resistance grounded system in the secondary of delta-wye incomer transformer. The fuse will protect the feeders from high short circuit currents i.e. 3phase and Line to line while the contactor will be opened only for overloads and ground faults. The contactor should be latched-in type so that it will not open during 3phase and line to line short circuit.
In this system, fuse cannot be used on the primary side of delta-wye incomer transformer because it cannot backup for a ground fault on the secondary side of transformer, because the ground fault at the secondary side of delta-wye transformer is seen as a line to line fault on the primary side.Instead of fuse use circuit breaker on the primary side.