mj, maybe this might help:
If Transformer has only ONE OUTPUT VOLTAGE, the Primary OCPD will be sufficient to protect the Secondary Side Feeders. Remember that we are not protecting the Secondary Windings - only the Feeders on the Secondary Side.
If the Secondary has TWO OUTPUT VOLTAGES, the Secondary Conductors need OCPD.
FYI: Examples of Systems with:
[*]Secondary having ONE OUTPUT VOLTAGE - 480x240 VAC 1 ph. 2 wire, 600x440 VAC 3 ph. 3 wire.
[*]Secondary having TWO OUTPUT VOLTAGES -
480x120/240 VAC 1 ph. 3 wire, 600x208Y/120 VAC 3 ph. 4 wire.
Simplified description: On Secondaries with only ONE output voltage, the Amperes on the Secondary side will correspond to the Amperes on the Primary side (KVA wise), so if the OCPD on the Primary side is rated properly to the maximum KVA for the Primary side, this in a vicarious way will protect the Secondary Conductors (if sized properly).
So much for a simple description!