I agree with Don and Bob- It's possible to proptect the xformer secondary and the secondary conductors with the overcurrent protection on the primary but we need more details about the xformer and the associated conductors on both sides of the xformer.
Look at it this way, A 1PH transformer with either a 240/120 or a 120/240 secondary providing power to a 1ph3w load requires secondary protection. A 3ph transformer that supplies power to a 3ph4w load must have secondary protection. A 1ph transformer that simply feeds a 1ph2w-load need only to have primary protection is the pri. OCPD is sized correctly according to art 450. It is the same for a 3ph transformer, which simply feeds a 3ph3w load. In both of these examples the secondary current is proportional to the primary current so a pri. OCPD can protect the secondary. Not so with a 1ph-3w or a 3ph-4w secondary. Think of it this way, if it is possible for the secondary to provide power where there are unbalanced L-N unbalanced loads if primary only over current protection was provided it would not see the unbalanced load and would not provide adequate transformer overload protection.