One other benefit of having the primary and secondary protection is the increased flexibility in the primary OCPD to grow larger (250% max) to overcome high inrush, possibly due to k-ratings or other design consideration.
Transformer inrush per ANSI standards is approximately 12xFLA or 1080A, so the primary breaker may trip, even if the actual load is low. Inrush is unrelated to actual connected load.
Edited for calculation error.
[This message has been edited by Ron (edited 09-09-2005).]
