If the service / main breaker was 1000 amps or larger, I would say the GFPE [Ground Fault Protection for Equipment], which is intregal with the main breaker, tripped along with the branch circuit OCPD. That would be a normal situation.
Since you described a 400 amp main, this most likely does not have GFPE [maybe it does??].
Sounds like from what you have described, you have in effect, a "Non-Selective" system.
Both of the breakers have similar ratings for Peak Let-Thru / Time-Current curves.
Could be a few reasons for this behavior, from adding a high load to an already loaded 400 amp main, to creating an arc acrossed the contacts of the 15 amp breaker, to coming very close of exceeding the AIC rating on the 15 amp breaker.
Typically this situation occurs where there's a very high AIC level available at points away from the Transformer, or systems with a lot of Motors running during the time of the fault.
I have seen this happen on 480Y/277 VAC systems before, plus on 208Y/120 VAC systems. Each time there was an extremely high level of SCA available [Short Circuit Amperes].
They also turned out to have incorrectly rated breakers for the available L-L current, and were indeed setup to be Non-Selective.
The Non-Selective problem, for example - on the 208Y/120 system - was due to using 10KAIC trip rated subfeed breaker [225 amp frame] and branch breaker [100 amp frame].
They both had equal Peak Let-Thru / Time-Current curve plots, which made them both trip.
In one way, this is a good thing [in case the smaller breaker fails to open], but normally this type of system is a result of poor designing / installations.
This situation can be a major nuisance when it brings down an entire building from a fault on a simple 20 amp circuit. That's why the point of designing a "Selective" system is pressed, as opposed to a "Non-Selective" system.
*note: had to fix typos again
[This message has been edited by Scott35 (edited 10-29-2001).]