Ed,
Unless the exact combination of fuses and breakers have been tested, listed and marked on the equipment by the breaker manufacturer, you can't use the X-Y system shown in your drawing. The code does not permit the use of a current limiting fuse to protect downstream breakers with a lower interrupting rating. The breaker interrupting rating must be equal to or exceed the fault current that is available at the line side of the breaker without the reducing effect of a current limiting fuse. Often the length of the feeder from the source will provide enough limiting to permit the use of lower rated breakers. The fuse people have been trying to get the use of current limiting fuses for this purpose into the code for the past few code cycles and have not succeeded.
Don
