30A Air Conditioner Disconnect
Are you using conduit that is weather proof between the building penetration and the bottom of the disconnect?
If you're using EMT, I don't know if that would be considered "water proof" and therefore not suitable for what I assume is a "wet" location (outdoors in the rain). The disconnect for my AC uses some sort of gray colored flexable conduit between the house and the disconnect. Since there is NM 10/2 running through the basement, and no junction box at the wall penetration, I assum the NM wire continues through this weather protection conduit to the disconnect box.
My disconnect box is then something of a pair of knife like blades (one knife for each of the two hots). The blades are positioned such that you disconnect the power by pulling the "thing" the blades are mounted to. The blades are positioned such that you spin the "thing" around and insert it back into the disconnect. That way, the hots in the disconnect are covered, but the unit is disconnected.
As a guess, the HVAC doesn't want a fussed disconnect because that way you don't have to worry about blowing the fuses (requiring that you either stock the fuses, or run to the local supplier when one of them activates). It's much simpler to only have to worry about tripping a breaker rather than worring about blowing a fuse.