This is perhaps a safety question. Most of the disconnects to 220v appliances are two hots plus ground (I have an air conditioning compressor, a well pump, and an accessory heater wired this way) with no neutral wire. The neutral isn't "necessary" as each hot is 180 degrees out of phase negating the need for a separate neutral.
But doesn't this mean if there is a short the appliance case AND the separate disconnect box (if metal) are BOTH electrified since their grounds are connected and there is rarely if ever a separate ground-to-earth at the appliance?
If so is this why 220v household clothes dryers are now four wire (hot hot neutral ground) or is there something in the dryers that needs 120v and thus needs the neutral for the 120v circuit?
If the disconnect were to a subpanel that is a different issue since the subpanel would require a distinct neutral (old school was to combo neutral and ground at the downstream panel but I think code frowns on that now).