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