We worked in a plant that had 250 volt DC for various equip (Winders/unwinders) After a lot of thought, I found a NON NEMA twistlock cap & plug and designated that for 250 volt DC. They also came up with 125 volt DC equipment....found another NON NEMA pair for that. Alas, the maintenance man came along with his 125 volt DC drill (with a 120 volt AC 15 amp, 3 minus 1 male plug.
He actually had regular 120 volt, 15 amp, AC duplex receptacles in his 'shop' with 125 V DC feeding them.
That plant was a nightmaer of caps...120 to 480 volt; every combo. And a LARGE budget for replacements.
John