Technical question - VERY technical question.

We call 'em closets, electrical rooms, vaults, etc. (Storage rooms [Linked Image] )

Building codes have a distinct difference between electrical closets and vaults, but try to find a hard definition of vault and you can run into a stone wall.

Does anyone have a definition, from an authority, of the difference between a vault and an electrical room? I've always held oil filled equipment as the difference, but there is truly no proof of that from NFPA, IEEE, or ANSI that I can find.

Any help is appreciated.