Every transformer that is used underground shall be air-cooled or filled with nonflammable liquid or inert gas.
Va. Code Ann. § 45.2-805, under Chapter 8. Requirements Applicable to Underground Coal Mines; Electricity, Safety, Etc..
Va. Code Ann. § 45.2-805
Every transformer that is used underground shall be air-cooled or filled with nonflammable liquid or inert gas.