Section 690-322 - Grounding(a) General rule.--In a direct-current electrical system, grounding shall consist in so connecting any part of an electrical system, including frames, to the earth that there shall be no difference of potential between them.(b) Negative side to be grounded.--Only the negative side of the direct-current circuit shall be grounded.(c) Rectifier diodes.--Rectifier diodes used at any bituminous coal mine shall be connected to the supply circuit through an isolating winding in order that isolation between alternating current and direct-current systems is effective.(d) Initial installation.--The initial installation of rectifiers at any bituminous coal mine shall be approved by the department before being energized.2008, July 7, P.L. 654, No. 55, §322, effective in 180 days [ 1/5/2009].