(a)General. Except as provided in paragraph (b) of this section, each circuit, the functioning of which affects the safety of train operations, shall be kept free of any ground or combination of grounds having a current flow of 75 percent or more of the value necessary to retain a permissive state of a safety appliance.(b)Exception. Paragraph (a) of this section does not apply to the following: (1) Circuits that include any track rail;(2) The common return wires of single-wire, single-break, and signal control circuits using a grounded common;(3) Circuitry internal to microprocessor-based appliances;(4) Circuitry internal to semiconductor-based memory; or(5) Alternating current power distribution circuits that are grounded in the interest of safety. 79 FR 49715 , Aug. 22, 2014