Grounds.

(a) General. Except as provided in paragraph (b) of this section, each circuit that affects the proper functioning of a highway-rail grade crossing warning system shall be kept free of any ground or combination of grounds that will permit 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 track rail; (2) Alternating current power distribution circuits that are grounded in the interest of safety; (3) Circuitry internal to microprocessor-based appliances; (4) Circuitry internal to semiconductor-based memory; and (5) Common return wires of grounded common return single break circuits. [79 FR 49715, Aug. 22, 2014]