49 CFR 234.213: 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.