49 CFR §234.213

1. (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.
2. (b)Exception. Paragraph (a) of this section does not apply to the following:
   1. (1)Circuits that include track rail;
   2. (2)Alternating current power distribution circuits that are grounded in the interest of safety;
   3. (3)Circuitry internal to microprocessor-based appliances;
   4. (4)Circuitry internal to semiconductor-based memory; and
   5. (5)Common return wires of grounded common return single break circuits.