(a) Telephone service.--Telephone service or equivalent two-way communication facilities shall be provided in all mines between the surface and each working section that is more than 1,500 feet from the main portal.

(b) Telephone lines.--Telephone lines shall be carried on insulators, installed on the opposite side from power or trolley wires and insulated adequately where they cross power or trolley wires.

(c) Lightning arrestors.--Lightning arrestors shall be provided at points where telephone circuits enter the mine.

(d) Telephone cables.--Telephone cables permanently installed in power boreholes containing unarmored power cables shall be either armored or protected at top and bottom by insulating transformers.

(e) Precautions.--All proper precautions shall be taken to prevent electric signal and telephone wires from coming into contact with other electric conductors, whether insulated or not.

(f) Standards generally.--Bells, wires, insulators, contact makers and other apparatus used in connection with electric signaling underground shall be of suitable design and of substantial and reliable construction and erected in such a manner as to reduce the liability of failures or false signals to a minimum.

(g) Potential.--In the face areas of any mine, the potential used for signal purposes shall not exceed 24 volts, and bare wires shall not be used for signal circuits.

(h) Voltage on signal circuits.--The voltage on signal circuits confined to intake air and using insulated conductors may be greater than 24 volts, but shall not exceed 125 volts average. This shall not apply to haulage block signal systems.