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(a) A telephone system shall be installed and maintained in good working condition in every mine which:
(1) Has reached a depth of 500 feet.
(2) Is entered by an audit or tunnel in which mining or development work is being done at a distance more than 1,000 feet from the entrance.
(b) The telephone system shall be equipped with telephones on the surface and at each working level 100 feet or more below the surface.
(c) Mines opened by a shaft having an inclination of less than 20 degrees from the horizontal shall be considered, for the purpose of this Order, to be mines opened by tunnels or adits.