30 CFR § 57.12069
Lightning protection for telephone wires and ungrounded conductors.
Each ungrounded conductor or telephone wire that leads underground and is
directly exposed to lightning shall be equipped with suitable lightning
arrestors of approved type within 100 feet of the point where the circuit
enters the mine. Lightning arrestors shall be connected to a low resistance
grounding medium on the surface and shall be separated from neutral grounds
by a distance of not less than 25 feet.