30 CFR § 75.521
Lightning arresters; ungrounded and exposed power conductors and telephone wires.
Each ungrounded, exposed power conductor and each ungrounded, exposed
telephone wire that leads underground shall be equipped with suitable
lightning arresters of approved type within 100 feet of the point where the
circuit enters the mine. Lightning arresters shall be connected to a low
resistance grounding medium on the surface which shall be separated from
neutral grounds by a distance of not less than 25 feet.
[38 FR 4975, Feb. 23, 1973]