30 CFR § 57.4160
Underground electric substations and liquid storage facilities.
The requirements of this standard apply to underground areas only.
(a) Areas within 25 feet of the following shall be free of combustible
(1) Electric substations.
(2) Unburied, combustible liquid storage tanks.
(3) Any group of containers used for storage of more than 60 gallons of
(b) This standard does not apply to installed wiring or timber that is coated
with at least one inch of shotcrete, one-half inch of gunite, or other
noncombustible materials with equivalent fire protection characteristics.