46 CFR § 108.171 - Class I, Division 1 locations.
The following are Class I, Division 1 locations:
(a) An enclosed space that contains any part of the mud circulating system that has an opening into the space and is between the well and final degassing discharge.
(b) An enclosed or semi-enclosed location that is below the drill floor, and contains a possible source of gas release.
(d) A space that would normally be considered a Division 2 location under § 108.173 but where combustible or flammable gases might accumulate.
(e) A location in the weather, or a semi-enclosed location, except as provided in paragraph (b) of this section that is within 1.5 m (5 ft.) of the boundary of any -
(1) Equipment or opening specified in paragraph (a) of this section;
(2) Ventilation outlet, access, or other opening to a Class I, Division 1 space; or
(3) Gas vent outlet.