Lead entrances.

§ 18.37 Lead entrances. (a) Insulated cable(s), which must extend through an outside wall of an explosion-proof enclosure, shall pass through a stuffing-box lead entrance. All sharp edges that might damage insulation shall be removed from stuffing boxes and packing nuts. (b) Stuffing boxes shall be so designed, and the amount of packing used shall be such, that with the packing properly compressed, the gland nut still has a clearance distance of 1/8 inch or more to travel without meeting interference by parts other than packing. In addition, the gland nut shall have a minimum of three effective threads engaged. (See figures 8, 9 and 10 in appendix II.) (c) Packing nuts and stuffing boxes shall be secured against loosening. (d) Compressed packing material shall be in contact with the cable jacket for a length of not less than 1/2 inch. (e) Special requirements for glands in which asbestos-packing material is specified are: (1) Asbestos-packing material shall be untreated, not less than 3/16 -inch diameter if round, or not less than 3/16 by 3/16 inch if square. The width of the space for packing material shall not exceed by more than 50 percent the diameter or width of the uncompressed packing material. (2) The allowable diametrical clearance between the cable and the holes in the stuffing box and packing nut shall not exceed 75 percent of the nominal diameter or width of the packing material. (f) Special requirements for glands in which a compressible material (example—synthetic elastomers) other than asbestos is specified, are: (1) The packing material shall be flame resistant. (2) The radial clearance between the cable jacket and the nominal inside diameter of the packing material shall not exceed 1/32 –inch, based on the nominal specified diameter of the cable. (3) The radial clearance between the nominal outside diameter of the packing material and the inside wall of the stuffing box (that portion into which the packing material fits) shall not exceed 1/32 –inch. [33 FR 4660, Mar. 19, 1968, as amended at 57 FR 61210, Dec. 23, 1992]