MIL-DTL-917F(SH)
3.2.22 Electrical creepage and clearance distances. Creepage (see 6.5.3) and clearance (see 6.5.2) distances
between electrical circuits, between each electrical circuit and ground, and across lines and between circuit elements
that operate at differences in potential levels shown in each row of table III shall be not less than those values shown
in table III for equipment conforming to this specification unless otherwise approved by NAVSEA (see 6.2).
UL 840 should be used for design guidance for creepage and clearance, but the creepage and clearance values in
UL 840 tables have been found to be too low for the Navy environment. It is emphasized that the values shown in
table III represent the minimum acceptable limits for non-arcing rigid construction based on normal volt-ampere
ratings (see 6.5.6) and that they take into consideration only the average degree of enclosure and service exposure.
Therefore, the designer shall employ creepage and clearance distances in excess of these minimums where it is
probable that structural features, contaminants, lack of maintenance, environment, exposure, or application
overstress will create service conditions more severe than normal. When necessary, insulating barriers may be used
to interrupt continuous electrical creepage paths. Cemented or butted joints will not be accepted as techniques to
obtain the minimum creepage distances in table III. For voltage levels greater than 1,000 V, but less than or equal to
15,000 V, refer to IACS (UR) E11 for electrical creepage and clearance distances.
3.2.22.1 Distance from enclosure. Exposed non-arcing current-carrying parts within enclosures shall have an
air space of not less than ¾ inch between them and the uninsulated part of the enclosure. However, the values
shown in table III may be applied to the creepage and clearance distances between uninsulated parts of enclosures
and exposed non-arcing current-carrying parts of devices whose mounting is sufficiently rigid and so designed to
prevent decrease of the clearance distance through a blow on or distortion of the enclosure as specified (see 6.2).
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