Thanks for your reply. I found in ANSI/API RP 14F-91, table 4.11 the description of various NEMA enclosures available, which says,
" An explosion proof enclosure (NEMA type 7) is desgined to meet the application requirements in art 500 of NEC for Class I locations and is designed in accordance with the latest specifications of UL for particular groups of gases. Certain NEMA 7 enclosures are approved for sdeveral groups (such as group B,C and D), while others may be approved only for a particular group (such as group D). NEMA 7 enclosures are not necessarily suitable for outdoor use".
As I understood, NEMA 7 denotes to only "explosion" type and not to any Ingress protection of enclosures.