I am interested to know about Type 'X' Purging & a standard called
NEPA 496. It is related with purging of control panel to be located in
Zone-1 & 2, Gas Group B,C,D. I like to manufacture a pressurised purge
type panel containing elctrical accessories inside & few indicating
instrument mounted on the front sheet. Please also let me know
possible suppliers of such
enclosures as well as suppliers of Purge Controllers.

I think this is actually in reference to NFPA Technical Standards and
not the NEPA.  NEPA is the National Environmental Policy Act, however
NFPA is National Fire Protection Association.  Since the NFPA
publishes safety standards - one of which is NFPA 496, regarding
control systems in pressurized vessels - this is likely the correct
source of information.

NFPA 496, “Purged and Pressurized Enclosures for Electrical Equipment
in Hazardous Locations,” 1993.
http://www.nfpa.org/Codes/NFPA_Codes_and_Standards/List_of_NFPA_documents/NFPA_496.asp

The standard is not freely available on the net, however copies are
available for purchase at:

NFPA - National Fire Protection Association, 
1 Batterymarch Park 
Quincy, MA 02269-9101 USA 
Telephone: (617) 770-3000 
Fax: (617) 770-0700
http://www.nfpa.org/

You should be able to purchase the hardcopy or subscribe so that you
can read it online.

A possible supplier for products related to this standard is:

http://www.okbebco.com/html/eps.html