API RP 505 is the "Zone" version and API RP 500 is the "Division" version of recommended practices for classification of locations in major petrochem facilities. NFPA 497 deals with the same subject (Divisions and Zones) in chemical plants. Several other Standards are listed in Sections 500.4(B) and 505.4(B) Reference Standards; especially in their respective Informational Note No. 2
API 540, Electrical Installations in Petroleum Processing Plants deals with more general petrochem installation applications.
NFPA often has training classes for Hazardous locations.
Nothing will beat OJT under an experienced professional.