As Don said, the Area Classification drawing is key. You should have one. If not, you will need to get one drawn up.
My usual reference for area classification is API RP-500, [FONT=Arial,Bold][FONT=Arial,Bold]Recommended Practice for [FONT=Arial,Bold][FONT=Arial,Bold]Classification of Locations forElectrical Installations at PetroleumFacilities Classified as Class I,Division 1 and Division 2.
I don't know the skill set for an EHS manager, but if you are not familiar with NEC 500, and API RP500, you will need someone that is. Doesn't have to be an engineer. If you have an electrical tech with site specific knowledge, they may well have the knowledge. An general electrical contractor - not so much.
Truly - unless the context/mission is security for above ground gas station tanks, I highly suspect the cost of the cameras is going to be way less than the wiring installation. [/FONT][/FONT][/FONT][/FONT]