PERMIT REQUIRED - Confined Space Marking Regulation 

Permit Required Confined Space, Do Not Enter 1910.146(c)(2)

Exposed employees must be informed of the existence and location of and the danger posed by permit-required confined spaces through the posting of danger signs or by any other equally effective means. A sign reading DANGER - PERMIT REQUIRED CONFINED SPACE, DO NOT ENTER" or other similar language satisfies the requirement.