Definition(s)
Open Drain
An open drain is defined as a drain that is open to the atmosphere [at its entrance (drain opening) or elsewhere] before (or where) its discharge enters a sump, pit, or other containment device.
API RP 500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2, Third Edition, December 2012, Global Standards