A three-dimensional space enclosed by more than 2/3 of the possible projected plane surface and of sufficient size to allow the entry of personnel. For a typical building, this would require that more than 2/3 of the walls, ceiling, and floor be present. Refer to API RP 500 and RP 505.
Source: API RP 49, Recommended Practice for Drilling and Well Servicing Operations Involving Hydrogen Sulfide, Third Edition, May 2001. Global Standards