NFPA 13 2010 Edition states that modifications affecting more than 20 heads must be hydrostatically tested unless the system cannot be isolated. To what extent is "isolated" considered. If I have a multi floor office building that is getting a renovation to a large portion of a single floor space, is isolating the floor itself sufficient to require a hydrostatic test or does the specfic work area need to be isolated? Should the contractor install a control valve to isolate the system before he begins the work with the anticipation of being able to isolate his work? What if the system is a gridded system and cannot be isolated?