Your most pressing National Electrical Code (NEC) questions answered
Q. What are the requirements regarding ground detection systems and labeling for ungrounded systems?
A. Systems that aren't required to be grounded in accordance with 250.20(B) [250.21(A)(4)] must have ground detectors installed [250.21(B)(1)]. Thisground detection sensing equipment must be connected as close as practicable to where the system receives its supply [250.21(B)(2)]. In addition, ungrounded systems must be legibly marked "Ungrounded System" at the source or first disconnecting means of the system, with sufficient durability to withstand the environment involved [250.21(C)].