The Sunspec protocol is a common implementation of rapid shutdown in systems with string inverters. In this scheme the inverter generates an AC "keep alive" signal that is superimposed on the DC conductors, and RSD components like those manufactured by Tigo are connected in series in the DC circuits with one or two modules each. When the inverter shuts off as it would when the grid goes down, the "keep alive" signal is no longer present on the DC lines, so in response the rapid shutdown devices disconnect the PV modules from the inverter and from each other. Since nearly all PV systems these days are only functionally grounded on the DC side, I surmise that both the positive and negative conductors from the modules are disconnected, but I do not know that for sure.I may have a little bit of a misunderstanding of Rapid shutdown. I was under the impression that rapid shutdown is a "stop generating" command via the RSD. Does this RSD also disconnect the ungrounded conductors? It seems like the RSD alone would not satisfy 2023 690.13(E). Maybe I am missing somehting on the inveter operation?
