According to another aspect of the present invention, there is provided a protection method in a distributed power system including DC power sources and multiple power modules each of which include inputs coupled to the DC power sources. The power modules each include outputs coupled in series to form a serial string. An inverter is coupled to the serial string. The inverter converts power input from the string and produces output power. When the inverter stops production of the output power, each of the power modules is shut down and thereby the power input to the inverter is ceased. When the inverter is connected to and supplies the output power to the electrical grid, one or more electrical parameters of the grid are monitored. When the one or more electrical parameters of the grid are out of a predetermined specification, the inverter is shut down and thereby production of the output power is stopped or the inverter is disconnected from the grid. When the inverter is shut down, a switch disposed between the serial string and the inverter is activated. When the switch is activated a change in current flowing through the serial string is sensed. Alternatively a signal is provided based on the shutdown mechanism. Multiple receivers are attached respectively to the power modules. The receivers receive the signals which enable the respective power modules to supply the input power to the inverter based on the presence of the signal or absence thereof. When the signal is a keep-alive signal, the respective power modules supply the input power to the inverter based on the presence of the keep-alive signal. When the signal is a shut-down signal, the respective power modules stop supply of the input power to the inverter based on the presence of the shut-down signal. The signal may be based on current in the serial string from the electrical grid and detected at the frequency of the electrical grid or detected at a higher frequency up converted from the frequency of the electrical grid. The signal in the serial string is optionally from the inverter or the output power therefrom, and detected at a switching frequency of the inverter. The signal is optionally actively superimposed on the power input to the inverter from the serial string. The signal may be wirelessly transmitted, and the receiver in each of the power modules, receives the wirelessly transmitted signal.