An intertripping station (10) of an intertripping system for use in the protection of power system networks and other applications where isolated intertripping is required. The station has output means (10A) for outputting either a supervision signal or an intertrip signal on the pilot circuit bus. The intertrip signal is of a higher level than the supervision signal and isolation means (TR) is provided between the output means and the bus. The station includes a supervision sensor (K1) for determining the presence of the supervision signal and for providing an alarm (K5) in the absence of that signal. An intertrip signal sender (K4) is also provided for enabling an intertrip signal to be provided by the output means and the station further includes a trip sensor (K2) for detecting the presence of an intertrip signal on the bus and for providing a trip control function at the station.
