A mains signalling network (1) for a mains power supply line (2) has a controller (10) that controls switching of appliances (3 to 6) connected to the supply line via respective nodes (30 to 60). Each node has a switch (36) by which a code can be set for the node, the controller having a keyboard (11) for addressing the desired node. When a node receives a signal from the controller, the node generates a reply signal along the supply line (2). The controller generates the signal for controlling the appliance after receipt of the reply signaL if the controller receives signals other than those initiated by the controller, it generates signals to prevent operation of the nodes. Signals from the controller (10) include a code identifying the controller followed by a code identifying a node, whereas reply signals from the nodes include a code identifying that node followed by a code identifying the controller.