Please note that the 240v changeover switches shown on the schematic are not physical switches, but plugs, which are plugged into one of 2 sources (inverter or external mains power). When I get a chance, I will update the schematic to show plugs and sockets, so as to avoid any confusion. Electrically the same, but physically different. The intent of the drawing is to show the electrical circuit rather then the physical connections. The solenoid supplying 12v from the main battery is controlled by the ignition switch, with an additional switch in series to turn the solenoid off when the main battery is low (eg just after starting). This gives the main battery a chance to charge up before applying the load of the trailer (up to 40A).