A method of operating an aircraft solid state power controller, the aircraft solid state power controller comprising a feed node configured to be electrically connected to a primary electric power supply, a load node configured to be electrically connected to at least one electric load, and at least one electric switching device coupled between the feed node and the load node, the method comprising:
- selectively switching the at least one electric switching device between (i) an on-state in which the at least one electric switching device provides an electric connection between the feed node and the load node and the primary electric power supply is connected to the electric load and (ii) an off-state in which the at least one electric switching device electrically isolates the load node from the feed node;
- applying a test voltage between the feed node and the load node, wherein the test voltage is provided by a secondary electric power supply independent from the primary electric power supply and galvanically isolated from the primary electric power supply;
- and
- determining a switching state of the at least one electric switching device by detecting a voltage drop between the feed node and the load node.