Apparatus for detecting faults in the delivery of electrical power to electrical loads, includes a plurality of load electrical connections arranged to deliver electrical power from an electrical power source to each of a plurality of electrical loads, a plurality of electrical switches, each connected to an associated one of the load connections, and a diagnostic device operable to detect a short circuit fault in the apparatus, wherein the diagnostic device is operable to apply a diagnostic procedure to detect a short circuit connection between at least two of the load electrical connections and includes a control logic unit operable to apply to each of the electrical switches in turn a test control signal causing operation of the switch to apply a test electrical signal to each of the load electrical connections in turn; and detector means connected to the load electrical connections and operable, while the test electrical signal is applied in turn to each load electrical connection, to detect whether a corresponding electrical output is produced in response on any of the other load electrical connections. Also described is a method of operation in the apparatus.