Apparatus for detecting faults in the delivery of electrical power to electrical loads, including: a plurality of electrical load connections (115, 117, 119, 121) arranged to deliver electrical power from an electrical power source to each of a plurality of electrical loads (123, 125, 127, 129); a plurality of electrical switches (107, 109, 111 and 113) each connected to an associated one of the load connections; a diagnostic device operable to detect a short circuit fault in the apparatus, the diagnostic device being operable to apply a diagnostic procedure to detect a short circuit connection between at least two of the electrical load connections and including a control logic unit operable to apply to each of the electrical switches in turn a test control signal causing operation of the switch; and detector means (131, 133, 135, 137) connected to the electrical load connections and operable to detect, whilst the test control signal is applied to each of the switches in turn, whether an electrical output is produced at a electrical load connection indicating a short circuit connection at the electrical load connection; wherein: (i) when the test control signal is applied to a first one of the switches connected to a first one of the electrical load connections, the control logic unit is operable to deliver a control ('Off Command') signal which puts the other switches (109, 111, 113) in an off state; and (ii) when the first switch is operated and the control signal has been applied to the other switches (109, 111, 113) to put those switches in an off state, the detector means (133, 135, 137) is operable to detect whether, in response, an electrical output indicating a short circuit connection to the first electrical load connection is produced on any of the other electrical load connections (117, 119, 121) connected to the switches (109, 111, 113) in an off state. a plurality of electrical load connections (115, 117, 119, 121) arranged to deliver electrical power from an electrical power source to each of a plurality of electrical loads (123, 125, 127, 129); a plurality of electrical switches (107, 109, 111 and 113) each connected to an associated one of the load connections; a diagnostic device operable to detect a short circuit fault in the apparatus, the diagnostic device being operable to apply a diagnostic procedure to detect a short circuit connection between at least two of the electrical load connections and including a control logic unit operable to apply to each of the electrical switches in turn a test control signal causing operation of the switch; and detector means (131, 133, 135, 137) connected to the electrical load connections and operable to detect, whilst the test control signal is applied to each of the switches in turn, whether an electrical output is produced at a electrical load connection indicating a short circuit connection at the electrical load connection; wherein: (i) when the test control signal is applied to a first one of the switches connected to a first one of the electrical load connections, the control logic unit is operable to deliver a control ('Off Command') signal which puts the other switches (109, 111, 113) in an off state; and (ii) when the first switch is operated and the control signal has been applied to the other switches (109, 111, 113) to put those switches in an off state, the detector means (133, 135, 137) is operable to detect whether, in response, an electrical output indicating a short circuit connection to the first electrical load connection is produced on any of the other electrical load connections (117, 119, 121) connected to the switches (109, 111, 113) in an off state. (i) when the test control signal is applied to a first one of the switches connected to a first one of the electrical load connections, the control logic unit is operable to deliver a control ('Off Command') signal which puts the other switches (109, 111, 113) in an off state; and (ii) when the first switch is operated and the control signal has been applied to the other switches (109, 111, 113) to put those switches in an off state, the detector means (133, 135, 137) is operable to detect whether, in response, an electrical output indicating a short circuit connection to the first electrical load connection is produced on any of the other electrical load connections (117, 119, 121) connected to the switches (109, 111, 113) in an off state.