Abstract: A fault detection apparatus for a public lighting system has a number of lamps. Each lamp incorporates an operating and control circuit which will emit signals on a fault occurring or conditions arising indicating a potential fault, for example, of a gas discharge tube forming part of the lamp. The apparatus, which can be incorporated in the lamp operating and control circuit or be provided as a separate plug in unit incorporates a host interface. A control unit and an externally visible signalling device such as a flag or flashing beacon will allow the detection of a fault or potential fault for example during normal daylight hours.