Abstract: A diagnostic device for use in the detection of a fault in an electrical apparatus, especially transformers that have electrical windings immersed in electrical insulating oil with a headspace above the oil. The device comprises a housing defining a chamber with a mouth adapted to receive a pressure relief valve on the oil tank. The device creates a fluid tight seal around the pressure relief valve and operates the pressure relief valve to extract a gas sample from the headspace into the chamber. The device may include a gas analyzer for analyzing the gas sample for the presence of acetylene, which is indicative of a fault in the apparatus.