Electrical cable sheaths are the most abundant fire load in nuclear power plants, mainly in rooms that are kept in under slight pressure. This configuration leads fires to grow in under-ventilated and vitiated conditions. Assessing fire threat involves characterizing the heat released, responsible for fire growth, and the smoke evolved, which may interact with sensitive components in the area. For that purpose, a revisited controlled-atmosphere cone calorimeter has been designed, set up, and coupled to a Fourier transformed infrared spectrometer and an electrical low-pressure impactor to measure simultaneously the evolved gases and aerosols, respectively. This bench-scale apparatus has been first qualified with polymethylmethacrylate. It has second been used to characterize polyvinylchloride cable sheath representative material reaction to fire in under-ventilated and vitiated conditions. It appeared that vitiation in under-ventilated fires lowers the heat release rate and the fuel mass loss rate.