This contribution presents an experimental study about pressure variations phenomenon observed during a pool fire in a confined and forced ventilated enclosure. This research is addressed to fire safety in the nuclear industry in which buildings are often compartmented in set of enclosures and equipped with ventilation air cleaning system able to ensure the confinement. The present work proposes to analyse the pressure effects and to emphasize the influence of the fire heat release rate (HRR), of the characteristics of the compartment and of the ventilation layout (especially the airflow resistances of the ventilation branches). This study is based on three sets of tests performed in three large-scale facilities of various dimensions (3600 m³, 400 m³ and 120 m³ in volume) and with several settings of the ventilation network. The analysis focuses on the conditions that lead to significant pressure peaks and identifies the level of fire HRR and airflow resistances for which pressure peaks may become a concern for fire safety. Conclusions bring practical criteria, which allow identifying the probability of having a significant pressure variation.