In 2021 problems with quench protection system resulted in two accidents with superconducting (SC)
magnets of the JINR Booster. Each accident resulted in extensive repairs. However, the recorded signals of
voltage and current in the magnets enabled detailed analysis of quench development well beyond normal
operating conditions. This paper analyses dynamics of quench propagation in the string of Booster SC
magnets and considers the safety margin in the string operation.