An air bag system for protecting the occupants of a motor vehicle in the event of a collision has a plurality of firing circuits comprising air bag igniters (3a,3b,3c) in series with respective power transistors (T.sub.1,T.sub.2,T.sub.3) which can be triggered in the event of an accident to inflate a corresponding plurality of air bags. A single energy storage capacitor (E) is connected to all of the firing circuits for supplying energy to activate the bag igniters over the upper common power stage (7) in the event that there is a loss of battery voltage when the system has been actuated. A comparator (9) monitors the voltage drops across respective resistors (R.sub.1,R.sub.2,R.sub.3) in series with the power transistors (T.sub.1,T.sub.2, T.sub.3) and reduces the current through these transistors in the event that an excessive current is detected. Steps are taken to ensure that the period of activation of the current supply to the igniters is limited.