a seat belt retractor having first and second serially arranged torsion bars and a switching mechanism provides two levels of load limiting . a bridge bolt rotationally fixed to the inboard ends of the torsion bars is threaded into a locking base , and the switching mechanism prevents bridge bolt translation relative to the locking base in a default condition to limit seat belt load with the first torsion bar . activation of the switching mechanism permits limited translation of the bridge bolt relative to the locking base to limit seat belt load with the second torsion bar . additional translation of the bridge bolt relative to the locking base is prevented to re - establish load limiting with the first torsion bar . the switching mechanism includes a set of detent wedges , a retaining band and an electrically activated cutting mechanism , all disposed within the retractor frame .