I do a variety of garage door work (openers, springs, hardware, even the door itself).  An older extension spring broke on this door (photo below), and although it did not have a safety cable, nobody was hurt, fortunately.  The safety cables are completely independent of the door operation.  They are attached at both ends and strung through the extension spring & loops to prevent pieces of metal flying through the air in case of a break.  I replaced all hardware on this system: two new springs, lift cables, connectors, pulleys, and safety cables.  This job usually takes less than half a day.  Call me for prices!  419-212-3200