The onset of fiber failure signals the commencement of catastrophic structural failure in fiber bundle systems, a process that is important in standalone bundles and in other material structures. In this work, we have demonstrated a technique by which the onset of fiber failure ran accurately be identified by monitoring the electrical continuity and resistance of filaments and bundles during loading. The dual sensor/loading functionality of the fibers differs from most prior approaches wherein separate sensors were added to the load-bearing fiber array, Individual fiber failure and bundle onset failures in 50 fiber bundles were reliably and reproducibly detected.