<p>Active stress corrosion cracking (SCC) poses a serious risk to the safety of prestressed concrete structures because it causes prestressing wire breaks (PWBs) and degrades the structural integrity. Not all structures at risk can be replaced immediately (e.g., due to limited resources) and need to be kept in operation—without putting passers-by at risk. Hence, reliable detection of PWBs is crucial for ensuring structural integrity and safety, e.g., by equipping these structures with an appropriate structural health monitoring (SHM) system. This study presents an experiment on a decommissioned prefabricated large-scale post-tensioned concrete girder. Up to 20% of the total area of the girder’s parallel strands in two bundled tendons were intentionally damaged by cutting to investigate the structural response of the girder, which was monitored using a distributed fiber optic sensor (DFOS) mounted to the girder’s surface It was shown that detection and localization of PWBs by means of distributed fiber optic strain sensing (DFOSS) is possible. However, the resulting signals are weak—requiring highly sensitive measurement equipment to enable their detection. Putting the signals relation with measurement principle-related disturbances and temperature influence highlights the necessity for compensating these disturbances for reliable PWB detection. The effect of different DFOS positions is discussed, showing that the signal is the more characteristic (more localized and stronger) the closer the DFOS is to the PWB.</p>