Disclosed is a primary-clocked switching power supply for converting an input voltage into an output voltage, comprising: a primary side circuit branch, where the input voltage can be applied; isolated from the primary side circuit branch, a secondary side circuit branch, where the output voltage is tappable; between the primary side and the secondary side circuit branch, a galvanic isolation; a fuse or circuit breaker arranged in the primary side circuit branch; a first switchable switch element arranged in the primary side circuit branch such that switching trips the primary side fuse or circuit breaker; and a monitoring unit connected with the first switch element and arranged in the primary side circuit branch and adapted to monitor a characteristic electrical signal determined by the second primary winding and, when the characteristic electrical signal exceeds a threshold value, to switch the first switch element.