The neutrino mass experiment KATRIN recently operated its complete chain of the superconducting magnets for the first beam test. A Magnet safety system (MSS) for the 16 m long source magnet system has been installed for quench detection and protection of the superconducting magnets. They are cooled by a liquid helium bath at 4.5 K. The magnet system consists of seven main superconducting solenoids with three different current circuits. External dumping of the stored magnet energy and quench heater activation are foreseen by the MSS for quenches. Quench validation is not trivial because of the inductive couplings of the magnets to their neighbors. The MSS was tested with the whole KATRIN magnet chain during the first beam test. The MSS is able to distinguish a quench of its own magnet circuit from a quench of other neighboring magnets and to discharge the magnets by the external dumping. The high-performance MSS allows one significantly to reduce both the helium consumption and the cryogenic recovery time. This paper reports on the first results of the quench detection performance of the MSS for the KATIN source magnets.