Apparatus (5; 105;...; 905) for braking a rotating element of an assembly, in particular for braking a rotating shaft of a rail vehicle, - with a rotating eddy current brake (6; 106;...; 906; 6a; 6b; 6c), which has a rotor (8; 108;...; 908) which can be connected to the rotating element in a non-rotating manner and a stator (9; 109;...; 909) which can be braced against the assembly, wherein the stator is provided with at least one excitation coil (11; 111; 211 to 218; 311 to 318; 411, 412; 511 to 518; 611, 612, 613, 614; 711 to 718; 811 to 818; 911 to 918) supplied by an electrical energy source (20; 120;...; 920) during braking operation, characterised by means (7; 107;...; 907) for eddy current testing of the rotating eddy current brake, which have at least one testing coil and one electrical signal source actively connected to the at least one testing coil, wherein the electrical signal source (125; 225;...; 925) applies a testing signal (U.P; U.P.1;...; U.P.9) to the testing coil during a diagnosis, which testing signal induces eddy currents in the rotor, and which have at least one measuring coil and one signal receiver (127; 227;...; 927) actively connected to the at least one measuring coil, wherein during the diagnosis the signal receiver taps a measuring signal (U.M; U.M.1;...; U.M.9) from the at least one measuring coil which is caused by the induced eddy currents and is thus suitable for diagnosis of the eddy current brake, - wherein the at least one excitation coil (11; 111; 211 to 218; 311 to 318; 411; 511 to 514; 611, 612; 711 to 714; 811 to 818; 911 to 918) forms the at least one testing coil and - wherein the at least one excitation coil (11; 111; 211 to 218; 311 to 318; 811 to 818; 911 to 918) and/or a further excitation coil (412; 515 to 518; 613, 614; 715 to 718) of the eddy current brake forms the at least one measuring coil. - with a rotating eddy current brake (6; 106;...; 906; 6a; 6b; 6c), which has a rotor (8; 108;...; 908) which can be connected to the rotating element in a non-rotating manner and a stator (9; 109;...; 909) which can be braced against the assembly, wherein the stator is provided with at least one excitation coil (11; 111; 211 to 218; 311 to 318; 411, 412; 511 to 518; 611, 612, 613, 614; 711 to 718; 811 to 818; 911 to 918) supplied by an electrical energy source (20; 120;...; 920) during braking operation, characterised by means (7; 107;...; 907) for eddy current testing of the rotating eddy current brake, which have at least one testing coil and one electrical signal source actively connected to the at least one testing coil, wherein the electrical signal source (125; 225;...; 925) applies a testing signal (U.P; U.P.1;...; U.P.9) to the testing coil during a diagnosis, which testing signal induces eddy currents in the rotor, and which have at least one measuring coil and one signal receiver (127; 227;...; 927) actively connected to the at least one measuring coil, wherein during the diagnosis the signal receiver taps a measuring signal (U.M; U.M.1;...; U.M.9) from the at least one measuring coil which is caused by the induced eddy currents and is thus suitable for diagnosis of the eddy current brake, - wherein the at least one excitation coil (11; 111; 211 to 218; 311 to 318; 411; 511 to 514; 611, 612; 711 to 714; 811 to 818; 911 to 918) forms the at least one testing coil and - wherein the at least one excitation coil (11; 111; 211 to 218; 311 to 318; 811 to 818; 911 to 918) and/or a further excitation coil (412; 515 to 518; 613, 614; 715 to 718) of the eddy current brake forms the at least one measuring coil.