LED unit for an optical signalling device, comprising: - a lighting unit, comprising a three-pole (31, 32, 33) terminal, a first lighting means with a first LED string, a second lighting means with an arbitrary connection direction and a second LED string, a first switching device on a first pole of the three-pole terminal, with which the first LED string is connected in series between the first pole and a third pole of the three-pole terminal, and a second switching device on a second pole of the three-pole terminal, with which the second LED string is connected in series between the second pole and the third pole, - a return conductor between the third pole and a power source connection pole - a controller for the first and second switching devices (21, 22), which comprises a first control function for activating the first and second LED strings through the first and second switching devices (21, 22) in an alternating manner, and - a measurement unit (51, 52, 61, 62, 71, 72, 81, 82) of the controller with a function for detecting a failure of the lighting unit, characterized by - a third switching device on the power source connection pole, wherein the second lighting means is connected between the third pole and the third switching device, - a second control function of the controller for connecting the first lighting means in a non-alternating manner to the first pole with the first switching device and for connecting the second lighting means to the power source connection pole with the third switching device on the disconnection of the return conductor from the second pole with the second switching device, and - a switch-over function of the controller, for switching from the first to the second control function on detection of a failure of the LED unit with the measuring unit as a result of the disconnection of the return conductor. - a lighting unit, comprising a three-pole (31, 32, 33) terminal, a first lighting means with a first LED string, a second lighting means with an arbitrary connection direction and a second LED string, a first switching device on a first pole of the three-pole terminal, with which the first LED string is connected in series between the first pole and a third pole of the three-pole terminal, and a second switching device on a second pole of the three-pole terminal, with which the second LED string is connected in series between the second pole and the third pole, - a return conductor between the third pole and a power source connection pole - a controller for the first and second switching devices (21, 22), which comprises a first control function for activating the first and second LED strings through the first and second switching devices (21, 22) in an alternating manner, and - a measurement unit (51, 52, 61, 62, 71, 72, 81, 82) of the controller with a function for detecting a failure of the lighting unit, - a third switching device on the power source connection pole, wherein the second lighting means is connected between the third pole and the third switching device, - a second control function of the controller for connecting the first lighting means in a non-alternating manner to the first pole with the first switching device and for connecting the second lighting means to the power source connection pole with the third switching device on the disconnection of the return conductor from the second pole with the second switching device, and - a switch-over function of the controller, for switching from the first to the second control function on detection of a failure of the LED unit with the measuring unit as a result of the disconnection of the return conductor.