already detectable even by conventional systems e g through the use of threshold value sensors which detect a short circuit or a line interruption according to the invention in the time period t the output signal produced by the speed sensor 12 is sampled n times this occurs according to the flowchart of fig5 after the start of the procedure according to step s 1 in step s 2 at a discrete instant e g t 1 t 2 etc in each case the actual sensor current is measured at the same time the amplitude of the sensor current is measured and an amplitude counter to which an amplitude range including this amplitude is assigned is incremented by the value 1 in step s 3 the respective values of the amplitude counter are entered into a histogram according to fig3 in this histogram the respective counter readings n are plotted against the amplitude values in other words to each amplitude value between 0 and 19 ma with an amplitude range width of in each case 0 5 ma an amplitude counter is assigned the interrogation of the actual sensor current according to step s 2 and the entry into a histogram according to step s 3 is carried out n times this means that in step s.