a and 4b in the former an output signal released from the trigger 69 and passed through the time delay circuit 70 is also applied to the memory 66 so that it may cause release of the audible alarm 49 and it is fed to the collective memory m in the embodiment of fig4 b the output of the trigger 69 is directly connected to the or gate 64 so that its function is substantially similar to that of error signals a1 a2 a3 or a4 as indicated in fig4 the actual tension signal u derived from the sensor 15 and the amplifier 54 is fed to the collective memory m also fed into the memory are the mean value mu derived from the filter 55 and the difference signal du generated at the output of the subtractor 58 in accordance with the invention the oldest recorded signals are continually written over or replaced by later signals two kinds of errors are detected these are firstly errors in the mean value occurring whenever the continuously monitored mean value mu exceeds or drops below upper and lower limits umu and lmu of the predetermined range and secondly errors in the difference signal generated whenever the continuously monitored difference value du is in excess of or below upper and lower maximum values udu and ldu of the.