of an alarm can be suppressed by control device 26 for the duration of the pulse a self test of spark sensor 22 especially of its optical receiver 28 can thereby be performed by means of a purposeful activation of source 40 a periodic checking of the operation of spark sensor 22 is thus possible corresponding to the repetition frequency of the test pulses without the sensor having to be removed from suction air system 10 for example in addition to this checking of the optical operation of spark sensor 22 a checking of the electrical operation can take place in a known manner by monitoring the resting current if the resting current exceeds a set value e g 20 ma a short circuit inside connecting lead 24 or inside spark sensor 22 is indicated if the resting current is interrupted for example an interruption of an electrical lead is indicated control device 26 comprises for example an optical display which displays a failure of spark sensor 22 an appropriate replacement of the defective spark sensor 22 can then be performed by the operating personnel furthermore an automatic error signal can be transmitted to a central data processor serving to control the rotor spinning machines so that in addition to a documentation of the error signal a defective spark sensor 22 can be identified at the same time instead of or in addition to actuating the test pulses via timing element 48 source 40 can also be actuated manually via a separate.