Applies to inductive and capacitive proximity switches that sense the presence of metallic and/or non-metallic objects, ultrasonic proximity switches that sense the presence of sound reflecting objects and photoelectric proximity switches that sense the presence of objects. These proximity switches are self-contained, have semiconductor switching elements and are intended to be connected to circuits the rated voltage of which does not exceed 250 V 50 Hz/60 Hz a.c. or 300 V d.c.