A protection assembly with an operating knob (1) for an electrical potentiometer comprises a substantially tubular housing (2) defining a longitudinal axis (L), an electrical adjustment device (3) held within the housing (2) and having an adjustment shaft (4) rotating about the longitudinal axis (L) and an internal variable resistor interacting with a longitudinal end (5) of the shaft (4), an operating knob (6) attached to the other end (7) of the shaft (4), limit stop means (8) for restricting the rotation of the shaft (4) within a predetermined maximum angle of rotation (a). The knob (6) comprises an annular anchoring member (9), which is designed to be removably coupled to the housing (2) and an operating member (10) coupled to the shaft (4) and rotating relative to the anchoring member (9). The anchoring member (9) and the operating member (10) have mutually facing cylindrical surfaces (11, 12), with said limit stop means (8) interposed therebetween to limit the infiltration of external impurities.