A ball shaft assembly comprising a cam shaft as a drive shaft, a ball shaft, as a driven shaft, a motor arranged to drive the ball shaft via the cam shaft, wherein the ball shaft has a head part defining a hole therethrough defining a flow passage, and a shaft part extending from the head part for engagement with the drive shaft, characterised in that the ball shaft assembly comprises a dielectric insulating insert assembly arranged to be positioned between the drive shaft and the driven shaft, the insert assembly comprising a plurality of elongate pins of dielectric material located between the drive shaft and the driven shaft and configured to engage, respectively, with the drive shaft and the driven shaft in torque transfer engagement, the pins providing a dielectric barrier between the drive shaft and the driven shaft, wherein the assembly further comprises mating interfaces at the drive shaft and the driven shaft shaped to engage with the pins in torque transfer engagement, and wherein the mating interfaces comprise a protrusion shaped to correspond to the shape of the pins provided at one of the drive shaft or the driven shaft and a recess shaped to correspond to receive the pins and the protrusion provided at the other of the drive shaft and the driven shaft, wherein the recess defines inner recess portions configured to receive the pins and the protrusion is configured to fit tightly within the recess when the pins are received therein, the protrusion defining outer recess portions sized to receive the pins; and further wherein the pins are sized to engage with the drive shaft and the driven shaft such that an air gap is formed between the drive and the driven shaft when the shafts and the pins are assembled together. arranged to drive the ball shaft via the cam shaft, wherein the ball shaft has a head part defining a hole therethrough defining a flow passage, and a shaft part extending from the head part for engagement with the drive shaft, characterised in that the ball shaft assembly comprises a dielectric insulating insert assembly arranged to be positioned between the drive shaft and the driven shaft, the insert assembly comprising a plurality of elongate pins of dielectric material located between the drive shaft and the driven shaft and configured to engage, respectively, with the drive shaft and the driven shaft in torque transfer engagement, the pins providing a dielectric barrier between the drive shaft and the driven shaft, wherein the assembly further comprises mating interfaces at the drive shaft and the driven shaft shaped to engage with the pins in torque transfer engagement, and wherein the mating interfaces comprise a protrusion shaped to correspond to the shape of the pins provided at one of the drive shaft or the driven shaft and a recess shaped to correspond to receive the pins and the protrusion provided at the other of the drive shaft and the driven shaft, wherein the recess defines inner recess portions configured to receive the pins and the protrusion is configured to fit tightly within the recess when the pins are received therein, the protrusion defining outer recess portions sized to receive the pins; and further wherein the pins are sized to engage with the drive shaft and the driven shaft such that an air gap is formed between the drive and the driven shaft when the shafts and the pins are assembled together.