I have a problem on a milking parlour. When the motors are running they are giving a reading of 2.9v Earth to Neutral. The motors are 3hp and 1.5hp single phase. A third motor is 1hp, giving a reading of 0.9v E to N.

This stray voltage is causing the farmer's cows to have a very high somatic cell count (mastitis). I have done loop impedance test and insulation test and all show good results. This problem seems to have arisen since the installation of a new substation, as part of an upgrading of the system. Earthing is by PME. Any help would be appreciated. Thank you.