It seems the permitted amount of shaft voltage is related to some design criteria which shall be recommended by manufacturers. Normally, voltage levels below 0.5 V will not cause harmful currents. When expected or measured shaft voltage is above 0.5 V, provisions are made to open or break the circuit to prevent current flow. The normal method is to insulate the motor bearing at the end opposite to the coupling end where the motor is coupled to the driven equipment. And, of course, other connections to the motor shaft at the end opposite the drive coupling must also be insulated to maintain the open circuit. Special provisions are necessary in special cases to maintain an open circuit, such as insulated drive couplings in large tandem motor arrangements, or when driving in auxiliary oil pump or other device on the outboard end of the motor.