Method for starting and operating a plant for the liquefaction of a gaseous product, said plant comprising at least a first and a second machine string, each string (2a, 2b) comprising an electrical motor and a load mechanically connected to the electric motor, the plant comprising a variable frequency drive connected to a power supply grid having its own frequency and associable to the electrical motors, the method comprising the steps of: electrically connecting the variable frequency drive to the motor of the first machine string; increasing the speed of the motor of the first machine string up until a first predefined threshold is reached, wherein the first predefined threshold is function of said frequency of the power supply grid; electrically disconnecting the variable frequency drive from the motor of the first machine string and electrically connecting the motor of the first machine string to the power supply grid wherein the speed of the motor of the first machine string is substantially maintained at the first predefined threshold; electrically connecting the variable frequency drive to the motor of the second machine string; and regulating the speed of the motor of the second machine string by means of said variable frequency drive; characterized in that during a trip condition where the electrical motor of the second machine string stops unexpectedly, the variable frequency drive is reconnected to the electrical motor of the first machine string to regulate the electrical motor of the first machine string until the trip condition is overcome, and once the trip condition is overcome, the variable frequency drive is disconnected from the electrical motor of the first machine string and reconnected to the electrical motor of the second machine string to restart the electrical motor of the second machine string. electrically connecting the variable frequency drive to the motor of the first machine string; increasing the speed of the motor of the first machine string up until a first predefined threshold is reached, wherein the first predefined threshold is function of said frequency of the power supply grid; electrically disconnecting the variable frequency drive from the motor of the first machine string and electrically connecting the motor of the first machine string to the power supply grid wherein the speed of the motor of the first machine string is substantially maintained at the first predefined threshold; electrically connecting the variable frequency drive to the motor of the second machine string; and regulating the speed of the motor of the second machine string by means of said variable frequency drive; characterized in that during a trip condition where the electrical motor of the second machine string stops unexpectedly, the variable frequency drive is reconnected to the electrical motor of the first machine string to regulate the electrical motor of the first machine string until the trip condition is overcome, and once the trip condition is overcome, the variable frequency drive is disconnected from the electrical motor of the first machine string and reconnected to the electrical motor of the second machine string to restart the electrical motor of the second machine string.