A method of controlling application of electrical power in an electrically driven oxygen separation device, said method comprising: applying part of the electrical power in at least one separation zone of the electrically driven oxygen separation device having at least one composite membrane element (10, 12) by applying an electrical potential difference to anode and cathode electrodes of the at least one composite membrane element (10, 12) while heating the at least one composite membrane element (10, 12) to an operational temperature, thereby to separate the oxygen from the oxygen containing feed stream by inducing oxygen ion transport in an electrolyte situated between the anode and cathode electrodes through the application of the electrical potential difference; the at least one composite membrane element (10, 12) having a resistance increasing over time and during operation of the at least one composite membrane element; characterized in that the method further comprises: controlling the electrical potential difference applied to the at least one composite membrane element (10, 12) such that an electric current drawn by the at least one composite membrane element (10, 12) remains at a substantially constant, predetermined electrical current level by increasing the electrical potential difference as the resistance increases until the electrical potential difference reaches a predetermined voltage level and thereafter, maintaining the electrical potential difference at a constant voltage level when the electrical potential difference reaches the predetermined voltage level, the constant voltage level being below a maximum voltage level at which the at least one composite membrane element (10, 12) will fail. applying part of the electrical power in at least one separation zone of the electrically driven oxygen separation device having at least one composite membrane element (10, 12) by applying an electrical potential difference to anode and cathode electrodes of the at least one composite membrane element (10, 12) while heating the at least one composite membrane element (10, 12) to an operational temperature, thereby to separate the oxygen from the oxygen containing feed stream by inducing oxygen ion transport in an electrolyte situated between the anode and cathode electrodes through the application of the electrical potential difference; the at least one composite membrane element (10, 12) having a resistance increasing over time and during operation of the at least one composite membrane element; characterized in that the method further comprises: controlling the electrical potential difference applied to the at least one composite membrane element (10, 12) such that an electric current drawn by the at least one composite membrane element (10, 12) remains at a substantially constant, predetermined electrical current level by increasing the electrical potential difference as the resistance increases until the electrical potential difference reaches a predetermined voltage level and thereafter, maintaining the electrical potential difference at a constant voltage level when the electrical potential difference reaches the predetermined voltage level, the constant voltage level being below a maximum voltage level at which the at least one composite membrane element (10, 12) will fail.