Breathing assistance device comprising: - an electric motor turbine comprising a gas inlet and a gas outlet, the gas outlet of the turbine fluidly communicating with a patient circuit, the gas outlet of the turbine being fluidly connected to the patient circuit by way of a gas supply line, - an air inlet duct comprising an air inlet orifice fluidly communicating with the ambient atmosphere, said air inlet duct moreover fluidly connected to the gas inlet of the turbine, - an oxygen supply duct connected to the air inlet duct, in a connection site situated upstream of the gas inlet of the turbine, the oxygen supply duct fluidly communicating with the air inlet duct, - controlling means cooperating with the turbine so as to be able to detect any stop in functioning of said turbine, and - a duct blocking member arranged on the air inlet duct between the air inlet orifice and the connection site of the duct for supplying oxygen to the air inlet duct, characterised in that: - the controlling means are configured to detect a stop in functioning of the turbine and to control the duct blocking member so as to block the air inlet and to prevent any gas circulation towards the air inlet orifice when said controlling means detect a stop in functioning of the turbine, - a first solenoid valve is arranged on the oxygen supply duct, - the gas supply line comprises a flow sensor and a pressure sensor, and - the controlling means are furthermore configured to control the first solenoid valve, in case of the turbine stopping, in response to signals coming from the flow sensor or from the pressure sensor so as to adjust the oxygen flow circulating in said oxygen supply duct by alternating: - gas delivery phases during which the oxygen can pass through the first solenoid valve at a given delivery flow, and - gas non-delivery phases during which the oxygen does not pass through the first solenoid valve or passes through it at a limited flow (d). - an electric motor turbine comprising a gas inlet and a gas outlet, the gas outlet of the turbine fluidly communicating with a patient circuit, the gas outlet of the turbine being fluidly connected to the patient circuit by way of a gas supply line, - an air inlet duct comprising an air inlet orifice fluidly communicating with the ambient atmosphere, said air inlet duct moreover fluidly connected to the gas inlet of the turbine, - an oxygen supply duct connected to the air inlet duct, in a connection site situated upstream of the gas inlet of the turbine, the oxygen supply duct fluidly communicating with the air inlet duct, - controlling means cooperating with the turbine so as to be able to detect any stop in functioning of said turbine, and - a duct blocking member arranged on the air inlet duct between the air inlet orifice and the connection site of the duct for supplying oxygen to the air inlet duct, - the controlling means are configured to detect a stop in functioning of the turbine and to control the duct blocking member so as to block the air inlet and to prevent any gas circulation towards the air inlet orifice when said controlling means detect a stop in functioning of the turbine, - a first solenoid valve is arranged on the oxygen supply duct, - the gas supply line comprises a flow sensor and a pressure sensor, and - the controlling means are furthermore configured to control the first solenoid valve, in case of the turbine stopping, in response to signals coming from the flow sensor or from the pressure sensor so as to adjust the oxygen flow circulating in said oxygen supply duct by alternating: - gas delivery phases during which the oxygen can pass through the first solenoid valve at a given delivery flow, and - gas non-delivery phases during which the oxygen does not pass through the first solenoid valve or passes through it at a limited flow (d). - gas delivery phases during which the oxygen can pass through the first solenoid valve at a given delivery flow, and - gas non-delivery phases during which the oxygen does not pass through the first solenoid valve or passes through it at a limited flow (d).