System separator having a housing which defines an inlet chamber, an intermediate chamber and an outlet chamber, having backflow prevention means which are positioned in the housing and are arranged one behind the other in the throughflow direction of a medium, specifically a first, inlet-side backflow prevention means, which is arranged between the inlet chamber and the intermediate chamber, and a second, outlet-side backflow prevention means, which is arranged between the intermediate chamber and the outlet chamber, and having a drain valve which aerates the intermediate chamber between the backflow prevention means (12, 13) and is controlled in a manner dependent on a differential pressure between the pressure in the inlet chamber and the pressure in the intermediate chamber, wherein each backflow prevention means (12, 13) comprises a nozzle body having at least one valve seat, and comprises a valve plate and a spring element, wherein the nozzle body of the inlet-side backflow prevention means and the nozzle body of the outlet-side backflow prevention means are of identical form and each have two valve seats, specifically in each case a first valve seat, which is active if the respective nozzle body is installed at the inlet-side backflow prevention means, and in each case a second valve seat, which is active if the respective nozzle body is installed at the outlet-side backflow prevention means, and wherein each nozzle body has a projection radially at the outside, characterized in that, only if the respective backflow prevention means (12, 13) is installed in its correct fitting position in the housing, that is to say the inlet-side backflow prevention means is installed between the inlet chamber and the intermediate chamber and the outlet-side backflow prevention means is installed between the intermediate chamber and the outlet chamber, on the one hand the respective projection of the respective nozzle body projects into a groove in the housing and on the other hand the respective backflow prevention means (12, 13) is able to be secured in said fitting position via a securing element which projects into the respective groove and which is in the form of a retaining clamp, whereas if the inlet-side backflow prevention means is erroneously installed between the intermediate chamber and the outlet chamber counter to the flow direction and the outlet-side backflow prevention means is erroneously installed between the inlet chamber and the intermediate chamber counter to the flow direction, the respective retaining clamp is not able to be fitted, and if the inlet-side backflow prevention means is erroneously positioned between the intermediate chamber and the outlet chamber in the flow direction and the outlet-side backflow prevention means is erroneously positioned between the inlet chamber and the intermediate chamber in the flow direction, dimensions of the spring elements of the backflow prevention means (12, 13) prevent the erroneous installation thereof into the housing. only if the respective backflow prevention means (12, 13) is installed in its correct fitting position in the housing, that is to say the inlet-side backflow prevention means is installed between the inlet chamber and the intermediate chamber and the outlet-side backflow prevention means is installed between the intermediate chamber and the outlet chamber, on the one hand the respective projection of the respective nozzle body projects into a groove in the housing and on the other hand the respective backflow prevention means (12, 13) is able to be secured in said fitting position via a securing element which projects into the respective groove and which is in the form of a retaining clamp, whereas if the inlet-side backflow prevention means is erroneously installed between the intermediate chamber and the outlet chamber counter to the flow direction and the outlet-side backflow prevention means is erroneously installed between the inlet chamber and the intermediate chamber counter to the flow direction, the respective retaining clamp is not able to be fitted, and if the inlet-side backflow prevention means is erroneously positioned between the intermediate chamber and the outlet chamber in the flow direction and the outlet-side backflow prevention means is erroneously positioned between the inlet chamber and the intermediate chamber in the flow direction, dimensions of the spring elements of the backflow prevention means (12, 13) prevent the erroneous installation thereof into the housing.