Abstract: A series-fed light monitoring system, used for example for runway lights at airports, and fed by a low-frequency feeding current, comprises at least one series loop having one or more loading points connected in series in the form of fittings with lamps or the like, each loading point being connected to the loop via transformer coupling comprising in series a high-frequency transformer, a switch and a choke. The switch and the choke are connected in series in the series loop and the high-frequency transformer is connected on the primary side to the series loop and on the secondary side to the loading point. The choke has an inductance so as not to hinder a low-frequency feeding current in the loop but provide high impedance at the switching frequency.