Abstract: A monitoring device for use with a loop circuit including a plurality of lights, which includes a monitoring current transformer having a magnetic core with a primary circuit connected in series with the loop and a secondary circuit coupled to the primary circuit, the primary circuit being arranged such that the loop current saturates the transformer core and the secondary circuit being in open-circuit condition, the monitoring current transformer generating a monitoring voltage signal in response to each inversion of the loop current. The monitoring voltage signal is used to generate a window signal during the time interval when the monitoring voltage signal has a significant value and the window signal is then used to control the integration of a signal representative of the loop voltage with a view to producing a detecting signal which is indicative of the number of defective lights. The integration time interval is thereby determined whatever the loop current waveform may be.