The thermal detection and visual alarm unit includes a plurality of first expansion thermal detectors (17) with normally closed contacts that are connected in series with a first lamp (15) to form a first two-terminal circuit (21, 23), a plurality of second expansion thermal detectors (18) with normally open contacts connected in parallel, a second lamp (16) connected in series with the second thermal detectors to form a second two-terminal circuit (22, 24), the first and second thermal detectors forming a plurality of cells (19) each combining a first detector and a second detector having respective contact-opening or contact-closing temperatures that are substantially equal, each of the first and second two-terminal circuits being arranged for connection to the terminals of an electrical power supply (12).