Module to monitor the status of the loudspeaker line. It connects in parallel to the last speaker on the line and emits a non-audible 20 kHz tone that is used to detect anything from an impedance change to a short or open line. It prevents any connection error and line impedance variation due to aging of the installation and weather conditions. It also incorporates a thermal fuse that reduces the risk of a short circuit in the event of fire: at 135ºC it burns and causes an open circuit error on the line. Supplied in sets of 10 units.