Lc1-mfd metal flameproof cover, with innovative ceramic connector lc1-pib lead tone indicator board loudspeaker can be installed with optional lead frequency sound indicator board. The small PCB with LED indicator lights can be fixed to a support and connected to a standard optical conduit. After that, the user can visually check if there is a pilot tone by integrating the LED indicator light on the edge of the front mask. The required pilot tone signal level is 4 V RMS (20 kHz), so the load of these panels on the power amplifier is negligible.