Τhe BS-521 is an autonomous siren for fire detection systems that gives a strong sound output and an optical warning (only sound for BS-520). They have rechargeable Ni-Cd batteries so they are activated in the case of the main's interruption which can be caused by a fire.They can be connected with one or more break glass call points (model BS -536). By pressing the glass of the call point the siren is activated.The strong sound and the optical warning beacon covers many square meters.