Abstract: In a security system, detector apparatuses (101, 102, 103) transmit an alarm message to a central apparatus 100 via RF in response to detecting an alarm condition. The message comprises a source identification uniquely identifying the transmitting apparatus. The central apparatus 100 raises an alarm if the alarm message is sent by a detector apparatus, which is part of the system. To this end, the central apparatus 100 only processes an alarm message if the source identification of the alarm message is stored in a memory means 200 of the central apparatus. For a new detector apparatus to be accepted as part of the system, the identification of the detector apparatus needs to be stored in the memory means 200 of the central apparatus. To reduce the chance of identifications of neighboring apparatuses inadvertently being stored, a detector apparatus transmits a learn-detector message in response to a learn trigger, for instance from a user.