A security alarm system for selectively detecting and signalling abnormal or emergency conditions, such as robbery, assault, fire, smoke, burglary, medical emergencies, etc. in a home, apartment, institution, plant or other place of business via digitally-coded messages, to a central data station. This central station monitors or services a plurality of areas to be guarded or protected and manually or automatically directs or dispatches appropriate aid to the location or area from which the signal originated. Essentially, the system comprises sensors responsive to the occurrence of abnormal or emergency conditions which transmit digitally-coded messages including information on self-identification and the nature of the emergency to a line converter. The line converter decodes the signal and then adds on information identifying its own location (e.g., room number, apartment number), and synthesizes a combined digital message which is then transmitted along power lines, such as the 110 Volt or 220 Volt AC power circuits commonly used in homes, apartments, businesses and institutions, at transmission frequencies and voltages substantially different from the power frequency and voltage, to a master controller. The master controller receives and decodes the digitally-coded messages transmitted by the line converters and adds further location information (e.g., street address) and synthesizes an appropriate digitally-coded message which it communicates to one or more central stations using one or more of a variety of transmission media: telephone line, coaxial cable, radio and external power line. Each central station services a plurality of master controllers in different protected areas.