counts halfcycles 50 or 60 hz of the supply current occurring after the receipt of the synchronizing signal and has an individual address expressed as a certain number of such counted halfcycles assigned to the monitoring unit when this individually assigned number of halfcycles has been counted in a light monitoring unit 10 and if this light monitoring unit has a functioning lamp connected it transmits a status signal to the associated communicating unit 14 the signal is transmitted through the corresponding isolation transformer 6 and via the power cable 2 to the communicating unit 14 a row of such status signals will thus be transmitted to each communicating unit 14 from the light monitoring units 10 connected thereto a light monitoring unit 10 connected to a field lamp 4 cannot send any signals as described more closely below thus for each light monitoring unit 10 having a failed lamp connected thereto there will be a gap in the row of status signals the communicating unit 14 detects the responses from the light monitoring units and reports the addresses of non responding light monitoring units via the local concentrator unit 16 to the central concentrator unit 22 in the central concentrator unit 22 the addresses are stored in a database accessible to the computer 24 in the control room 27 on the display 25 the number of failed lamps 4 and the position of each failed lamp can be displayed different alarm criteria can be set in the central concentrator unit 22 via the computer 24 as will also be described more closely below the communication between the light monitoring units and the associated communicating unit.