A lighting system and a corresponding method are provided. The method comprises: sending a change operational state command to a lighting device group (3) comprising several lighting devices (9); at each lighting device, applying a randomized delay within a predetermined delay interval or an individual predetermined delay within the delay interval; changing the operational state in accordance with the change operational state command at each lighting device at the end of each respective delay; detecting changes in the total drive power fed to the group of lighting devices within the delay interval and counting the total number of changes; comparing the total number of changes with a nominal number corresponding with the number of lighting devices within the group of lighting devices; generating a lighting device error signal if the number of changes is smaller than a predetermined fraction of the nominal number, including the nominal number.