Planned outages of transmission systems are required to allow planned work to be done with the minimum impairement to the service concerned.All tests,measurements,rearrangements,etc.,which are not attributed directly to a failure - and are known in advance-are considered to be planned work.Such work will include installation of new equipment,routine maintenance,work on power supply equipment and in some cases, work for the clearance of faults which at first could only be remedied provisionally (mainly cable faults).In the event of planned work which results in the complete or partial interruption in a transmission system,efforts are at first made to reroute the telecommunication traffic as required.If special restoration plans exist for cases of faults,these pians can also be used in the event of planned outages.Should rerouting be impossible,planned work is generally carried out during periods of light traffic,e.g. at night.To allow appropriate measures to be taken,all stations affected by the planned outage must be informed in good time.