I can't reproduce this on Triskel 5.5 with a nVIDIA 9800 GTX+ graphics card. Perhaps your settings have got corrupted. Please try creating a new userid / account for test purposes and adjusting the settings in:
System Settings > Display and Monitor > Screen Saver >> Start Automatically after
and
System Settings > Power Management > Power Profiles >> Screen Energy Savings
If you can't recreate the problem in a fresh account I suggest you move ~/.kde/share/config/kscreensaverrc and ~/.kde/share/config/powerdevil* out of the way and see what it does when you recreate the settings.
You can delete the test account once you have tried to recreate the problem.
The monitor has been switching off since I restored default settings. I had changed it to 15 minutes.
I can't reproduce this on Triskel 5.5 with a nVIDIA 9800 GTX+ graphics card. Perhaps your settings have got corrupted. Please try creating a new userid / account for test purposes and adjusting the settings in:
System Settings > Display and Monitor > Screen Saver >> Start Automatically after
and
System Settings > Power Management > Power Profiles >> Screen Energy Savings
If you can't recreate the problem in a fresh account I suggest you move ~/.kde/share/config/kscreensaverrc and ~/.kde/share/config/powerdevil* out of the way and see what it does when you recreate the settings.
You can delete the test account once you have tried to recreate the problem.
Please also check if you have this upstream bug https://bugs.kde.org/show_bug.cgi?id=270053
This also happens in Debian.