Vielen Dank @joekamprad xset war das Stichwort 
In der Wiki Dokumentation “hebelt” XFCE die Settings aus.
Note:
- XScreenSaver and xfce4-power-manager use their own DPMS settings and override xset configuration. See XScreenSaver#DPMS and blanking settings and Xfce#Display blanking for more information.
Xfce Power Manager controls blanking and DPMS settings. These settings can be configured in the Power Manager GUI within the Display tab.
Note that when Display power management is turned off, DPMS is fully disabled, it does not mean that Power Manager will simply stop controlling DPMS. It does not disable screen blanking either. To disable both blanking and DPMS, right click on the power manager system tray icon or left click on the panel applet and make sure that the option labelled Presentation mode is ticked.
Der Presentation Mode, ja gar nicht einfach zu finden 
konnte ich nur erreichen, in dem ich die Energieverwaltung in der XFCE Panelleiste gelegt habe. Anbei die Screenshots
PS: In der Panelleiste heisst das Icon “Rechner” 
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