Selector switch detect if source is switchoff or setlevel [Solved]

[elgringo]

I have a dummy selector switch. With (for exmaple) 3 levels OFF(0), A(10), B(20)

Let say the selector is in B. When it is selected with OFF the level will become 0.
However when it is switched off it remains the level B, dzVents gets a trigger but the level has not changed.

When switch off, the active state remains true and the level remains unchanged. How can I detect if the the source is a set level command or a switch off command?

Edit:
I think the nValue contains what I want. This one is set to 0 when switch off and when setlevel = 0. combined with the current level I can determine it