I have read a lot of information about the clearance and creepage strategies, distances, materials, etc..
I am designing my first HV PCB, main LV circuit will be on TOP and HV (6KV) on the bottom. But I can' t avoid some low voltage points on the same side. Here I will be using insulation barriers to increase creepage and clearance at the same time. But I would like to use slots in order to separate and give some air gap between "hot" points. But when you put slots, the only factor that increase will be creepage. Clearance stills being the same with or without the slot.
So, here are my questions:
When to use slots instead of barriers?
Easier to produce.
When can be the clearence shortest than creepage?
Clearance is always at most as long as creepage path. That's the simple geometric triangle inequality.
Is it the same to put a 3cm slot between two points and to simply put a distance of 3cm without slot between 2 points?
No, as your pictures clearly illustrate.
How can it helps to avoid breakdown if clearance is the same at two both cases?
Not quite sure what you mean.
Not an answer, still an important thing to show (and comments don't show images):
When the bottom layer has the high voltage, there is another creepage way, e.g. the one like shown in purple in image below.