Cast when damage taken mechanics

Hi,

is the damage needed for Cwdt added together till it hits the trigger-value or do you need to get hit one time for that damage? Is there an internal cooldown for this addition process? (do you have to get x damage in y time?)

The damage accumulates. There is no internal cooldown for damage received, as long as you are still in the same map it should keep adding up.
There is an internal cooldown for re-casting, but it's not very long.
Only damage from hits is counted.

PS: Also, each gem keeps its own record of 'damage taken'. If you have 2 gems, only one can proc at a time. But the other one will still record the damage taken, meaning it could be ready to go off the next time you took any damage at all.

Thanks for the info!

So if you got multiple Cwdt its smart to have them at different levels, so they go off after each other instead of blocking one oneanother?!

Depends.
If you want to space out casts, then yes.
I do it out of habit.
But if you have no spell twice in a CwDt setup (over the char, not that specific item), you needn't worry.

For more in-dept understanding, consider this:
If you have a CwDt with EK, and after you take 500 damage (let's assume it hasn't triggered yet), you add a second EK, they will effectively be on different timers, though they'll possibly equal again over time as you might take more damage at once than necessary for triggering.