Where does overflow occur?

[Jordan]

I understand that when I top off my tank, the only place the water level rises is in the return pump section of the sump. How does this work when taken to the extreme? I mean, If I add enough water to overflow the tank/sump, where does the overflow occur? The return pump section of the sump is what I assume, but I want to make sure? I am trying to determine where to install my two leak detection sensors.

 

[Aquarian]

If the sump water level is to high and your drain lines get clogged your pump will over fill your tank, if the return pump fails your tank can over flow your sump.