How does this overflow system not siphon out if pump dies?

[JUReefer]

This is the tank in question. I just want to understand the plumbing. Sorry if this is a noob question. I'm just confused.

 

[Dread Pirate Dave]

Once water falls below the slots across the top of the overflow it will stop. This is as long as the return lines have check valves or are not too deep to prevent back siphon.

 

[Fish Fan]

Dread Pirate Dave is right on, the water will stop once the bottom of the weir (slots) is hit, and/or when the return nozzles break the siphon.

I'll add, you need to make sure that your sump can hold all of the water that does drain back when the power to the return pump is off.

 

[JonoH]

The overflow chamber itself will fully drain also - so make sure your sump can take that volume of water.

Seems strange for the full siphon to be so low down!

 

[JumboShrimp]

Those nozzles seem positioned pretty shallow (basically just skimming and breaking the water level in the tank... which is good for aeration), so you should be good if the power cuts out. As others have suggested, have enough excess room in your sump to handle running a 'kill the pump power' test.

 

[Fish Fan]

The overflow chamber itself will fully drain also - so make sure your sump can take that volume of water.

Seems strange for the full siphon to be so low down!

It is, and one could certainly add a piece of pipe to make it higher, but it shouldn't be an issue as the siphon will break once the bottom of the weir and/or the return nozzles are reached.