I just installed an ATO last night in my sump. I have seen in various places where a relay has been used but I did not set my system up with one. I used a DA submersible switch with an Aqualifter pump. What is the advantage of using a relay? My power is on a GFCI circuit with surge protection. I have yet to come across an explanation as to why the relay is needed or just preferred.

Safer to run 12V through water than 120V is what I've been seeing.

yeah but if you have a cord in the water it still has 120 volts running through it

Float switch's for our use can not handle the higher initial amp draw of most pumps. They will essentially weld the circuit shut and not allow the pump to turn off (or worse short circuit). If by DA you mean a digital aquatics float switch it would be hooked up to a module on your system and running on about 9 volts. The switch is turning on or off a 120 volt outlet on your PC4 I'm assuming is how you have it set up? NO relay is necessary then. You already have a relay circuit because you are not running the power for the aqua lifter directly through the float switch only the control for it. DO NOT run 120 volts through a float switch unless they are rated for it. FYI the ones sold for aquariums are not it will ruin them.

Also your going to want the aqua lifter pluged into channel 1 or 4 since the are relay controlled outlets 2 and 3 are solid state circuits and will give you issues with small pumps i.e. not turning off when they should. just a warning.

Posted By ThyRaven on 07/30/2012 04:54 PM
Safer to run 12V through water than 120V is what I've been seeing.

That's true but isn't the reason for adding relays. You could just buy dc pumps if that is your concern but the selection is slim and they are expensive. Just look at the vortechs and tunze pumps they are DC (low voltage) pumps you can vary the speed of DC current. AC current is completely different and much harder to do it with.

The ato sold by autotopoff.com come with a relay built in fwiw.