If water comes out of that secondary drain, your main drain is plugged. It can happen at the coil drain pan at the opening going out to the trap, before it gets to the trap. For a permanent tattletail device, set a small bowl directly under the secondary drain so you can know if the secondary is flowing.
Hopefully the overflow switch is wired to protect from damaging overflow.
A forum community dedicated to Do it yourself-ers and home improvement enthusiasts. Come join the discussion about tools, projects, builds, styles, scales, reviews, accessories, classifieds, and more! Helping You to Do It Yourself!