Secondary condensate drain installation

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Both of my AC units are in my attic so I want to make sure the secondary drain is working properly. I just had one unit replaced. The primary drain is working and piped to the house sewer system. The secondary drain on the collector pan is capped, but there is a large pan under the entire evaporator which would capture any water dripping out of the bottom of the evaporator. If the collector pan overflows, then the condensate would just have to find its way out however it could. My other unit has the secondary drain hole open so that if the level in the internal pan gets above the primary drain, then it will flow out the secondary drain into the overflow pan and out to the telltale pipe. However, because the hole is open, some cold air is always flowing out that hole and into the attic. My question is, can the secondary drain be connected in such a way that it will it will work reliably, but without the air leakage? What's the best practice for the secondary drain?

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I have learned there is a product called a wet switch which will detect the presence of water in the secondary pan and turn off the AC when it happens. My evaporator case is such that I could install one of these on the inside of the condenser. Is this ever done?

[gator37]

If you have an aux condensate drain pan under the indoor unit you can put a float switch that set so when detects a certain water level it will shut of the unit. This is in lieu of the aux drain to the outside.

[garya505]

I would use a Safety Overflow Switch. I didn't need a pan in mine, so I just put the SOS on the secondary drain port. Diversitech makes a nice one for that.