Normally, Ducted Air Conditioning units have two drain pans, one is called Main Drain Pan which is inside the FCU to collect condensation water in cooling/dry mode, while the other is called Safety or Emergency Drain Pan which is placed beneath the FCU in the ceiling. Safety Drain is connected to the Safety Drain Pan and normally one can see that as a small pipe poking out of the fascia.
The reason for fitting a Safety Drain Pan is for the occasions that the Main Drain gets blocked by debris, sludge, slime or any foreign object. In this occasion, water will overflow into the Safety Drain Pan and it can save the ceiling.
The point is that in some installations, the installer tell the customer that they must always take care of that Safety Drain and if they see that it is dripping, turn the A/C off and let the installer/service technician know. The problem is that some customers cannot always keep an eye on the Safety Drain to see if it is dripping and they sometimes miss the point.
Imagine a scenario that the main drain gets blocked and water overflows into the Safety Pan and the Safety Drain starts to drip and the customer does not notice it for a while. If it keeps going, the accumulated water can easily overflow and damage the ceiling, especially when the Safety Tray is not level due to different reasons.
The more professional and safer way to protect the ceiling from overflow damage is to fit a Float Switch on the Main Drain Trap. So, if the drain gets blocked, the Float Switch gets activated and turns off the unit automatically.
Always ask the installer to fit a Float Switch for your ducted units even if the installer tries to put you up to ignore it.