Protect output of op-amp from overvoltage
When you put 15V into a "very low impedance", one of two things will happen.  


The low impedance will sink the current from the voltage source, and the voltage source will be dragged down to a low voltage.  In this case, you will have a large current, and it's the large current you need to detect to save your circuit.  
The voltage source "wins", and basically destroys the circuit to get to 15v.  In this case, it's too late for protection.


Other than the instructive insights here, the point is that "something that reacts to 15V" isn't really what you're after.