How to protect against line transients?
In any shunt overvoltage protection (OVP) design, there must be a way to limit series current if a low impedance clamp is used.  This requires knowledge of expected local , norms for peak voltage swings that can be tolerated by your equipment specs and levels of voltage and duration that might be expected.  this includes Lightning surges and power interruption surges.  


If the energy is excessive then a PTC fuse or thermal fuse is added.
a good way to attenuate kV peak  spikes is with a line filter.
Only you can determine if your grid is within 10% above expected nominal voltage which depends on location.  
Bear in mind RMS to peak conversion and use bipolar clamps.


The best design starts with the correct specifications of OVP and impulse to be protected in your application and location.