Protecting a low voltage (RS485) bidirectional bus from direct mains connection
In the real world you can never make things totally idiot proof. 

If these things are designed to be installed and then left alone, it is foolhardy to add more components than are necessary to protect it from normal operational faults. 

If it is meant to be routinely moved around, and or reconnected, then I'd say it maybe more prudent to invest in more protection. Use of TVS though is intended for transients. They will not protect your circuit from being connected to a multi-megawatt power supply. If I really had to do this I would be looking at some form of crow-bar circuit, perhaps using a triac, to detect over voltage, short the line to mains ground, and blow a fuse, possibly a resettable one.

Instead of spending development money on making it bullet-proof and reducing the reliability of the unit as a whole, invest time in preparing clear and concise installation manuals and trust that the electrician can read / do his job properly. If you are really paranoid, add paper tags to the low voltage connections indicating "NO MAINS HERE" or "LOW VOLTAGE CIRCUIT".

If you are getting lots of field returns then analysing what can be done to prevent that is prudent. However, fixing the connectors and improving the documentation is the better approach. Preventing the problem ALWAYS trumps protecting from it.