If the receiver is defined in its data sheet to be "fail safe" then it will automatically make its output to the idle level when the inputs are open.

A direct hit of lightning will easily take out even 600W tranzorbs. So for them to be effective in this case there also has to be another scheme for adding some type of series resistance to limit the overall current. There us a great way to protect sensitive level signal lines while still being able to protect them via a transil type device. Put the transil to ChGnd near the I/O connector. Attach the positive end of the part to dummy power rail connection of copper along the edge of the board. Then use a diode such as a BAT54S at each signal line to clamp the signal line to ChGnd and to the dummy power rail.

Mike Karas