Overvoltage protection RS485 inputs
My advice is to use bidirectional zeners or TVS diodes and fuse the lines. If 48 volts is inappropriately applied, then the fuses burn to protect the zeners and, the zeners protect the circuit from over-voltage.
It's not ideal but at least you can change the fuses. You can get nice little SMD fuses and holders that might be suitable: -

Picture from Farnell.
Also, Analog Devices has got a good article on 485 protection methods here - it's entitled: -

RS485/RS422 Transceivers Operate from 3V to 5.5V Supplies and
Withstand ±60V Faults

Picture from the aforementioned document as an example: -