I sell access systems and one of my suppliers has a RS485 optical Mux. One in and eight out that are optically isolated. Can you add a link to your units? I will pass it by the manufacture for their consideration. The theory is something is putting voltage on the ground. The entire system uses RS485 two wire with ground. The ground is connected at one spot to earth ground. Bad juju if the ground is feed voltage not dissipating it.140326-1944 EDT
The RS485 circuits need isolation. RS485 chips do not have a large common mode capability. Just in the few volt range.
On I232 isolators I make RS232 is optically isolated and converted to RS422, and then with another unit back to RS232. With this arrangement RS232 data can be sent error free at 115.2 kbaud while a 1000 V RMS 60 Hz sine wave is applied to the RS422 common relative to the RS232 common. The 1000 V limitation is base on the maximum voltage capability of the optical coupler. RS422 and RS485 are essentially the same type of signal. RS485 is a half-duplex connection whereas RS422 is full-duplex.