Peter, here are a few links for worthwhile choices of Ethernet Isolators that are not optical. I am still trying to find the thread on CA but I am still looking for the right one. So this will have to make do for now.
http://www.emosystems.com/product/en-70e-ultra-compact-network-isolator/
http://industrialcomponent.com/baaske/mi1005.html
http://www.artistic-fidelity.de/index.php/en/giso-isolator
Some me of us like the Emo best and others like either of the other two.
Of course this is not an exhaustive list but these are the most respected and popular ones.
You would put put one of these just before the server.
I hope this helps.
http://www.emosystems.com/product/en-70e-ultra-compact-network-isolator/
http://industrialcomponent.com/baaske/mi1005.html
http://www.artistic-fidelity.de/index.php/en/giso-isolator
Some me of us like the Emo best and others like either of the other two.
Of course this is not an exhaustive list but these are the most respected and popular ones.
You would put put one of these just before the server.
I hope this helps.