ESD protection on an isolated circuit
250kbps is quite fast for RS485. How far apart is your nodes?
How big your EMI problem will be (and how strong protection is required) is significantly tied to location of your infrastructure. If they are all contained in the same building a tiny TVS diode, such as the SM712 will suffice. However, if you are outside, with a cable that is some hundred meters long you need to &quot;beef up&quot; with a GDT or MOV, and appropriate supporting components.
Boruns has some great articles on this topic: Link