Not strictly arduino related but the answers would certainly be helpful for similar arduino setups. I have a usb relay plug-n-play kit which allows the PC to send commands over the usb cable to turn relays on and off. Inductive AC loads on the relays cause it a lot of problems, despite it saying it can switch "anything" on ac power. Basically, what happens is the switching will work once or twice but then its onboard micro seems to get corrupted and will no longer communicate. Requires a complete power cycle and disconnection from usb to correct. Yes, I know inductive loads are notorious for causing this kind of EMI havoc.
From my research it appears either a mains rated capacitor or RC network will address the issue. I have actually solved the problem temporarily by putting a purely resistive load in parallel with the inductive load. Before this I had tried isolating with relays, but the relays being an inductive load themselves also naturally caused issues. Would an RC network or just a mains rated capacitor be the better solution? I'm trying to find something reasonably affordable since there are 4 relays on the board. Searching on digikey for RC network brings up this, and I am unsure how to further narrow down the parametric search for what I need in this application.