TomGeorge:
Hi,
It may be best to invest in an Opto-Coupled Isolated USB to RS232 cable.http://www.advantech.net.au/products/gf-5u7m/adam-4561/mod_92dc04b1-c0fe-4f2b-baf6-5c27e79900c6
Unfortunately they are not cheap, but how much does the gear you are hooked up to, plus the PC cost and any down time you may incur.
Tom...
And the problem with these off the shelf solutions is they don't make any claim about dv/dt tolerance because they have not actually considered it in the design. I have designed RS485 isolators because the existing solutions couldn't tolerate much dv/dt at all without getting data errors. They may well withstand 2k5V or 3kV but only if it gets there slowly or there is no error rate checking during the transition from low CM (Common Mode) voltage to the very high CM voltage.
This is why I suggested those specific isolators; they are inductively coupled as opposed to capacitively coupled and have a very high dv/dt rating as a result. The real challenge is getting a power supply from one side of the isolation barrier to the other side without mucking up the dv/dt rating of the completed thing. The withstand voltage is almost irrelevant because the average would only be a few volts. The noise however could be huge, fast and high frequency as the OP has said he saw when he probed around the circuits.
PE.