You are quite correct, if the PC is not connected to any patient connected device, then it is not a problem, but in many cases a PC is used to store data or display results when the medical device is in contact with the patient. The PC can comply with the relevent standards, but may also be interconnected to a number of other devices, monitors, hard drives, printers, and any number of other bits of kit, which all meet their own standards, but which taken together, can pose a significant risk. It is not unusual to find a PC plugged into an extension mains lead which is also feeding a monitor, printer, phone charger, and hard drive, so that the total earth leakage current flowing down the earth of the extension mains lead exceeds the maximum 3.5mA for a 13A plug.
If the PC is connected to a medical device that is in contact with the patient, then there should be an isolator in the connecting lead. In any case, I would check that there is no significant voltage (<10mV) on the medical device when it is connected to the PC, by measuring between all accessible conductive surfaces on the device and earth. Use a DMM and switch between AC and DC.You can sometimes get a DC touch voltage because the AC is being rectified.
On the same tack my report on switch mode power supplied has now been published, you can find it on
http://www.mhra.gov.uk/Safetyinformation...sSMPS/index.htm
