USB powered handheld medical device - Isolation requirements

[Fred1977]

Hello,
I have to develop a medical device which is connected to a standard PC by a USB cable.
The device will take power from the USB port. The cable will do directly into the handheld section (no connector) - which is a large lump of plastic, and there are no accessible metal parts. The closest a fingertip could possibly get to any internal conductors would be if the user held the cable with their tingertips.
Am I correct in thinking that there would be no requirement for galvanic isolation from the PC USB port, as the plastics already provide all the galvanic isolation required?
Any advice gratefully received.

 

[Peter Selvey]

The decision is mostly based on the way the device interacts with the patient. It's OK for the user to contact the USB circuit directly, it is "safe" on the assumption that the PC complies with the relevant safety standard (IEC 60950-1).

However, 950 isolation is not good enough for the patient, so to comply with IEC 60601-1 there will need to be some additional barrier to the patient. Sometimes this is inherent in the design (the parts that contact the patient are naturally plastic), sometimes another isolating barrier is needed. And in some cases it could be argued that even though it technically fails IEC 60601-1, the type contact with the patient is nothing special so the additional requirements in that standard don't make sense. Anyway, it all comes down to the type and nature of patient contact.