USB-Connection - Creepages and Clearances

Benjamin Weber

Trusted Information Resource
Dear community,

I came across the following situation for USB connections in ME equipment.

Usually maximum mains voltage (let's say 250 Vrms) is considered normal condition at the USB port (you never really know what will be connected to the device). Usually digital isolators are used to isolate the USB from patient circuits on the internal PCBs. But what about creepage distances and air clearances on at the connection itself?

Here the requirements should apply as well (material class IIIb, pollution degree 2):
Let's say it's used in homecare, we need patient safety:
-> 2 MOPP: 8 mm CD, 5 mm AC, 4000 V test voltage

Even if no patient safety is needed, for 250 Vrms operator safety is still an issue:
->2 MOOP: 5,0 mm CD 4,0 mm AC, 3000V test voltage

Looking at a conventional USB connection (be it type A, B or C) only individual USB-socket designs (where the plug must be inserted quite far into the device) can be used to meet the required distances from the conductive parts to the test finger.

And even if the connection's side at ME equipment is fine, what about the other end of the cable? If this is a standard USB cable to be connected to any PC, laptop or USB-charger, the requirements will not be met there.

I have seen several test report for such ME equipment, where this requirement were not even listed in the insulation diagram and as a consequence not tested.

Do I misunderstand something here, or are off-the-shelf USB-connections a very bad idea for ME equipment?
 

AbelVV

Starting to get Involved
Honestly unless you have a very good reason to use it I would use something with a smaller chance of misuse. The risk is fairly high and easily avoidable.
 

Marc

Fully vaccinated are you?
Leader
Do I misunderstand something here, or are off-the-shelf USB-connections a very bad idea for ME equipment?
On some tech forums I visit USB connectors are discussed - They are problematic because of the various possibilities because of the variations in the specs. And the potential of being able to connect an incorrect cable (adding in that identification of cable type is inconsistent at best).

I would say stay away from USB if you can.
 
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