Leakage current tests on USB ports

[PeteH1]

Hi,
I'm designing a medical device which uses USB ports for applied parts, and may be connected to mains-powered accessories like HDMI monitors or USB hard drives. I'm trying to work out how to get the device through leakage current tests. As far as I can see, 60601-1 figure 14 tells me that to measure touch current, the test house will connect mains voltage via an unspecified resistor R to each USB port and to the HDMI port. All ports have the ESD protection you'd expect. Clearly, if R=0, the mains voltage will fry my device. Is the test house free to choose the value of R? I've read suggestions that R = 1k, but I don't know where that comes from.
Looking at 8.4.2 c), any connection to the patient during normal use is highly unlikely. In my case, it's likely that several of the ports won't have anything plugged in, so there will be accessible contacts of connectors. However, the condition for applying 8.4.2 c) is the voltage to earth <= 42.4V peak a.c. (or 60V d.c.). Does this not depend on the value of R?
What am I missing?

Thanks.

 

[Peter Selvey]

The trick is hidden back in 8.1a, which says mains on SIP/SOP is only if you have no instructions how to set up a ME system to avoid untoward accumulation of leakage currents from other devices. And the test labs allow a one paragraph statement that generally refers back to the standard, something like make sure to use certified devices (e.g. IEC 60950-1, IEC 61xxx can't remember the new number) and follow Clause 16 of IEC 60601-1 when setting up a system. You can make the instructions more specific like don't use multiple socket outlets, use an isolation transformer, or additional earths, or it's also possible to actually specify the peripherals e.g. if they are known to have low leakage so an isolation transformer/earths are not needed. But to be honest, most test labs will accept the one paragraph and leave it at that, without worrying how practical it is for the user to follow Clause 16.

The test itself should only be performed if the manufacturer chooses to have an isolated SIP/SOP, which eliminates any concern about the set up of the external stuff. This is a valid design option in high risk cases, e.g. things like dialysis machines, high risk surgical instruments. In that case, the value of R is irrelevant, it's the isolation barrier that limits the current. The test should never be blindly applied to an non-isolated SIP/SOP. But of course there will always be test labs that don't understand what they are doing so take care ...