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Isolation requirements for USB/Ethernet

eezud

Inactive Registered Visitor
#1
I have general question about required isolation, creepage and clearance distances for medical devices with patient connections but having also connections available to USB for data and power and/or Ethernet. I searched other topics but couldn't find an exact answer, although this seems to be a general and common design issue.
Both USB and Ethernet operate in low voltage, 5V supply. Does it mean that the medical device need 2 means of protection with working voltage of 5V? In many topics, I saw people recommending 240V as working voltage because the USB can come into contact with the mains. But wouldn't that be a single fault condition? So, you need only 1 MOP for 240V?
 
#2
Hello,
Please check the figure J.4 from the standard. There is specified that there shall be the 2 x MOPP relating to working voltage) and 1 x MOPP relating to maximum mains voltage between SIP/SOP (e.g. USB) and patient connections.
 

Benjamin Weber

Involved In Discussions
#3
It is very common to regard mains voltage at SIP/SOPs as normal condition.

See cl. 8.1 a) for the background:
"Normal condition (...)
- the presence on any SIGNAL INPUT/OUTPUT PART of any voltage or current from other electrical equipment that is permitted to be connected according to the ACCOMPANYING DOCUMENTS as specified in 7.9, or, if the ACCOMPANYING DOCUMENTS place no restrictions on such other electrical equipment, the presence of the MAXIMUM MAINS VOLTAGE as specified in 8.5.3;"


The problem is, even if you specify that only a certain device shall be connected to the SIP/SOP (e.g. medical grade PC), you cannot be sure, that the operator will really adhere to this. And in the context of risk management acc. 14971 this only safey by information, the least favourable risk control measure. As a result for USB-connector mains voltage is considered as the working voltage in normal condition and an appopriate seperation should be implemented.

Additionally you will end up with much more problems regarding the necessary creepages and clearances. I discussed this her:
USB-Connection - Creepages and Clearances
 
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