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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: EMC compliance question
Date: Sun, 13 Oct 2024 06:34:27 -0700
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On 10/13/2024 4:15 AM, Klaus Vestergaard Kragelund wrote:
>>> In IEC60730 (safety for household products), 2.1.5 SELV is defined as 
>>> maximum 42V. Note states that for the US and Canada the SELV voltage is max 
>>> 30VRMS (which equates to 42.4Vpeak). Those numbers are when dry, when wet it 
>>> reduces to 15V/21.2V peak.
>>>
>>> Normally things are dry, so US is 30V. I do not know how it is possible to 
>>> allow 48V warts.
>>>
>>> Searching a little, it seems the 48V systems are approved against 
>>> telecommunication standards which may not use the SELV nomenclature
>>>
>>> NEC has higher voltages, up to 60VDC, but matters little since most product 
>>> needs to comply to 60730/60950, and now 62368 has replaced 60950.
>>>
>>> The touch voltages are defined in yet another standard, IEC61201. I do not 
>>> have access to that one.
>>>
>>> The 48V warts are also strange in that when the product is tested for peak 
>>> SELV voltage a single fault must be introduced. So if you mess with the 
>>> feedback of the SMPS, the trip voltage determines the maximum voltage, and 
>>> that is most likely significantly higher than 48V.
>>
>> But, is the constraint on the "wall wart package"?  Or, on the presence
>> of ~48V on conductors that are accessible to the user?
> 
> POE voltage is directly on the pins of the ethernet interface.

Yes.  And those pins are physically accessible on the exposed 8P8C
on the end of the patch cord that connects to the PD.  (though power
/should/ be switched off by the PSE when the load "disappears")
Imagine a pet chewing a flimsy patch cord (or child putting a faulted
end in its mouth!)

> The designer 
> insources the external wart with 48V nominal voltage (which can be more under 
> single fault)
> 
> There may be a loop hole
> 
> If you ship the adapter/wart with the product you should test as a system, right?

A PoE *switch* can have the power supply built in -- no wall wart.  Yet,
the same potential fault conditions at the end of that patch cord.

> But if you just state it needs 48V in, you can blame the wart manufacturer if 
> it puts out more voltage.
> 
>> E.g., an N-port PoE switch looks like the (output) power cords from
>> N 48V wall warts.  (technically, this is only the case while the
>> cables are physically connected to their PDs as the PSE should
>> power down the unconnected port).
>>
>> Because the switch "isn't a wall wart", is it exempt?
>>
>> Or, is all this moot because PoE switches aren't "household kit"?
> 
> I just took a random POE ethernet switch which uses a 54V external adapter:
> https://www.proshop.dk/Switch/Netgear-GS110TPv3-8-Port-Gigabit-PoE-Ethernet-Smart-Switch-with-2-SFP-Ports-and-Cloud-Management/2871263
> 
> No mention of standards in the datasheet. But found a reference in the hardware 
> manual:
> https://www.downloads.netgear.com/files/GDC/GS108Tv3/GS108Tv3_GS110TPv3_GS110TPP_HIG_EN.pdf
> 
> Page 2, link to netgears compliance document:
> https://www.netgear.com/about/regulatory/
> 
> Then searched for the model no in the Declaration of conformance:
> https://kb.netgear.com/11621/EU-Declarations-of-Conformity?article=11621
> 
> Finally here:

(sheesh!)

> https://www.downloads.netgear.com/files/DoC/204-11529-04_CE_GS110TPv3_EN-EP-FR-IT-GR-SP_19SEP22.pdf?_ga=2.171448224.1872638720.1728816211-1033670545.1728816210
> 
> Mentions use of 60950 and 62368
> 
> I am doing EMC tests tomorrow at a test-house, so will ask them whats the deal ;-)

It will be amusing if they've *not* thought about it!

Be sure to ask what *they* think PoE (and PoE+ and PoDL) "means" as there
are multiple standards as well as legacy implementations.

I think it is still relatively rare in "homes" (our modem is PoE
powered) but likely to become increasingly so as it makes
powering devices less annoying than having wall warts *at* each
such powered device.  Likely for new construction as running cable
is too costly after-the-fact in most homes.

[Good luck with your testing!]