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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: EMC compliance question
Date: Sat, 12 Oct 2024 16:33:11 -0700
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On 10/12/2024 4:02 PM, Klaus Vestergaard Kragelund wrote:
> On 12-10-2024 12:00, Don Y wrote:
>> On 10/12/2024 2:38 AM, Klaus Vestergaard Kragelund wrote:
>>> Somebody was talking about 48V warts. Some standards only allow 24V (for wet 
>>> environments), and 32V for certain parts of the world
>>
>> 48V wall-warts/bricks are typically used in midspan PoE
>> injectors (and as standalone power supplies for PDs
>> without PSEs).  As such, almost always in dry locations.
>>
>> Can those "certain parts of the world" use PoE products
>> with nominal 48VDC delivered over the twisted pairs?
>> Is the limit on the "packaging" or on the potential?
> 
> 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?

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"?