Gentlemen:
Please recall that Ethernet is isolated from the sender/receiver
circuits at each end of the Ethernet circuit (where the isolation
in the equipment usually is a part of the Ethernet RJ45 connector).
The isolation is 1500 V rms. Why? Because building transients
magnetically couple into the Ethernet cable within the building
wiring.
Consider two buildings. The NEC requires the neutral to be
grounded at each building service entrance. Therefore, the
neutral is grounded at two points, which places the ground
in parallel with the neutral, and, VOILA! the ground between
the two buildings is a current-carrying conductor. Isolation
from ground is required because, otherwise, the Ethernet
conductors would be in parallel with the ground and would
become a third current carrying conductor between the two
buildings. And would quickly open due to the current.
As I understand POE...
http://hw-server.com/images/supply_diagram.png
POE is a common-mode circuit between two Ethernet
pair conductors. As shown in the diagram, the "powered
device" must have a DC-DC converter which is isolated
from ground (again, 1500 V RMS).
http://www.poweroverethernet.com/associated/files/file_334_1114621476.pdf
POE is a maximum 48 V DC. This circuit comprises SELV
between poles, and a Limited Current Circuit between
each pole and ground.
The Ethernet standards require isolation between:
(1) the Ethernet circuits and the internal equipment circuits
(provided by the RJ45) and
(2) the DC-DC converter input and output (provided by the
equipment).
However, 60950-1 does not require clearance or creepage
across these isolations.
Best regards,
Rich
-----Original Message-----
From: Bill Owsley
Sent: Monday, November 26, 2012 5:48 PM
To: ralph.mcdiar...@schneider-electric.com ; EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] POE creepage & clearances
SELV is 60 vdc but not if outdoor or wet environment - right ???
Then it drops to 30 vdc?
So Ethernet is inside only for easy compliance ??
Maybe one can extend the inside environment to an outside application by
careful use and implementation of conduit and enclosures...
We get lazy and apply the creepage and clearance and keep moving.
ps. I am not a safety engineer and have no clue...
I suspect that the 1500 v isolation came from a single fault introduced at
the PoE injector that tied primary to secondary and thus the PoE device
needed to also have the isolation to withstand that fault.
Add up how many isolation transformers are in the circuits back to primary
and/or TNV-1.
From: "ralph.mcdiar...@schneider-electric.com"
<ralph.mcdiar...@schneider-electric.com>
To: EMC-PSTC@LISTSERV.IEEE.ORG
Sent: Monday, November 26, 2012 7:21 PM
Subject: Re: [PSES] POE creepage & clearances
The external DC power supply needs to be SELV too, not just energy limited.
_______________________________________________________________________________
Ralph McDiarmid | Schneider Electric | Solar Business | CANADA |
Regulatory Compliance Engineering
From: Ron Pickard <rpick...@equinoxpayments.com> To:
EMC-PSTC@LISTSERV.IEEE.ORG, Date: 11/26/2012 12:38 PM Subject: Re:
[PSES] POE creepage & clearances
Hi Joe,
I agree that an Ethernet circuit would be considered to be an SELV circuit
and that would be true if the POE circuit were completely on premise
generally, which would categorize this as a Class III product according to
IEC/UL 60950-1 2nd Ed. given that the external power supply is a limited
power source (LPS). However, TNV-1 creepage/clearance requirements would
apply if the POE were to be exposed to overvoltages from telecommunication
networks (essentially off premise) requiring Basic insulation. I make this
distinction as Ian did not mention where the POE circuit originates.
Best regards,
Ron
From: Joe Randolph [mailto:j...@randolph-telecom.com]
Sent: Monday, November 26, 2012 12:43 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] POE creepage & clearances
Hi Ian:
I think the short answer to your question is that under 60950 there are no
requirements for creepage and clearance distance between the Ethernet
circuit and ordinary SELV circuits that are user-contactable. This is
because under 60950, Ethernet is classified as an SELV circuit. There are
no requirements to separate one SELV circuit from another.
Where this issue becomes confusing is that under the Ethernet 802.3
standard, an Ethernet port must provide a 1500 VRMS isolation barrier
between the Ethernet conductors and earth. This is a legacy requirement
whose origins seem to have been lost in the sands of time. However, I
believe the consensus view is that the original intent of this requirement
was to protect against ground loops and was not related to user safety.
Nevertheless, the 802.3 isolation requirement has sometimes been treated as
though it is a safety requirement. Note that the 802.3 requirement is for a
simple dielectric test, with no reference to creepage, clearance, or
distance through solid insulation.
So, in principle, there are no safety isolation requirements for your
application, since both of the proposed power supplies have a SELV output.
This makes the entire device a SELV circuit (unless there are other ports on
the device that are not SELV).
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com
-
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