On 9/11/2009, Jim Goedderz wrote:
Are the Additional Electrical Specifications, Isolation, para 33.4 of
IEEE802.3af (1500Vdc hipot) satisfied by the input power connection and the
router. It wouldn’t seem necessary that the cameras themselves need to
provide 1500Vdc isolation from their Ethernet connection to SELV. Bottom line,
would the system as described be considered compliant with IEEE802.3af? Would
the camera be considered compliant with IEEE802.3af?
Hi Jim:
I think the answers to your questions depend on some additional details about
the system design and the customer involved. Following are some points to
consider:
1) IEEE 802.3af is not a regulatory requirement. It is an industry standard
and compliance with any or all of its requirements is optional.
2) For safety compliance per EN 60950, a conventional Ethernet connection,
even a POE version, is classified a SELV circuit.
So, it is important to keep in mind that the 1500V requirement you are
describing is not a regulatory requirement for UL 60950 safety compliance. It
is a legacy requirement that has been embedded in the Ethernet specifications
since the early days of Ethernet, at a time when the industry thinking about
safety analysis was less precise than it is now. I believe that the main
reason for the 1500V requirement was to prevent ground loops in a system where
each node might have a different local ground potential. I'm not sure how
they arrived at 1500 volts.
If you want the POE port on the camera to be "IEEE 802.af compliant," you will
have to implement the specified 1500V barrier. However, you might be able to
meet the requirement by careful physical design that prevents user contact
with the internal circuitry. Similarly, if you want each of the POE ports on
the router to be "IEEE 802.af compliant," you will need to implement the
specified 1500V barrier on each of those ports as well.
The situation changes if your cabling arrangement, camera, and router are not
specified to be "IEEE 802.af compliant" and compatibility with generic
Ethernet ports is not required. In other words, the cabling and signaling use
a proprietary scheme that is not intended to be compatible with
industry-standard Ethernet interfaces. In that case no isolation barrier
would be required in the camera or the POE-like ports on the router that
connect to the camera. However, if the router has an Ethernet port that is
intended to connect to the internet via the home's conventional Ethernet LAN,
that one port should have the 1500V barrier if you want to call it "IEEE
802.af compliant."
I'm sorry if this seems confusing at first, but I think it can be sorted out
by using a step-by-step approach to define more clearly what requirements you
are trying to meet and why.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
[email protected]
http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
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