On 9/14/2009, Don Umbdenstock wrote:
802.3af indicates the isolation requirement. Does the concept of the
defined
use of the product to be within a building, not connected to systems outside
of the building, exclude the product from the isolation requirement? If the
product is defined to be used only within a building, would it be considered
in conformance with 802.3af without isolation provisions?
Hi Don:
I don't interpret the 802.3af standard that way. The standard assumes that
the cabling will be with a building, but it still imposes the 1500V
requirement. If you want to declare a product to be "802.3af compliant," you
will have to implement the 1500V barrier.
As I noted earlier, I view the 1500V requirement to be a legacy requirement in
the 802.3 family that pre-dates some of the current thinking regarding safety.
I'm not sure what rationale was used to arrive at 1500V for the requirement
when it first appeared many years ago for 10 BASE-T.
It seems perfectly reasonable to me to require an isolation barrier of some
sort just to prevent ground loops between connected equipment that may have
different ground potentials, but I would think a 500V requirement would be
adequate for that consideration. Somehow the committee that wrote the
original standard arrived at a higher number.
One thing to keep in mind is that virtually any Ethernet interface transformer
that you can buy already has 1500V isolation build in, so the additional steps
required to comply with the 1500V requirement are minor. Typically, the only
additional steps are some attention to the board layout and selection of
appropriate high voltage capacitors for any EMC caps that bridge the barrier.
The isolation requirements becomes much more difficult and expensive to
implement for POE applications, because power must be transferred across the
isolation barrier. However, the industry chips sets that implement POE
interfaces are all designed to be used with an appropriate isolation
transformer. So, as with the standard Ethernet interfaces, any POE design
that is based on an industry chip set will already have provisions in the
circuit design for an isolation barrier. To maintain compliance with the
1500V requirement, all the board designer has to do is pay attention to the
board layout and choose appropriate parts for components that bridge the
isolation barrier.
Another thing to keep in mind is that the 1500V isolation requirement is a
simple hipot test. There are no requirements for creepage distance, clearance
distance, or distance through solid insulation. So, as long as the interface
can withstand the 1500V test, it complies with the requirement. This differs
>from the way that UL60950 typically specifies an isolation barrier.
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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