On 3/26/2009, Peter Tarver wrote:
What if the power supply is Class 2 Equipment? Then the
isolation requirements are somewhat larger than 1500 V.
Also, doesn't the supplied power to the PD have to continue
to be isolated from earth, as do the ethernet circuits?
Hi Pete:
A Class 2 supply is the most common type of supply that I have seen for POE
injection. And yes, the output should be isolated from earth ground to comply
with IEEE 802.3af.
Personally, I do not see any safety compliance problems with this construction
when used as intended (inside lines).
By the way, one problem that I *have* seen with this construction is
performance related. Most of these POE injection power supplies are switching
converters, and the Y-caps used on the input for EMI suppression create a
voltage divider that places a very high 50/60 Hz common mode noise signal
(typically one half of the AC mains voltage) on the Ethernet outputs. This
means that an IP telephone powered by the POE injector will be riding on the
50/60 Hz common mode noise. The analog audio circuits in the IP phone, such
as the microphone for a hands-free mode, can be sensitive to this common mode
noise and produce audible hum.
Grounding the chassis of the POE injector fixes the problem, but most of these
devices are ungrounded. So, the IP telephone has to be designed with an
extraordinary level of 50/60 Hz common mode immunity in order to avoid hum in
the analog audio path.
Joe Randolph
Telecom Design Consultant
Randolph Telecom, Inc.
781-721-2848 (USA)
j...@randolph-telecom.com
http://www.randolph-telecom.com <http://www.randolph-telecom.com/>
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