On 3/27/2009, John Woodgate wrote:
How can Y-caps do that? Y caps go from both mains poles to earth.
Hi John:
I will try to explain the mechanism, but I don't want to take this thread too
far off track. Most of the discussion here has been about the safety aspects
of POE. My comment about common mode noise was an aside, since it does not
relate to a safety issue. It's simply a consequence of using POE injectors
that have an ungrounded chassis.
Perhaps I used the term "Y caps" too loosely. The caps I refer to are
Y1-rated safety caps that connect from each side of the AC mains input to the
internal chassis of the POE injector. You are correct that these caps do not
actually connect to earth ground in a Class 2 arrangement. They would only
connect to ground if the chassis were grounded, and it is the ungrounded
chassis that causes the problem. With the chassis floating, the two caps
create a voltage divider. If one side of the AC mains is grounded, the entire
chassis rides on the center point of the capacitive voltage divider, and with
respect to earth, it carries a 50/60 Hz potential of one half the AC mains
voltage.
All of this is a high impedance circuit and there are no dangerous touch
currents involved, but since the Ethernet cable is also floating with respect
to earth, the 50/60 Hz noise typically carries through to the Ethernet output
via similar EMI caps in the second power supply that generates the DC applied
to the Ethernet port within the POE injector. An IP phone that is powered
exclusively by an ungrounded POE injector will typically be floating on this
very large (but high impedance) common mode noise.
In theory, a floating IP phone will be immune to this common mode noise, but
there are parasitic capacitances to earth that allow small common mode
currents to flow. I have seen this become a problem for the microphone input
of the hands-free mode.
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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