On 11/19/2008, Fred Townsend wrote:
I'm wondering if something was missed here. In the telephone world +48
is
grounded... NOT -48V. Mixing telephone systems with other systems where there
are negative grounds like POE could certainly lead to smoke. The fact you have
a resistor to ground is meaningless. What is the value of the resistor? Isn't
one side of all shunt loads connected to ground?
Hi Fred:
You are correct that in the telecom world, the positive side of the 48V
battery is typically connected to earth ground at the FXS end of a 2-wire
analog loop. This convention was adopted to reduce galvanic corrosion
influences on buried cables.
However, in the POE world, the 48V battery for the POE supply is floating with
respect to earth ground (if the POE ports were properly implemented). So,
there should not be any opportunity for ground loops even if some of the
terminal equipment is powered by a ground-referenced -48V supply.
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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