Gary,
WHY have the floating system? In order to reduce corrosion in the Outside
Plant wiring, telephone lines are held negative with respect to Earth. This
imposes a need to hold circuitry directly connected to them negative as
well. Specifically, both the tip and ring conductors are maintained below
Earth potential most of the time (ringing voltages may go positive on
peaks). Logic is not necessarily treated in this manner, and you may find
it sharing a connection locally to frame ground. However, frame is NEVER
used as logic or signal return path. Vcc return is provided by a separate,
properly sized copper conductor, and signals are provided with discrete
returns of their own. Considerable effort goes into thus insuring chassis'
are not used as return paths for power or signal currents.
There is as well a need to dump often substantial transients (lightning and
power-cross) to a ground that is isolated from logic. I recommend you look
at Telcordia GR-78, GR-63 and even GR-1089. Maybe also SR-3580. These are
all available from Telcordia. They should agree with what you are seeing in
UL1950, but shown in context may be more illuminating as to the rationale.
Cortland
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On 13-Oct-00, Gary McInturff asked:
Confused and missing something pretty basic here, I think.
As I look at the diagrams in annex NAB for UL 1950 or UL60950 I
am pretty confused.
What it appears to show me is a floating system -48 and -48
reference lines into the power supply system. Do I really have to float my
circuit boards etc, (ungrounded SELV?), and use the -48 reference line as
the return path rather than chassis and its bonding wire (grounded SELV).
Logic and chassis are not tied togeather until after a considerable
distance down the line?
Am I stairing into the obvious and missing the point?
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