Paul Hovnanian P.E. wrote:
Think of the shore power ground as a
connection to the rest of the
world. A world with structures and
ground rods that may be in contact
with sea water, but have no provisions
for galvanic protection
themselves. If you have zincs on your
boat, there will be a current (and
your zincs will dissolve). If you break
that circuit, you will see the
voltage of that galvanic cell (the one
between your zincs and the rest
of the world).
Well, that's only half of the situation.
Your prop and any underwater metals
will also form galvanic cells with the
rest of the world, of different polarity
and potential difference. There is a
burden here of demonstrating that on
average these do not cancel, and a
greater burden of demonstrating that
their net effect is a 1.2 volt cell. But
the 1.2 volts measured was NOT an
open-circuit voltage measurement, but
one across a functioning GI.
The discussion has gone open loop. Here
is a recap.
1. An assertion was made that 0 VDC
across a GI means the diode(s) are shorted.
2. I suggested that 0 VDC also meant the
absence of galvanic currents through the
shore power ground wire; a good thing.
3. A counter-assertion was made that
there is always 1.2 VDC across a
properly functioning (NOT open-circuit)
GI due to the boat's zinc.
4. I suggested there was no path for the
boat's zinc/bronze galvanic current to
pass through the shore power grounding
wire and some other explanation was
called for.
5. An assertion was made that the
galvanic couple in 4 made the boat
"live" and that led to the measured 1.2
VDC across the GI.
6. I responded that the assertion failed
to identify the current path by which
this occurred. Further, I observed that
a measured forward voltage of 0.6 volts
per diode was equivalent to a forward
current on the order of 100 mA, and that
was far in excess of the currents found
in typical yacht-based galvanic couples.
7. A contrary assertion was made that at
0,6 volts, the forward current in a
diode is on the order of only microamps.
And so here we are. I don't know your
basis for that assertion, Paul.
Disregarding what has come to be the
normal protocol for some in the group, I
actually measured a 1N1190A (I use them
in the GI's I build) a few moments ago.
The forward current at 675 millivolts is
100 milliamperes. Using a Keithley
electrometer, I measured a forward
current of about 10 microamps at a
voltage of 300 mV, consistent with the
675 mV/100 mA measurement.
Even the 1N914 signal diodes pass almost
one mA (about 700 microamps per the
datasheet) for a forward voltage drop of
600 mV.
Here is where the discussion stands:
I. An observed anomalous current of ~100
mA DC is measured across a GI. (Actually
the measurement was 0.6 volts DC and
there is disagreement over every aspect
of that measurement.)
II. The current path (of 100 mA) between
the boat's zinc/bronze couple and the
shore power ground has not been
identified although much hand-waving has
transpired.
III. There is seemingly irreconcilable
disagreement about metrology, Ohm's law,
diode VI characteristics (e.g., the
switch analogy), and the properties of
galvanic currents.
Your patience with me is appreciated,
but there are other callings.
Chuck
PS: Paul, I inadvertently seem to have
sent an earlier draft of this directly
to you rather than to the group. No idea
how that happened, but my apologies.
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