Chuck, you're confusing current and voltage.
First consider the DC voltage measurement.
You don't have to have any current flowing in order to have a voltage
across the Galvanic Isolator. Think of the isolator as an on/off
switch. If the voltage across the switch is less than 1.2 volts the
switch stays off and a meter across the switch will read the voltage
with no current flowing. The zincs are used because they create a
voltage in the right direction to protect your underwater metal. They
also make your boat "alive" at a very low voltage to the water and it
is this (plus other influences) that you measure across the Galvanic
Isolator.
It is theoretically possible, but extremely unlikely, that other DC
voltages in the vicinity could completely cancel out the zinc but like
I said above, I've never seen one in 20 years of working on boats.
So if you read exactly zero volts across the Galvanic Isolator it is a
pretty good indication that the "switch" is shorted out and it is not
functioning.
Of course if you measure greater DC than 1.2 volts that indicates the
GI is open circuit.
Now consider the AC voltage measurement.
AC voltage across the GI can come from two sources. The first is AC
return or leakage from your own boat going or attempting to go back to
the dock. The second is AC voltages on the ground wire on the dock
attempting to go through your boat to the water. This latter can be
caused if a boat on the same dock circuit is mis-wired and returning
neutral current through the ground wire. The ground wire is typically
not designed to carry "working" currents so there is a voltage drop
along it back to the distribution point. This voltage can often be in
the range of 1 or 2 volts but occasionally I've seen higher and it
appears on the ground terminals on the dock and the dock side of your
GI.
AC voltage on its own does not create electrolysis however if it is
excessive it can cause blisters on metal boats where chlorine forms
under the anti-fouling paint. But that AC voltage is arriving across
the Galvanic Isolator. In one half cycle the AC voltage is subtracting
from the DC voltage but in the other half cycle it is ADDING to it. So
if your AC has a peak of 0.5 volts your DC protection is reduced from
1.2 volts down to 0.7 volts for a percentage of the time that increases
with the AC voltage to a worst case of 50%.
Putting a large capacitor across the Galvanic Isolator shorts out the
AC voltage while making no change to the DC and this prevents the AC
from piggybacking the DC through with it.
Regards,
Ann-Marie Foster,
chuck wrote:
Thanks for the comments, Ann-Marie.
Andina Marie wrote:
Chuck, in over 20 years working on boats I've never encountered a boat
in the water on shore power that had zero volts across the Galvanic
Isolator. Just the zincs on your underwater metal will introduce about
0.5 volts DC.
I don't see how the zincs on your boat
can cause a current to flow through your
GI (and thus the shore power ground). In
the common case of a bronze prop and a
zinc on the shaft, for example, the
galvanic current passes through the
shaft. It does not pass through the GI.
A neighboring boat using the shore power
ground to complete a galvanic circuit
with your zinc would definitely cause a
current to flow in the GI.
Even if the neighboring boat was
protected with its own zinc, there could
still be a small galvanic couple created
with your zinc, especially in sal****er.
Not nearly as likely in fresh water.
Other than that, I would have a
difficult time explaining the voltages
you measure.
Chuck