Take a fairly decent digital voltmeter and measure the voltage across
the
Galvanic Isolator with the boat hooked up and normal loads turned on.
Check
the readings when refrigerators and AC are running and drawing current.

First measure the DC voltage (reversing the leads if necessary to get
the
right polarity if the meter doesn't correct automatically). This
voltage
should measure somewhere between about 0.l volts and 1.2 volts. If it
measures exactly zero volts (less than 0.1) this indicates the isolator
is
shorted out and needs replacing. If it measures above 1.2 volts this
indicates the isolator is open circuit and needs replacing or your
ground
circuit is broken somewhere.

Second, measure the AC voltage across the isolator. This should read
0.25
or less. If it is higher than this voltage then AC current flowing
through
the isolator will be decreasing the efficiency as much as 50% depending
on
the voltage.
AC voltage can come from:-
1. on-board AC equipment that is wired incorrectly and using the ground
return instead of the neutral.
2. on-board leakage that tends to build up on older boats due to salt
deposits on electrical items.
3. (the most likely one) a neighbors boat is using the ground for the
return
instead of the neutral and this introduces AC volts back into the
ground
connection of all the other boats on that ground.

Cu-
1. One by one turn off all the AC items and see if turning one of them
off
makes the AC voltage on the isolator go away. If you find a culprit,
then
check the wiring at the point of connection and in the device itself to
make
sure it is wired correctly.

2. There is no easy way to correct accumulated leakage. You have to
bypass
the AC leakage around the isolator with a galvanic capacitor. Some
Galvanic
Isolators have a capacitor built in but they are typically just a
woefully
inadequate size so they can advertise it. You will note they never
quote
the current carrying capacity of the capacitor in their specifications.
We
make a Galvanic Capacitor rated for 5 amps AC. No others come even
close to
this capacity. You wire the capacitor in parallel with the isolator -
see
the link below.

3. Turn off the main breaker at the dock. If the isolator AC voltage
remains across the isolator then it is coming from another boat through
the
ground lead. In cases like this I've had someone watch the meter about
mid-day when no one is on board and then turn your neighbor's dock
breaker
off for a couple of minutes and then back on. (Wait at least 2 minutes
so
you are not re-starting compressors under load). If that made the AC
voltage go away, THEY are the culprit with the mis-wired boat and you
need
to report it to the marina. I've had criticism for recommending this
in the
past but the bottom line is the same thing happens in a thunderstorm
blackout and that is an act of God so the boat should be able to
survive.

If you can't get rid of the AC voltage across the isolator you need a
Galvanic Capacitor.
http://www.yandina.com/GIsolCap.html

Finally, if the DC voltage is within limits and the AC voltage is less
than
0.25 volts then it is time to consider an instrument to detect galvanic
stray current issues. Rather than purchase one and attempt to do the
measurements yourself I would suggest getting someone who has a meter
and
extensive experience in tracking galvanic currents because the meter
just
gives you a stack of voltage readings. Analysis of these readings can
take
years of experience unless there is something very obvious.

Regards,

Ann-Marie Foster,