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. Getting a voltage reading does not in itself indicate
electrolysis unless it is over about 1.1 volts since that is the
purpose of the GI to block DC current from sources up to about 1.2
volts.

So a reading of zero is a fairly good indication that either the GI is
shorted internally or more likely there is another connection somewhere
on the boat that is by-passing the GI and rendering it inoperative.
One source to watch is the television cable if you have one. The
shield on this will probably be grounded at the dock end and depending
on how it couples to your TV or cable box it can easily be grounded at
the boat end too. This shorts out or bypasses the GI and voids the
electrolysis protection.

However you are correct that an AC reading of zero is a good indication
that there is no AC leakage.

Regards,

Ann-Marie Foster,




chuck wrote:
Some observations:

SNIP
A DC voltage reading across the isolator
of zero or near-zero volts is a highly
desirable situation! It means that
galvanic (and other DC) currents passing
through the shore power ground system
are negligible, and that is always
desirable. A reading of zero volts does
not automatically mean the isolator
diodes are shorted!

Similarly for AC voltage readings of
zero or near-zero volts. A reading of
zero volts AC means there is no evidence
of appliance leakage on your boat and no
reason to suspect problems with AC
wiring on neighboring boats.

Chuck