On Sun, 6 Jan 2008 08:12:46 -0800 (PST), Andina Marie
wrote:

On Jan 6, 6:06 am, "Steve Lusardi" wrote:
Andina,
I found your synopsis of this issue very good. This problem is not
understood by most people. Please explain what a Galvanic Isolator is.
I
find absolutely no reason to connect shore earth to your boat, ever.
There
should be a ships earth and it should never be the hull, either on the
AC
side or the DC side. The ships earth should be isolated from the hull.
There should be an LED mounted in an obvious location connected
between the
hull and ships earth. Normally this LED would be off, unless there is
a DC
leak to the hull. This would then allow the LED to turn on and alert
the
crew to locate the fault immediately. On the AC side, an isolation
transformer is a necessity. The ships safety earth is then bonded to
neutral
at the panel only and the main ships ckt breaker should be of the GFI
type.
Steve

REPLY from ANDINA.

A Galvanic isolator consists of back to back diodes which still
provide an electrical path for AC current but block DC voltages up to
about 1 volt. So the ground protection for electrical faults is
retained but the small DC voltages that cause electrolysis are
blocked. ABYC specifications require that the Galvanic Isolator be
able to carry 130% of rated current continuously. This is required
because if an appliance on a boat was wired incorrectly and using the
ground instead of the neutral, the total current would be flowing
through the isolator. Should the isolator then fail under load, the
AC would now be connected directly to the ground that has become
disconnected from the shore ground. This puts 120 volts AC on all the
underwater items on your boat which can KILL SWIMMERS IN THE VICINITY.

Your statement that:-

I find absolutely no reason to connect shore earth to your boat, ever.

is very dangerous without explanation.

Omitting this connection can kill people.

On most boats you cannot isolate the DC ground from the underwater
metal because the starter motor is bolted to the engine block which in
turn connects to the propeller shaft and the water. So without the
ground, an internal failure of the battery charger could put 120 or
230 volts AC on your DC ground and into the water.

Relying on indicators or alarms is not a satisfactory solution, it
only takes milliseconds to kill someone.

Relying on a GFI Circuit breaker is not satisfactory. In salt water
the current flowing though a GFIC can kill a swimmer before the GFIC
can trip.

Now I agree that the probability of these worst case scenarios is
remote, however the consequences are grim. The ABYC specifications
err on the conservative side to avoid leaving the risk/consequence
decision up to the boat owner.

If using an isolation transformer the risk is miniscule. Isolation
transformers are extremely reliable, the only risk would be an
uninformed installer making an error in wiring up equipment. Without
the isolation transformer and with no ground you are betting the
reliability of your equipment against the life of swimmers in the
vicinity of your boat.


Good advice.

Bottom line: unless an isolation transformer is (properly) installed,
the shore power green grounding wire should ALWAYS be connected to the
boat's AC ground, preferrably through a galvanic isolator.

Chuck


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