it's a problem I've struggled with, being the maintenance person for 6
permanently moored steel vessels here in the Netherlands.
even if the safety earth wire is left disconnected, the hull of the boat
will be effectively earthed through the water [especially in salt water of
course].
any stray currents that find their way to the hull will flow into the water,
taking some metal with it. this is bad.
any significant current will switch off the ground fault interrupter [GFI,
whatever. a boat connected to shore power without a gfi is a dangerous
vessel to everyone].
there is the idea of putting a pair of diodes in the line, but this merely
raises slightly the threshold of where the problem begins.
for very small systems [a few appliances], double insulation is probably
good enough. the safety earth circuit is, after all, not used on many
consumer appliances these days anyway.
however, if there's going to be a breaker board and multiple AC circuits,
then an isolation transformer is the only option that can be responsibly
recommended.
I've bitten the bullet and bought mine.
regards, Mark Holden
www.amsterdamhouseboats.com

"Jim Woodward" wrote in message
om...
That's the reason I think an isolation transformer is essential.
Without one, however, I must admit I tend to agree with you about the
shoreside green wire. What I objected strongly to, howver, was the
suggestion that you should connect the neutral to ground on the boat
while on shore power. That's just asking for more of the kind of
problem you want to avoid, as if the neutral going back to the marina
service entrance is carrying a lot of load from other parts of the
marina, by the time it gets to your boat it will be a little above the
ground potential at the service entrance, where it is connected to
ground.

If you then have neutral connected to ground on the boat, the neutral
current will tend to flow through the water rather than the neutral
wire. This will possibly cause electro-chemical problems, possibly
give swimmers a "tingle" or worse, and certainly trip any GFCI (RCCB
in British usage, I don't know the EU term) in the line.

Jim Woodward
www.mvfintry.com



"Steve Lusardi" wrote in message
...
Jim,
You are correct about the isolation xformer and the code, but I will
never
connect the safety line from the shore. This may be against code, but I
would never jeopardize my hull to someone else's electrical fault. This
action presents only risk and zero value. It certainly does not add more
safety. A safety earth from your hull provides both adequate safety and
eliminates the risk of electric corrosion.
Steve

"Jim Woodward" wrote in message
om...
Steve:

Joao started this thread in three places, so we have more going at
rec.boats, but I wanted to respond here to one thing:


"Steve Lusardi" wrote in message
...
You should not use the hull as either a return path or a safety
earth.
In
all cases, somewhere on the inside of the hull, there should be a
welded
plate with multiple threaded holes or studs for all your earth
references AC
or DC. Never use the safety earth connection from shore power, you
must
supply your own from this plate. Only connect the phase and the
neutral
line
from shore power. Never connect neutral to safety earth anywhere,
but at
this plate for on board AC generators. snip

This is a violation of ABYC and NFPA rules. If you are connected to
shore power without an isolation transformer (which I view as
essential on any boat), you must use the shore power green ground and
white neutral. Never make an onboard connection from neutral to
ground when on shore power. You must use the shore ground because it
is to that ground (at the marina's service entrance) that the neutral
is referenced.

If you have an onboard AC supply, either genset or inverter, this
requires switching, because when you are off shore power you will
connect green to white at one point on the boat. Most larger
inverters (those intended to be wired permanently) do this for you
with a relay.

Jim Woodward
www.mvfintry.com