mortond wrote:
Hi,

Sorry, I guess I wasn't as clear as I thought!

The marina only has a 50 amp receptacle.
I wanted to avoid the cost of buying a second 30 amp or a single 50 amp
isolation transformer.
Basically, I wnted to hook-up the convection oven and fridge to the 50 amp
shore power receptacle, like an extension cord, except, the extension cord
would be routed via a 30amp breaker on the boat (revised that from a 50
amp) to one leg of the 'Y' branch of the shore power cord.
The appliance grounds would not be connected to the boat at all. I assume
the shore power ground would be isolated from the boat by the existing
isolation transformer on one leg of the "Y" and because there is no boat
connection on the other "Y" leg.

It is true that with what you are considering, the shore power ground
would be isolated from the boat. Partly true, anyway. Problem is that
unless you electrically insulate your steel boat from the water, it will
make a connection of sorts through the water back to the shore power
ground. At least near the dock, which is where your shore power
connection will be.

Here's the problem. Suppose you have a conventional (i.e., NEC/ABYC
approved) system and there is a short from the hot wire to the fridge
case (which is most probably connected to your hull). Normally, such a
short will trip a breaker instantly, giving protection as well as notice
that something potentially serious is amiss.

If the shore power ground is not attached to the hull, as you are
considering, that same short will probably not trip the breaker. The
path through the water will limit the current through the short, but not
enough to avoid possible electrocution. If the fridge case is not
grounded to the hull (as you are also considering) it is now at 120
volts relative to the hull!

If the boat is at the dock in sal****er, you have a potentially lethal
situation. It will work electrically, but unfortunately will introduce
serious hazards that include fire and electric shock.

The safe solutions are 1) isolation transformer(s), properly installed;
2) shore power ground connected to the hull (hopefully with a galvanic
isolator); and 3) a plastic boat. I don't recommend #3 as a way around
#1 or #2.

Hope that helps.

Chuck


Thanks for the interest.

David



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