Larry wrote:
chuck wrote in news:haikf.8489$N45.4470
@newsread1.news.atl.earthlink.net:


providing an opportunity for a
potential difference between your DC ground system and the grounded

case

of an AC appliance. An improperly wired boat in the next slip can cause
real safety problems aboard you vessel.



Show me how. The water is earth ground, no matter how the joker's boat
is wired next door. If he's wired wrong, the AC current through the
water will trip the AC breaker on the dock. Try it.


Well, the classic example of this is when the guy on the boat next to
yours uses a 2-wire AC cord on an automotive-type battery charger. The
charger develops leakage between the hot AC wire and the 12 volt DC
ground. Won't trip any breakers because the current is too small. The
circuit could easily deliver 10 amps forever with nothing tripping.
Remember that the breakers are sized to trip before the wire causes a
fire; NOT before enough current has passed through a person to cause
electrocution. We're not talking about someone savy enough to use GFCI
outlets here which can detect small current leakages.

Alternatively, an automotive-type charger is plugged into a makeshift
extension cord with the hot and neutral wires swapped. No green wire at
all. Even without a leakage or short, the hot AC wire is now connected
directly to the neighbor's DC ground system to his propeller. Again,
nothing trips because the path through the water to the AC system's
ground/neutral junction is too high a resistance to carry 15 amps (or
whatever the breaker is rated at) with 120 volts applied.

Next, you grab your engine with one hand and open the door of your
fridge with the other as you reach for a cold one and you receive an
unhealthy dose of 60 Hz.


There is NO WAY for the guy's boat next door to raise the AC potential of
the whole ocean up off the AC shore ground potential my fridge is
connected to! He'd need a thousand amp breaker to raise it a tiny
fraction. The shore power ground is hooked to the earth/seawater.


What happens, Larry, is that your fridge cabinet is connected to the
green equipment grounding conductor and to an AC ground/neutral junction
on land. Your engine is connected through the water to the hot AC wire
on the neighboring boat. The voltage between those two pieces of metal
could approach 120 volts. Even a zinc/copper galvanic couple will result
in a measurable potential difference in seawater. No need for massive
currents and no need to think in terms of raising the potential of the
whole ocean. :-)

Draw out the circut and it will pop right out at you.

There are documented cases of swimmers and boaters suffering
electrocution from these problems. How probable? Different subject.
First let's establish how it could happen.

Chuck