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From: Richard Goodwin (no email)
Date: Wed Oct 03 2001 - 17:12:08 EDT
Just read a bunch more about this subject, and I'd like to clear up one
thing by making a strong, non-theoretical statement:
Do not simply cut the ground wire between the shore power and your
boat's 120v system, without doing some other things in conjunction with
cutting that connection. Especially if your boat is in fresh water.
People on board might be safe, but people in the water or people getting
on/off the boat might not.
The reason is: IF you cut the shore power ground, even if you leave the
on-board 120v system safety grounded to the engine and sea (this won't
do any good in fresh water, only in salt water), then IF there is a
short inside an appliance between 120v hot and the safety ground, then
this will put 120v on the boat's engine, and on all the grounded metal
parts of the boat, and then IF someone should step off or on the boat
and touch a metal dock, IF the dock is grounded, they could get a shock.
Not only that, but there will be current flowing through your engine
into the seawater, which just has to cause some problems somewhere
eventually. If nothing else, the gradient around your boat would be
dangerous to divers.
The problem is simply that the seawater doesn't make all that great a
safety ground, most likely not great enough to blow the 120v breaker
that is protecting the faulty device with the short. And if the boat is
in fresh water, then there is no significant ground protection at all
through the water.
A GFCI would definitely help, but that is reacting to the problem. We
need to prevent the problem altogether if possible.
So what can you do?
1. Install a galvanic isolator. These things would solve the problem
that started this thread, because they do block low DC and AC voltages
such as the .45 DC volts described in the post, while allowing anything
more than that to pass through to the shore power safety ground line.
So cut the safety ground between the shore power wire and your boat's
120v system, and insert the galvanic isolator between those two points.
These things should be cheap -- all they are is back-to-back diodes,
which you can get at Radio Shack for a buck or two each.
2. Install an isolation xformer, and cut the ground to the shore power
cord. Your boat is in its own separate electrical world entirely then,
and an internal appliance short would not put 120v on the boat's ground
system. To be safer, though, install a galvanic isolator anyway between
the boat's safety ground and shore power cord ground.
3. Protect the boat with GFCI, so that in case all else fails, the GFCI
will shut off power. Theoretically this should be enough by itself, and
you could disconnect the ground from shore power.
If you do 1 OR 2, and 3, you should be in pretty good shape. One source
recommends cutting the shore safety ground if your boat is fully GFCI
protected or if you have an isolation xformer. But a galvanic isolator
will eliminate the stray low-voltage currents, but still provide
protection for higer voltages if the right kind of fault should occur.
The third link below goes into much more detail.
Interesting links:
http://www.yandina.com/galvanicIsolator.htm
http://pkys.com/galvanic_isolators.htm
http://www.islandnet.com/~robb/marine.html#ACSystem
Dick
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