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From: Richard Goodwin (no email)
Date: Mon Oct 01 2001 - 08:08:04 EDT
> Richard
> The advice you posted to the lv-ab list regarding how to deal with a stray
> current problem is very dangerous!
> Cutting the ground wire is not recommended!
> In fact it violates several standards because it will increase the risk to
> occupants in the boat and anyone in the adjacent waters.
A general phrase like "cutting the ground wire" contains more dogma than
information. I'll try to make this clearer:
Let's use a concrete example of why there are safety grounds and what
they can do for you. Take a simple case like an electric heater plugged
into a grounded outlet. Let's say that the heater develops a short
inside so that the hot 120v wire comes into contact with the metal case
of the heater. Without the ground connection, if you were to touch a
ground, like your engine block, and the case of the heater at the same
time, then 120v would go through you. But with the ground connection,
the fuse will blow instead, or the gfi, and you will be spared the
experience.
So if you disconnected the ground wire from the heater to the outlet you
would indeed be in danger of a shock. Or if you disconnected the ground
connection between your 120v outlet system and your boat's ground you
would still be in danger of a shock if you touch the engine and the
heater case at the same time.
Appliances with AC motors can also generate induced current in their
cases, even without there being any short circuit internally, so a
safety ground will protect you from that as well.
But this case is different. In this case the 120v AC outlets are
grounded properly to the boat's ground, and therefore to the surrounding
seawater. The problem is that they are ALSO grounded through the shore
power's ground wire to some ground somewhere on shore. There are TWO
separate grounds!
And that's where the problem is coming from. You don't need TWO
grounds, you only need ONE. And you especially don't need TWO grounds
which are at different electrical potentials from each other. This
could create the very danger to people on board or in the water that you
want to avoid.
I have many years of experience with matters electric, including just
this sort of problem on boats. By the way, this problem -- differing
ground potentials -- is not just a problem with boats, I have seen
exactly the same problem on land many times, leading to some very
strange computer malfunctions.
So again, I would disconnect the ground connection between the shore
power and the boat where the shore power comes into the boat, or the
equivalent. And of course make sure your 120v AC system is grounded
properly to your boat's ground and therefore to the surrounding
seawater.
The next question is: Isolation xformer or not?
If you put in an isolation xformer, then you are truly isolated from
anything that can happen to the power or ground beyond your boat. This
is the best idea. You would want to pick one side of the isolated 120v
circuit to be neutral, and tie it to the safety ground, and make sure
all outlets are wired correctly, same as what you would find on shore.
But do you need an isolation xformer? That's the intriguing question,
and personally (this part is not advice, but is only my personal
opinion) I would not bother with one, but I would protect the 120v
system with a gfi that can shut down the 120v supply to the entire
boat. I would do that in any case, actually.
You would have the same electrical potential between neutral and ground
as you had previously between the two grounds, but that should not cause
problems, unless you have an appliance that internally connects neutral
and ground for some reason. I have not seen this done since the last of
the old cheap tube type televisions. People used to call up and say, "I
get a shock whenever I touch my TV!" We would tell them, "Turn the plug
around." Things have improved much since those days.
Dick
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