"johnhh" wrote in
:

So are you complaining that the AC ground and DC negative are both
connected to the boats ground
"Boats ground"? What's that? The boat is made of PLASTIC, an insulator.
It has no "ground". It has a negative battery bus. It has an AC supply
ground connected to the shore power ground system. What's the "boats
ground"?

I hear this a lot. It doesn't exist. The battery negative bus has no
relationship, whatsoever, to the AC shore power ground as the two systems
are totally isolated from each other unless there is something wrong with
your battery charger, dual-voltage fridge or other AC/DC components.

Touching the AC shore grounded appliance, already hooked to AC shore
ground, with one hand and touching the engine block connected to the sea
and DC ground bus results in no hazard...unless the AC shore ground is
defective PLUS the appliance or device is defective. Touching an AC
grounded device with one hand and a defective 2-wire toaster with the hot
lead of the AC line hooked to its metal case has the SAME SHOCK as
touching the DC ground bus hooked to the engine block....unless, of
course, the boat has a proper GFI that should, but is not, required
aboard the boat....just like proper NEC-approved breaker panels are not.
(A bunch of little plastic breakers on a plastic panel screwed into a
flammable wooden box made of some exotic, expensive wood is NOT up to the
National Electric Code of the USA or any country with any brains. AC and
DC panels in boats are made that way to SAVE MANUFACTURERS MONEY. They
are NOT "safe", from fire (flammable) or explosion (not sealed against
fumes) at all!

Ok, back to the buses connected together for some stupid reason.....

If the DC bus hooked to the zincs by virtue of the DC bus being connected
to the engine block....is connected directly to the AC bus ground back to
shore power ground through the boat's impressive yellow shore cable, this
connects the boat's zincs STRAIGHT TO THE BOTTOM OF THE HARBOR...not to
mention every other boat wired this stupid way. YOUR zincs are hooked to
HIS underwater metal parts through the common-connected AC shore power
ground wires.

So? What happens now?........(you ask).....

Ever notice the metal conduit hanging loose under every dock on every
marina in the country? See how it hangs into the water? If I hook MY
zincs to that metal conduit (by the AC shore power ground wire from the
boat with the DC negative bus hooked to AC shore power ground), MY zinc
will fizz away protecting the rotting plating on that sagging conduit,
every little ground wire touching anything conductive hooked to the
water, and even the ground rod back at the feed point from the power
company! Isn't that nice of me?

Is it any wonder some people are going through those amazingly-expensive
zincs at an alarming rate in your marina? Zincs protect what they are
CONNECTED to. They are a virual shorted battery, the zinc one plate, any
metal part making connections with the water the other plate and seawater
as the electrolyte. In order for the battery to DISCHARGE, eating the
zinc, it has to have a DIRECT DC PATH from the zinc to that plate.
Unhook the path, you've opened the shorted battery circuit. The only
battery plate the UNHOOKED boat makes a battery with is the metal part
the zinc is mounted on and is protecting, not every neighboring boat
connected to the AC power shore ground.

Every AC device in the boat, with, of course, the exception of insulated,
UL-approved, appliances-electronics-doubleinsulated drills-etc....should
be hooked for safety to the SHORE GROUND, to prevent the AC line from
appearing on the metal cabinet parts of it. Of course, it would be nice
if the damned AC panel, ITSELF, on pleasure boats were so connected.
They're not. Look into the wooden box. Metal breaker panel hooked to
green wire? Not most.

If you disconnect the DC negative bus from the AC Shore Ground green
wire, you measure the DC potential caused by the zincs...the
electroplating voltage trying to destroy the zincs. It CAN'T have any AC
voltage between the Shore Ground hooked to a ground rod somewhere and the
DC ground bus hooked to the engine block....doesn't wash.