Larry wrote:
"Glenn Ashmore" wrote in news:l_2kg.112247$Ce1.77235
@dukeread01:

http://www.bluesea.com/Article_detai..._ID=290&id=299

What this manufacturer of diode isolators DOESN'T tell you is that their
product does NOT protect your underwater metal parts from AC LEAKAGE
caused by connecting this wire.

These "isolators" are simply two rows of silicon diodes in series with
the boat's ground wire. Like this:

|--|-|-|-------|
ac gnd--| |----boat gnd
|--|-|-|-------|

Each diode has a forward breakover voltage of around .6V. By having two
rows in parallel, either a positive or negative voltage over, in my
example, 1.8V (AC or DC it matters not) across the device will make it
conduct. 120VAC will make one row conduct on one half cycle, the other
on the other half cycle, a virtual short if there is any kind of short OR
AC LEAKAGE in any appliance. Any leakage that will trip a GFCI, for
instance, will make this device ALWAYS conduct, rendering it useless, a
fact Blue Sea will never tell you, of course. Once it is conducting the
AC leakage current from the hot water heater's leaky element to the
grounded cabinet of the hot water heater, all the DC electrolysis
currents it blocked destroying your prop and zincs now pass through it as
if it never existed. The AC current through it provide the breakover
voltage that was supposed to block the electrolysis currents...either
way.


GFCI protection at the pedestal of the neighboring
boat causing the problem would have prevented the
problem at the source if it were a 125 volt circuit.

The solution to THIS malady is an isolation transformer of sufficient
power to isolate the boat without overheating, itself. Big yachts have
them in the bilge. They're beasts humming away in my bud's Hat 56 FBMY.
The only connection between the power company and a boat protected by an
isolation transformer is magnetic power in the core. NO DC current can
pass between the windings, at all, even if the hot water heater leaks
like a sieve.

By the way, if the boat had had an isolation transformer, the only way
the kids would have gotten shocked is if TWO appliances connected to the
water had a short on L1 on one of them and L2 on the other. The whole AC
power system on the secondary of the isolation transformer has NO PATH to
earth. The only way you can get shocked is if you get right across the
AC power leads, L1 to L2. Too bad the whole power grid doesn't simply
have NEUTRAL isolated from ground, making it impossible to get a shock by
touching one of the wires and ground.


ABYC and common sense would have you ground the
neutral of the isolation transformer secondary and
use that as your equipment grounding conductor.
That way you remain isolated from the AC systems
of the rest of the world, but you maintain the
safety benefit of equipment grounding. Even GFCI
protection on the secondary wouldn't work without
that ground. You don't need the green wire for
GFCI to detect a ground fault, but you DO need a
connection to "ground" for a ground fault to
occur. So if you leave off a secondary ground, you
prevent ground faults by allowing the hot wire to
short to an equipment cabinet undetected! Not
likely to be very popular, Larry.


Chuck

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