"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.

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.