[R-390] The ground round

[Bill Hawkins]

While we are talking about power near ground ...

My son bought a 55 foot power boat for fishing parties in
Ocean City, MD. The wiring needed some work, and I learned
some new stuff about power and grounds.

A common ground point is used to control noise fed to radio
and navigation (autopilot, GPS, LORAN) electronics. Wired to
it are the engine blocks (and battery negative), a 'quiet'
battery negative for the electronics, battery charger negative,
ship's AC generator green, shore power AC green, a copper tape
from the SSB and a copper tape from the other electronics.

When the ship is at sea, it is 'grounded' to seawater by a
couple of porous bronze plates. This is mostly for the
benefit of the 200 watt SSB transmitter and its quarter
wave whip and antenna tuner. The SSB also has 100 square
feet of copper screen for a counterpoise, which reduces
the amount the autopilot changes course while transmitting.

When the ship is in a marina, the bronze plates are still
there, but shore power adds a green wire for ground to the
mix. There are two interesting things about shore power.
First, the white wire isn't always neutral - sometimes it
is hot and black is neutral. So the shore power breaker has
to break both wires. Second, the green wire could be carrying
electrolysis (corrosion) current, perhaps from another boat.

So you can't just tie the shore green to boat ground. Worst
case, the electrolysis current is strong enough to evaporate
one of your through-hull fittings in a day, leaving a hole
for seawater to enter the boat. Or weaken it enough so that
it breaks at sea and finally turns on the high water alarm.

The fix is called a "Galvanic Isolator" in the green wire.
Two diodes in series begin serious conduction at about 1.2 VDC.
That's high enough to prevent most seawater electrolysis. You
need 4 diodes, two in each direction, big enough to pop the
shore power breaker if there is a fault from hot to ground,
perhaps from the block heaters or battery charger.

And then there's the problem of lightning, but that's another
long story.

The Coast Guard building code for boats does not allow any
kind of aluminum wire or terminal in a power circuit.

My son's boat does not have an R-390. He already has an anchor.

Happy new year,
Bill Hawkins.