Chuck,
You are not misunderstanding. In fact you have an excellent understanding of
the problem. Our discussion has led us to your statement below and while you
are correct that the same code is used for millions of residences, they
don't float and are not subject to galvanic damage and because of that, it
does bear on our discussion. My point is that code is not appropriate for
marina use. So it comes down to personal choice. I use a custom made
isolation transformer, they are cheap insurance.
Steve

Perhaps I'm misunderstanding, Steve, but every one of the millions of
residences in the US subject to the NEC parallels the shore based grid and
ground connection. While not a perfect system design, I don't believe it
bears on our discussion.

So
the correct advice is, if you don't want to waste your metal bits below
the waterline due to electrolosis and galvanic protection is your goal,
never direct wire to shore power. Instead, you should couple your energy
magnetically through the use of an isolation transformer and leave
other's faults to themselves.

Again, I fully agree that a properly installed isolation transformer is
one of several ways that galvanic currents through the shore power
grounding conductor can be mitigated. I continue to maintain that the
grounding conductor should be utilized from the dock pedestal to the
isolation transformer connection at the primary. At the secondary, you
should create and use your own grounding conductor by connecting to the
secondary's neutral for that purpose.


Chuck

Steve
Hello Steve,

I would caution strongly against using the water-earth path in lieu of
the green equipment grounding conductor. Ever.

The purpose of that conductor is to provide a low impedance path for the
purpose of clearing a ground fault.

In proper wiring, the impedance is no more than a few ohms and the fault
is cleared at a breaker. Fault current does NOT flow through the earth.

By contrast, the water-earth path may have a resistance of 25 ohms or
more for salt water, and tens of thousands of ohms or more for fresh
water. This path will NOT clear a ground fault. It will not trip a 20 A
breaker. Witness the 120 volt power cords dropped off a dock without
tripping a breaker!

If one has a problem with wiring impedances in the marina, deal with it
by getting the wiring inspected. Ask the marina operator when it was
last checked. Yell. Scream. Buy your own tester and tell the marina what
you measured. Tell the other boaters. Take names and publish on the
Internet. But don't throw out the safety net because even with corroded
connections, the wiring may still allow a ground fault to be cleared.

If there is a problem with a nearby source (boat or otherwise) then an
isolation transformer would be a good idea.

But even with an isolation transformer, always use approved wiring
practices. There is probably no more thoroughly reviewed standard than
the NEC. The ABYC standards for AC wiring are similarly solid.

And install GFCIs for the smaller ground fault currents that won't trip
a regular breaker but can electrocute.

Good luck.

Chuck



















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