Jim, is not the galvanic isolator for the wire going to ground/earth in the water? I have no such thing. The 115vac runs only when plugged into the dock, and I rely on the green wire going to the dock for that. I have a crossed-polarity indicator on the panel to indicate when stray current is going to the green (which the isolator would protect against, if it happened). The light went off only once-- when I first hooked up the 115vac on the boat-- and after inspecting every single connection on board I found the fault was in the yard's outlet box (no kidding).
My cousin Rick (ABYC) says the biggest problem is for boats with on-board 115vac (generators or inverters) needing to ground the green wire to 'earth' when there's only a ground plate in the water. So at the dock, these people are running two green-wire grounds (if they don't remember to turn one off). So they have a complete circuit. I don't have anything like that going on... hence my question.
Because our boats AC grounding wire (green) is
supposed to be bonded to the DC grounding system, for human safety, this means that when you plug into shore power your underwater metals are connected to your neighbors underwater metals via the green AC grounding wire. If you have no underwater metals, and no ground path to the ocean, then there would be no galvanic corrosion. However if you leave your OB in the water, have a bronze ground plate, your centerboard is bonded, your gudgeons are bonded etc. then you will want a GI at a minimim.
A galvanic isolator is a device that is inserted, in series, into the green grounding wire (safety ground) of your shore power feed to help minimize or reduce the effects of
galvanic current from
flowing between your vessel & your neighbors. While blocking
galvanic level current it also has to allow for the passage of AC fault current. This is why any galvanic isolator should be of the "fail safe" type or have idiot lights that tell you you still have an AC grounding connection.
Bottom line: If you plug into a marina without a GI
your anodes will be protecting your neighbors boat who's anodes have already eroded away.
This
blockage of low voltage galvanic current is achieved by using two diodes in-series in each direction. Each diode drops approximately .6V or
requires more than .6V to open and "
Flow". Two of them in series results in approximately a 1.0V - 1.2V threshold for blocking DC galvanic level voltage & current.
GI's normally have two diodes in each direction so the AC green wire is not "check valved" and acts just like a wire normally would. The only difference is it acts as one that won't pass voltages below 1.0V - 1.2V. Simple and pretty effective at blocking
galvanic level current. GI's do not however block
stray current that exceeds 1.2V...
Think of a diode as an electrical
check valve. It allows current to flow in only one direction but not in the other direction. One of the inherent traits of diodes is the voltage drop associated with them, which is usually around 0.6V. In a galvanic isolator application they have used this
often assumed bad trait of a diode to an
advantage. By wiring two diodes in series you now have a device that can block any galvanic level voltages below 1.0V - 1.2V from flowing into or out of your vessel.
If we understand the voltage potential spreads, between underwater metals, which is usually below a 1.0V differential, it becomes easy to see how a GI works.
Because the dissimilar metals connected together in the electrolyte can't really create more than 1.2V the GI stops your vessels anodes from protecting your neighbors underwater metals.
If plugging in at a marina, with underwater metals that need anode protection, a GI is the
absolute bare minimum level of protection that every boater should have.
On personal level you'd never catch me plugged into any marina without an isolation transformer (IT). A simple GI won't cut it for me. Of course the conversation of
true isolation versus
galvanic isolation only takes on another life and is a whole other discussion & topic. Without knowing what your anode is actually protecting it is impossible to say what to do...
At a bare minimum you should have a fail safe
galvanic isolator if you have below water metals.
The standards for AC ground
ing (green AC wire) are simple and clear:
ABYC E-11
11.5.3.3 The main AC system grounding bus shall be connected to:
11.5.3.3.1 the engine negative terminal or the DC main negative bus on grounded DC systems, or
11.5.3.3.2 the boat’s DC grounding bus in installations using ungrounded DC electrical systems.