A “Galvanic Isolator” is an electrical device that is installed aboard the boat. It is connected inline in the green safety ground conductor, at the inlet(s) of the shore power service to the boat. The device consists primarily of a solid-state full-wave bridge rectifier. Since it’s a full-wave device, it has two diode junctions in series in the circuit path at all times. The electrical nature of these diode junctions causes them to drop 0.6V across the junction, so the full-wave device will drop – or “block” – 1.2V, either AC or DC.
It is possible for there to be both AC and DC currents flowing in the safety ground wire. For the purposes of this discussion, there are three kinds of currents that could flow in the boat’s safety ground. They are 1) an AC fault current, 2) a DC fault current, or 3) a DC galvanic current. In a correctly wired boat with no AC or DC faults, there should never be any AC or DC fault currents flowing in that safety ground wire.
The current carrying capacity of the Galvanic Isolator diode pack must be matched to the AC electrical service(s) installed on the boat. A Galvanic Isolator needs to be rated to continuously handle the maximum worst-case current that could flow through the device. That would be 30A for a single 30A shore power service, at least 50 amps with a single 240V/50A shore power service, and at least 60 amps with two 120V/30A shore power services. If those current levels are exceeded, as they certainly would be in an AC short circuit fault, the shore power overload protection breakers and/or the boat’s main disconnect breakers should disconnect the shore power source and terminate the fault.
The DC current that a Galvanic Isolator is intended to stop, however, is called a galvanic current. The boat in salt water is a natural galvanic cell. The boat IS, ITSELF, a battery, producing a small DC voltage between its under-water anodic and cathodic metals. Salt water is this battery’s electrolyte, and the dissimilar metals of the propeller, drive shaft, reduction gear/transmission, rudder, thruster components, outdrives, trim tabs, thru-hulls, radio ground plane, speed and sounder sensor bodies, etc., etc., etc., are the anodes and cathodes.
Galvanic currents (in the form of electrons) flow from the under-water cathodic metals, through the water and into the earth, find their way to the AC safety ground wire via the shore power ground rod and associated electrical infrastructure, flow back onto the boat through the shore power safety ground conductor, flow into the boat’s bonding system and finally back to the under-water anodic metals. In the process, these small DC currents deteriorate the least noble metals they encounter. Hopefully, that will be zincs and not the more noble metals of props, outdrives, transmissions, rudders, thrusters, etc. And by the way, if you have good zincs on your boat, but your dock neighbors do not, you will be glad to know that the noble metals of the neighboring boats are also protected… by your zincs (via the shared dock-side AC shore power safety ground connection). Since your zincs are the sacrificial metals in this system, and are likely to deteriorate at a way faster than normal rate, you may or may not consider this generosity to be a good thing.
The cheapest way to stop the erosion of noble under-water metals is to zinc all the metals parts under the waterline. For this to be effective over time, however, the zincs must receive frequent, routine maintenance. The entire purpose of zincs is to absorb, and be destroyed by, galvanic currents. With a galvanic isolator, galvanic voltages are still present, but the device prevents galvanic currents from flowing. The physics of a full-wave bridge rectifier configuration is such that it takes 1.2 VDC to overcome the diode threshold conduction voltage and cause current to flow. Galvanic voltages are less than that threshold. They cannot overcome the diode junction conduction voltage, so the device effectively blocks them. Think of this as a battery sitting in its package in a drawer, not installed and not conducting. This has the effect of “disconnecting” the shore power safety ground connection for small DC galvanic voltages, while leaving the safety ground fully intact for larger AC fault currents. Ah! There we have it! The best of both worlds.
Conclusion: if the boat spends 325 nights a year at anchor away from shore power, there is no need for a galvanic isolator. That boat is still a battery, but without the shore power connection, there is not a complete circuit for the galvanic currents to flow through. However, if the boat spends 325 nights connected to shore power in a marina – like the vast majority of boats – then a galvanic isolator will pay for itself in saved zincs over a relatively short time. And sooner than that if it saves a prop, rudder, thruster or outdrive from galvanic deterioration. If your boat lives on an inland fresh-water lake, this issue is greatly diminished. Fresh water is a poor electrolyte, and galvanic currents are essentially not formed.
An isolation transformer will perform the same function of blocking galvanic voltages, and much more, too. However, isolation transformers are large and heavy, while galvanic isolators are small enough to be easily installed, in an electrical closet, near the shore power inlet(s). In initial construction, I’d recommend the isolation transformer over galvanic isolators. But to retrofit an existing boat, the much small and lighter physical form factor of the galvanic isolator makes it a very good choice.
Finally, center cockpit day boats connected to shore power to run a battery charger are also subject to this galvanic current phenomenon. If you have a boat with outdrives that are always in the water, you SHOULD also have a galvanic isolator on that boat to protect them.