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Galvanic Isolators Explained

Often overlooked, this piece of equipment is critical in the fight against corrosion.

Galvanic Isolator Illustration by Erich Stevens

Illustration: Erich Stevens

In the article "How To Control Electrical Flow On Your Boat", I explained the function of an isolation transformer in a shore power system. It basically eliminates any hard-wire connection between the dockside AC power system and the AC power system on board your boat while still maintaining electrical power and enhanced integrity all achieved by employing the principles of magnetic induction. In that same article I also identified some of the shortcomings of the transformer, namely weight, heat generation, and cost.

The issue for boats plugged into shore power is that each one is electrically connected to every other one in the marina via the system's grounding conductor — the green wire. This creates a natural phenomenon known as the galvanic cell. In this cell there will be winners and losers in the form of unwanted metal corrosion of the less noble metal in the cell, which is unfortunately often expensive underwater metals such as aluminum outdrives. To help mitigate damage from this natural electrochemical reaction, a typical boat will have sacrificial anodes installed to corrode rather than the metal in the galvanic cell. This is all well and good for a boat isolated from the dock and stored on a mooring. But any time that boat is plugged into shore power at a dock, those anodes, which are connected to the boat's grounding system, are also contributing to the corrosion protection for all the other boats plugged into that same dock sharing the same green wire. A boat that is plugged in that has insufficient anode area will naturally act as a drain to a boat that is properly equipped with adequate anodes and plugged in at the same dock.

Enter the galvanic isolator. Although not as capable as an isolation transformer, the galvanic isolator can go a long way toward extending sacrificial anode service life and mitigating corrosion, at much lower cost and nearly insignificant weight compared to the transformer. If your boat is plugged into shore power on a regular basis, you need a galvanic isolator installed in your shore power system.

How Galvanic Isolators Work

Remember that the galvanic isolator needs to be able to fulfill multiple roles in your boat's AC shore power system. Eliminating one of the components that make up a galvanic cell is one of its functions. But because it is installed in series with the green grounding conductor in your boat's AC shore power system, it must also ensure that electrical continuity is always maintained in that wire.


Be sure that any galvanic isolator that you install has a UL marine rating. These are certified as fail-safe units; even if the diodes fail the safety ground will still be connected.

For that reason, the American Boat & Yacht Council (ABYC) has some strict guidelines in its standard addressing galvanic isolators. The isolators must be rated for system amperage, usually 30 or 50 amps. They must be hard wired into the system without the use of any friction type connections and meet a series of design specifications to ensure that they can never inadvertently open circuit, effectively eliminating the all-important safety ground for your boat. The newest designs incorporate technology that can identify these units as "fail safe." Simply put, this designation ensures that even after something as significant as a lightning strike, the galvanic isolator will maintain continuity of the safety ground on board your boat. It may not continue to protect your underwater metals, but the safety ground will be intact, one reason why a galvanic isolator should be regularly checked for proper function.

Where And How To Install A Galvanic Isolator

The galvanic isolator is installed in the green grounding conductor in your shore power system. It works by introducing a low DC voltage drop in that conductor. Because galvanic voltages and currents run below that level, the net effect is that galvanic current flow is stopped, effectively isolating your boat from your neighbors on the dock, at least galvanically. Because the green wire's primary role is to act as a fault-carrying conductor in the event of a shore power equipment fault, the electronics used to block the galvanic current have no impact on AC current flow when there may be a fault.

If your boat is plugged into a dock regularly, you need a galvanic isolator (or an isolation transformer). If you have an older boat with an isolator installed, you should have a marine electrician test it to ensure that it is functioning properly and that it meets current standards that apply. Early isolators had inherent shortcomings that can make them unsafe. The problem is that you won’t have any idea whether it is functioning without testing the unit properly. If in doubt, get a qualified marine technician to test it or consider replacing with a new fail-safe unit.

Corrosion Warning

Aside from missing anodes, shore power is probably the most common cause of severe underwater metal corrosion. Without a galvanic isolator, your aluminum outdrive can become the anode of someone else's underwater metal because you're both connected via the shore power ground. Check your anodes (zincs) frequently, and if they seem to be suddenly wasting away, you may be a victim of galvanic corrosion. Installing a galvanic isolator can prevent expensive damage.

— Charles Fort

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Ed Sherman

Contributor, BoatUS Magazine

Ed Sherman is an avid boater and the author of "The Twelve Volt Bible for Boats," "The Power Boater’s Guide to Electrical Systems," and "Advanced Marine Electrics and Electronics Troubleshooting." He's the former vice president and education director of ABYC.