Galvanic Isolators and Isolation Transformers

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Most cruising boats employ some type of AC shorepower system. In fact, the newer the boat, the more complex and necessary this system is likely to be. More often than not, the most necessary consumer of shorepower is the battery charger. Even the most frugal and purist cruiser is unlikely to forgo this veritable necessity. For dockside vessels, keeping the batteries charged while the boat is unattended means having fully charged start and house batteries at all times—as well as an ample power supply for bilge pumps in the event of a hose, through-hull, or other leak.


Once you bring AC power aboard, you must also accept responsibility for ensuring that it is safely wired. This means meeting the American Boat & Yacht Council’s specifications as described in its Standards and Technical Information Reports for Small Craft, Chapters E-2, E-4, and E-11: “Cathodic Protection,” “Lightning Protection,” and “AC & DC Electrical Systems on Boats,” respectively.

Together, these guidelines form an intricate mosaic that tries to make the mixture of electricity and seawater—an inherently unsafe combination—as safe as possible. (For the purposes of this discussion, “sea water” includes any water your boat may be floating on—salt, fresh, or brackish). One of the primary tenets of these ABYC standards is the need for bonding. Bonding is just another name for electrically connecting (a connection usually associated in some way with grounding) selected metal objects on board a boat.

These objects (sometimes referred to as masses) include such things as the engine, metallic fuel and water tanks, steering gear, metallic hardware below the waterline, spars, shrouds, davits, arches, and so on. A bonding wire is used to connect the masses. This grounding circuit is separate from the green AC safety ground circuit, but as we will see later, it is important that these two circuits be connected.

There’s no shortage of controversy where the practice of bonding is concerned. However, along with many other boatbuilders and repair yards, I subscribe to this practice for several reasons. But one reason in particular stands out: A bonded boat is less likely to electrocute one of its crew.

Aboard a properly wired vessel, if an energized AC wire—often called “hot” and typically black or red—comes in contact with one of these bonded masses, a tank or a rudder post, for instance, the electricity is safely discharged to ground and will, in theory, trip the circuit breaker as well.

On a vessel that does not follow the ABYC guidelines on bonding, the hot conductor will energize the metal mass—for example, that same tank or rudder post. If an unsuspecting crew member comes along and touches this energized mass with one hand and then touches another mass that is grounded, such as the engine block or propeller shaft, with the other hand, the resulting electrical shock across the crew member’s chest will be a frightening, if not fatal, experience.

Detractors of the practice of having a “bonded boat” argue that bonding is much more likely to result in galvanic corrosion—sometimes incorrectly referred to as electrolysis. Unfortunately, they are correct.

Galvanic corrosion, also called dissimilar-metal corrosion, occurs when two different metals—for instance, aluminum and bronze, or stainless steel and brass—remain in contact either directly or through a wire while they are immersed in an electrolyte, in this case, sea water. The rate and severity of corrosion depends on many factors: the types of metals involved, the salinity and temperature of the water, and the presence of zinc anodes, to name a few. Typically, galvanic corrosion is a relatively slow process, causing cumulative damage over the course of months, if not years.

Even if there were no solution to this dilemma—and there is—please read on, because you have a clear choice: Suffer galvanic corrosion damage or electrocute yourself or one of your crew. This should be an easy decision to make.


In addition to the failure-to-bond problem, many vessels are plagued with other AC electrical land mines. The most notorious of these is connecting—aboard the boat—the AC neutral conductor (the white wire) and the safety grounding conductor (the green wire). Unlike in a house, aboard a boat, these two conductors should have nothing to do with each other. Rather, they must be connected only ashore.

This includes the internal wiring of appliances such as some domestic microwave ovens, coffeepots, clothes dryers, and so on. In many instances, boatbuilders, repair yards, or owners will obtain these common domestic appliances for installation aboard a vessel. These appliances must be tested, and possibly modified, to ensure safe shipboard use.

The reason for this idiosyncrasy is the potential for the transmission of current through sea water. All AC power coming aboard on the hot conductors (on a vessel equipped with 240-volt service there will be two hot conductors) must ultimately find its way back to ground. If the white and green wires are allowed to touch or are intentionally connected, ordinary current that normally returns to its source on the white wire will return to its source through the green wire as well. In a properly wired boat, the AC safety grounding circuit (the green wire) must be connected to the bonding circuit—making these two circuits essentially the same system. So current improperly flowing between the white wire and the green wire can also flow through the bonding circuit.

If a boat is wired in a way that connects the green and white wires, the current’s return could then take one, two, or all three of the following paths: through the white neutral conductor, through the green safety grounding conductor, and through the boat’s bonding circuit to underwater hardware and thence to the sea water. If high resistance develops in the neutral and/or AC safety ground conductors—this often happens when the shorepower cable contacts become corroded or wet—the sole return path for shorepower current could become the sea water.

A swimmer passing through this electrical path could be killed, even if the strength of current is not great enough to be considered lethal ashore. Sadly, this has happened on a number of occasions. Again, salt water or fresh—it doesn’t matter. In fact, studies and anecdotal evidence indicate that AC current may be more likely to be lethal in fresh water than in salt water. The reason for this is the directness of the path that current takes when it travels through fresh as opposed to salt water.

Because fresh water is less conductive than salt water, current takes a more direct path through fresh water. This more direct path results in a higher current density—that is, the amount of current present in the water at a given location. A good analogy is a lightning bolt passing through air—it’s concentrated and very direct because air (like fresh water) is a poor conductor. Greater current density means a greater difference in current potential from one location to the next, and greater potential for the current to electrocute a person or upset his or her heart rhythm, delivering a lethal shock.

In 1999, a nine-year-old boy was electrocuted as he swam in fresh water next to a marina dock. He was wearing a life jacket, and his face never touched the water. His mother jumped in to save him. As she did so, her limbs and extremities went numb. In spite of this, she was able to pull her son to the dock, where others helped pull them from the water. The ensuing investigation determined that this unfortunate tragedy occurred because of an electrical fault in a nearby unbonded boat. A melted wire allowed AC shorepower current to leak into the water around the boat and the nearby dock where the boy was swimming.

Because this was fresh water, the current had difficulty finding a path to ground, until the boy entered its path. The salinity of the human body makes it a much better conductor than fresh water. The boy’s mother was able to pass through the path of the current without being electrocuted because of her greater body weight and skin surface area, but the current was great enough to be lethal to the boy’s smaller, lighter body. Had the vessel with the offending electrical fault been bonded, it’s unlikely this tragedy would have occurred. The fault current would have passed safely ashore over the green safety grounding wire, where it would most likely have tripped the dockside circuit breaker. If no other lesson is learned from this sad tale, let it be this: Never swim in a marina or next to docks where shorepower is present.

Small amounts of AC current are sufficient to immobilize voluntary muscle reflexes, such as those needed to swim and stay afloat. Current as low as 5milliamps can cause muscle seizure. Higher current (50 milliamps for 2 seconds or 500 milliamps for just 0.2 seconds) can cause ventricular fibrillation and ultimately death. (This is probably what happened to the boy mentioned above.) Essentially, a swimmer can drown or suffer heart stoppage even in water that’s not over his or her head.

Another common AC error is severing the connection between the AC safety ground and the boat’s bonding system. Both are green wires, but of different gauges: one wired to most or all AC appliances and going ashore in the shorepower cable; the other, a series of wires aboard the boat that connects various pieces of hardware. As mentioned above, these two circuits must be connected (usually at the boat’s electrical panel) and remain at the same electrical potential at all times. Connecting the two ensures that fault current is safely conducted to the shorepower ground and that—ideally—the circuit breaker is tripped. (The boat that led to the nine-year-old’s death did not possess this connection.)

To understand the importance of this connection, consider the scenario in which a fault is created when a hot wire comes into contact with a grounded wire—usually by accident, for example, when a hot wire gets crushed between a metal tank and a support beam. Now fault current passes to the tank. If the tank is properly bonded and if the bonding circuit is connected to the AC safety grounding circuit, the current will be safely conducted to ground ashore, not through the water.

In an attempt to reduce the occurrence of corrosion, an unwitting skipper may disconnect this all-too-important connection. The reasoning is that if the underwater metal is not connected to the dock through the bonding circuit and AC safety ground, then the boat will no longer be plagued by galvanic corrosion induced by neighboring boats.

This is indeed true: Separating the bonding circuit and the AC ground circuit may reduce the likelihood of stray-current corrosion (this type of corrosion is caused by DC current that leaks into the bilgewater or the water surrounding the boat) and galvanic corrosion that travels over the shorepower green grounding wire. But disconnecting this wire will do nothing to mitigate the effects of these types of corrosion if they originate on board.

As an aside, many boat owners and even some marine professionals incorrectly assume that because itoften appears that this corrosion occurs or is exacerbated when a vessel is plugged into shorepower, the culprit must be the AC power, or the marina’s AC power supply itself. In fact, nothing could be further from the truth. Galvanic corrosion and stray-current corrosion are both strictly DC phenomena. Stray current from one boat may still enter and damage another boat’s bonded underwater fittings. Although this does occur, it’s not terribly common. But nothing can prevent stray current except eliminating the bonding system, and for reasons discussed earlier, that’s not a safe option. Once this connection between AC ground and the bonding system has been disconnected, the scenario of an electrocuted crew member or swimmer pays another visit.

I once encountered the following set of circumstances. A boat owner intentionally broke the connection between the bonding and ground circuits. The boat’s microwave oven developed a short between the hot conductor and the metal enclosure, and the ground contact on the shorepower plug was heavily corroded. The scene was now set. There was no low-resistance return path to ground for the energized metal enclosure, so it remained energized. The boat was afloat. And I was working on some galley plumbing. Each time Ibrushed the microwave with my shirtsleeved arm while I was touching a bonded piece of hardware, I felt a slight tingle.

Had my sleeves had been rolled up, you might not be reading this article today. If the microwave’s safety ground and the hardware’s bonding wire had been connected, there would have been no difference in potential, and thus no possibility for electrocution, even if the shoreside ground were faulty. The moral of this story is that the AC safety ground circuit and the bonding system must always be connected, and they must remain at the same electrical potential.


Now that we have established that your vessel should be properly wired for AC safety ground and selected onboard hardware must be bonded, you might ask why anyone would not do this. The problem is that when all of the safety precautions I have mentioned are taken, the undeniable side effect is the increased potential for galvanic corrosion when the boat is plugged into shorepower.

When you bond underwater metals and dutifully connect them to the AC shorepower safety ground, you may have unwittingly invited aboard an unwanted guest—corrosion. The circumstances are simple: You conscientiously bond your boat and inspect the zinc anodes regularly, changing them whenever they are more than 50 percent depleted. You also remain plugged into shorepower to keep the batteries up and the reefer cold, and to run an air conditioner, a microwave, a coffeemaker, or other appliances. Your slip neighbor, however, hasn’t been seen aboard his boat in months, but his boat remains plugged into shorepower to keep the fridge cold and the air conditioner running.

You have now inadvertently connected the two boats together, electrically, through the AC shorepower safety ground. Galvanic current flows from one boat to the other. When the other boat’s zinc anodes are depleted, yours take over, protecting both boats’ underwater hardware. In this case, that’s not for long. This could happen with any number of boats, potentially an entire marina.

It’s important enough to warrant repeating: Galvanic corrosion is DC (direct current) in nature. Stray current corrosion, which is different from galvanic corrosion but sometimes confused with it, requires the introduction of “leaked” voltage from, once again, a DC-positive source, such as a wire whose insulation is damaged that is immersed in bilgewater.

I have encountered many people who will argue strongly that corrosion can be caused by “hot marinas,” that is, faulty dockside AC electrical systems. This belief is usually based on a “corrosive” experience they’ve had while visiting a marina, which was, in all likelihood, caused by common DC galvanic or stray-current corrosion. But they have misidentified the source because it occurred only when the shorepower cable was connected. Personally, I have yet to find any evidence that AC current is capable of causing stray-current corrosion (other than in a laboratory or in some commercial pipelines that have been buried under high-tension power distribution cables), although DC current may be superimposed on AC circuits and consequently cause corrosion. However, faulty AC shorepower wiring is quite capable of injuring or killing people, as previously mentioned, whether swimming or not.


Fortunately for cruisers using dockside shorepower, it is possible to have a safe, properly wired AC electrical system and simultaneously prevent rampant galvanic corrosion. Would it solve the problem if the damaging DC current could be prevented from sneaking aboard your boat, but the required AC safety ground current were allowed to pass unimpeded? Yes, it would, and this can be accomplished by using a device known as a galvanic isolator. Depending on the configuration, this device will prevent up to 1.2 volts from passing through the green AC shorepower safety grounding conductor, thus stopping most destructive galvanic current, which is usually less than 1 volt.

Remember, however, galvanic corrosion currents are DC in nature, so that’s all the galvanic isolator stops—again, up to 1.2 volts DC. It will not prevent AC voltages/currents from passing through it, so the safety ground remains intact, in accordance with ABYC standards. (Any galvanic isolator you purchase or have installed should comply with the latest and most stringent ABYC standard: section A-28 of the Standards and Technical Information Reports for Small Craft. If your vessel is already equipped with a galvanic isolator and it does not meet this standard, consider upgrading it with a compliant model.)

If the galvanic isolator solved the shorepower-induced corrosion problem completely, I could end my discussion here. Unfortunately, this is not the case. The Achilles heel of the galvanic isolator is twofold. Its primary weakness is that when it is subject to high DC fault voltage (this may be as little as the previously mentioned 1.2 volts), the isolator becomes essentially transparent, conducting any current that cares to pass through it. This effectively nullifies the corrosion firewall effect of the galvanic isolator. Unfortunately, unless your boat’s electrical system is equipped with a monitoring device, you may never know the galvanic isolator is not working. For that very reason, the latest ABYC standard calls for incorporating a monitoring device into every galvanic isolator.

The galvanic isolator’s other shortcoming is its inability to prevent other shorepower-induced faults, the most notorious being reversed polarity. If this situation exists, the galvanic isolator will have no effect on it. Undoubtedly, it is better to have a galvanic isolator—one must be installed on each shorepower inlet or circuit—than not. However, the prudent cruiser must be aware of its limitations.

The ultimate solution for most of these problems is the isolation transformer. Once installed, the isolation transformer acts much like its own power supply, similar to a generator or an inverter, or a utility company, for that matter. All voltage produced by the isolation transformer seeks a return to its origin, not just any ground. The importance of this feature cannot be overemphasized. Shorepower voltage, once it passes through the isolation transformer, will return only to that isolation transformer, through either the white neutral conductor or the green safety grounding conductor, whether by design or fault. Voltage that now emanates from the isolation transformer will never travel through sea water to seek a path to ground. This protects swimmers. Damaging galvanic voltages that normally would be allowed to come aboard via the green safety grounding conductor in the shorepower cable are also thwarted, because there is no longer any direct connection to shoreside grounds. This is where the isolation transformer and the galvanic isolator diverge. Where the galvanic isolator attempts to block DC current from coming aboard, like the walls around a medieval fortress, the isolation transformer severs this connection altogether, much like digging a moat around the same fort, filling it with water and crocodiles, and pulling up the drawbridge.

The isolation transformer achieves all of this through the principle of magnetic inductance. Here’s how it works. Shorepower voltage travels from the dock, through the shorepower cable or cables, and onto the boat’s shorepower inlet. As is the case for the galvanic isolator, one transformer is required for each shorepower inlet. But instead of allowing current to go from there to the shorepower circuit breaker panel, the isolation transformer interrupts the current before it can reach the circuit breaker. The incoming AC power travels through the primary or input winding of the transformer and back to shore. That’s as close as the dockside shorepower ever gets to the boat’s electrical system. Electricity is induced on the transformer’s secondary or boat side winding magnetically. There is no direct connection. This arrangement eliminates the possibility of reverse polarity and of unintentionally creating the potential for a swimmer either drowning—because the electricity paralyzes his or her voluntary muscle reflexes—or, if the current is strong, being electrocuted. (It’s telling that vessels equipped with isolation transformers are exempt from ABYC’s reverse-polarity indicator requirement.)

Additionally, much like the power sources mentioned above, the onboard AC green safety grounding conductor now originates at the secondary winding of the isolation transformer. As a result, shoreside grounds and the boat’s ground have nothing in common. This reduces the potential for foreign stray-current corrosion. Stray-current corrosion, which originates domestically—that is, aboard your own boat—is still potentially destructive and not prevented by the isolation transformer or any other device or practice except good wiring procedures. With the installation of the isolation transformer, all onboard bonding, DC grounds, and AC safety grounds remain unchanged, provided they previously met the ABYC standards mentioned at the beginning of this article.

The primary drawbacks of the isolation transformer are its size and weight. The average 30-amp unit may measure roughly a foot square and weigh 60 pounds. Although such a unit is not impossible to accommodate, all space aboard cruising boats is precious. Additionally, an isolation transformer has to be properly ventilated.

When shopping for a unit, the primary prerequisites are a marine UL listing (most isolation transformers are UL listed, but not all carry the “marine” prefix), full adherence to ABYC’s standards for isolation transformers, and a shield between the primary and secondary windings that is able to carry the full current rating of the unit in the event of a short circuit. The final requirement is that there must be no connection between the isolation transformer’s windings and shoreside ground. Beyond that, there are several case and shield grounding configurations and options. Some units are even capable of boosting low dockside voltage.

One point worthy of mention: Isolation transformers and polarization transformers are not the same thing. The latter only ensure correct onboard polarity in the event of a dockside fault. They will do little if anything to prevent corrosion, and the return path of the current to shoreside ground remains unchanged. If you have a polarization transformer, you are protected only from reverse-polarity scenarios—dangerous though they are—not from shore-induced corrosion or the possibility that you will electrocute a swimmer.

The isolator and transformer are not mutually exclusive systems. Some boats use both—the galvanic isolator supplements the case ground of an isolation transformer—but this is a belt-and-suspenders approach. In most cases, the economical approach is to use the galvanic isolator, and the all-inclusive approach, which affords the greatest corrosion prevention and some protection against electrocution, is to use the isolation transformer. Given the choice, I’d opt for the latter, but it’s not practical for every boat because of its size, weight, and expense. At a minimum, every boat that uses shorepower should have a galvanic isolator.

Only a small number of isolation transformer manufacturers produce units appropriate for the recreational cruising vessel, specifically, single-phase 120/240VAC, 30/50 amp service. Whichever product you may use, ensure that the installation instructions are followed to the letter. (If the isolation transformer is not installed property, you won’t get the benefits of this system and will have wasted your money.) Unless you are trained and experienced, AC shorepower wiring should be left to the pros—and preferably to an ABYC-certified marine electrician.