As more and more boats plugged into shore-power at crowded marinas, the problem became more common. There must be a connection... Actually, there is-the AC green grounding wire. Whenever you plug your boat into shore-power at a marina, you connect your boat to every other boat plugged into that pier via the green wire. That means your boat's underwater metal hardware is at risk, as is your neighbors', to the corrosive effects of galvanic current as it passes between two or more boats linked by the green wire, like an invisible trespasser. Invisible, that is, until you notice your aluminum sterndrive or some other metal fitting is looking a bit funky.
But before you storm down to your marina demanding retribution, you should know that the marina wiring plays only an indirect role in a problem born of correct wiring practices. The solution is a galvanic isolator. If you spend a lot of time plugged into shore-power, a galvanic isolator may solve some of your excessive corrosion problems and even preserve good relations among your neighbors. There is only one caveat: You must regularly monitor whether the isolator is working properly or the likelihood of an electrical shock hazard increases significantly.
Galvanic corrosion occurs whenever two dissimilar metals are immersed in an electrolyte, such as salt water, and those two metals are also connected either by an external wire or direct contact. An electrochemical reaction causes the flow of ions in the electrolyte to attack and dissolve the less noble metal, the anode, while protecting the more noble metal, the cathode. The coupling of dissimilar metals in an electrolyte is called a "galvanic couple."
Note that there must be a complete circuit around the two metals for the electrochemical reaction to occur (see figure 1). On a boat, a bonding wire connecting the different underwater metal fittings usually provides one side of the circuit, and the salt water electrolyte outside the hull completes the other side of the circuit. Without the bonding wire, they are only two or more detached metals sitting passively in an electrolyte.
Galvanic activity normally restricted to one boat can extend to affect other boats via the AC green grounding wire when ever two or more boats are plugged into shore-power. This is because the boat ground (including the bonding wire) and the AC green wire are connected together at a "common ground point" (often the engine), completing a circuit between multiple boats. Typically, the galvanic current passes from one boat's underwater fittings through a bonding wire inside the boat, onto the common ground point, onto the AC green wire that passes out to the shorepower chord to the dock pedestal, onto another boat's pedestal, power chord, boat, common ground, bonding wire, fitting, and then back through the water to complete the circuit. The boat with the least noble metal among the different boats will always protect the others (see figure 2).
In effect, the electrical field of the galvanic couple is enlarged by the external circuit of the marina's AC green wire. But the basic principles of the galvanic couple still apply. Two boats berthed side-by-side with all bronze hardware will not affect each other; there must be an imbalance of dissimilar metals between them. A few scenarios of marina environments likely to produce galvanic corrosion via the AC green wire help illustrate the problem: -If a large yacht or sportfisherman with bronze and stainless steel underwater fittings (rudders, props, thru-hulls, trim tabs) berths near a powerboat with an aluminum sterndrive (or outboard engine), the sterndrive will severely corrode after the zinc has been attacked by the neighboring metals. When several boats with bronze underwater fittings are berthed around an aluminum sterndrive, the sterndrive will provide protection for all the other boats with inadequate zincs or no zincs at all. -If one boat is protected by a zinc, but a boat nearby has no zinc, the protected boat's zinc will corrode rapidly while protecting both boats. After the zinc has dissolved, the least noble metal on either boat will begin to corrode.
Factors such as the condition of the insulating paint on an aluminum sterndrive; water temperature, current, and salinity; condition of zincs; proximity of boats; the relative surface areas of the respective anodic and cathodic metals (called the area ratio effect), all have an influence on the rate of corrosion. For the majority of boats that remain plugged into the marina most of the season, however, time is on the side of the cathodic boat.
How far can the galvanic corrosion travel between boats? Depending on the factors mentioned above, the voltage produced by galvanic activity (normally less than 1.0 volt) can affect boats two or three slips away before the current begins to dissipate through the water. On the other hand, galvanic current can be extremely far reaching in the right circumstances, and other boats may not necessarily be the cause. Paul Fluery, a marine corrosion specialist in the Chesapeake Bay area, reported a case where a twin-engine powerboat berthed at a private residence was losing up to three zincs on each propeller shaft each month-with no other boats on the pier. After noting almost a 1.0 volt difference between the unplugged boat and the shore, he determined that the galvanic current traveled from the boat, all the way down the pier to the ground plate buried on shore, and returned through the water to the boat's zincs - a distance of several hundred feet. In this case the earth, or some unknown buried metal object, was cathodic to the boat's zinc anodes. A galvanic isolator solved the problem.
Galvanic isolators connect to the green wire, close to the shore-power inlet. Simply put, isolators block the circuit of galvanic current flow between neighboring boats. Isolators contain diodes, which are like valves whose inherent resistance blocks the low DC voltage generated by galvanic activity (up to 1.0 volt), but which conduct higher voltages exceeding the initial resistance. The idea is that your boat becomes isolated from passing or receiving low-voltage galvanic current between neighboring boats while still being able to pass dangerous AC voltage through the green wire to the shore ground.
The most efficient isolators have a capacitor, which allows low levels of only AC current to immediately bypass the diodes to the shore ground-an important feature, since stray AC current is more common than not. Considering that marina wiring is often less than ideal, and that as little as 5 milliamps of stray AC current can imperil a swimmer, it makes sense to immediately pass as much stray AC current through the isolator to shore ground as possible before the stray current seeks alternative parallel paths to ground through the water.
In addition, without a capacitor to immediately bleed-off low levels of stray AC current, stray AC voltage and galvanic DC voltages can together add up enough voltage to easily drive the diodes to conduct, leaving the boat unprotected without the owner knowing it.
Also note that isolators are rated in two amperes sizes: A boat with a 30 amperes shore-power cable needs an isolator rated to carry 30 amperes; a boat with a 50 amperes cable needs a 50 amperes isolator. If a boat has two shore-power inputs, it needs a separate isolator for each input, unless a single 50 amperes isolator is rated to carry two 30 amperes inputs by itself. Having the correct isolator rating is vital, as an underrated isolator will blow like a fuse when it is needed most to carry ground fault current to ground.
When installed and maintained properly, galvanic isolators are a practical solution to a specific corrosion problem that attaches itself to a primary AC grounding wire. Put another way, the purpose of the green wire-to protect people-is paramount. That means the status of the isolator must be routinely monitored to assure it can still conduct AC current to ground whenever necessary. For example, if the isolator diodes are burned out by a lightning strike, a common cause of failure, they will no longer conduct AC current off the boat through the green wire. That means the AC current will seek an alternative path to ground through the water, endangering swimmers, or through some grounded part of the dock, threatening people stepping on and off the boat.
Question: Have you ever had your galvanic isolator tested? Do you know if you even have an isolator? If you answered "no" to both, you're not alone. Because the American Boat and Yacht Council (ABYC) requires that no part of the AC grounding system bypass the isolator, which would make it ineffective, the isolator is usually connected to the green wire close to the shore-power inlet (read buried). It should be in a well ventilated place because it can become extremely hot-as in start-a-fire hot-when it is conducting. In fact, one simple test anyone can perform is to lay your hand on the isolator; if it's warm or hot, current is being conducted through the diodes and is not protecting the boat.
More to the point, there is obviously a significant AC leak somewhere, which needs immediate attention. If the isolator is installed in the areas containing gasoline machinery, fuel tanks or fuel line connections, it must be ignition protected.
Testing an isolator with a multimeter is beyond the capability or patience of most people, which means monitoring its status is even less likely, increasing the danger. Fortunately, there are a few other options. The best option is to invest in a status monitor that can be easily glanced at whenever your boat is plugged-in to shorepower. At least two companies offer monitoring devices that have easy-to-interpret LED displays: Guest offers an isolator (with a capacitor) with a built-in monitor and LED display (Model 2433P, available through BoatU.S). Separate LED indicators can also be wired to a remote location where people can see it, which is required by ABYC.
KCB Technologies offers an isolator (with capacitor) with a separate "shore-power status monitor" remote panel, which can also be connected to any isolator you might already have; the LED display panel can then be mounted where you are most likely to see it (call (905) 934-5628, or go to www.kcbtech.com).
As required by the ABYC, both monitors test for proper functioning of the diodes and capacitor, failure to block galvanic current, continuity of the green wire, and failure of the status monitoring system. In addition, the KCB shore-power status monitor verifies that the diodes are strong enough to carry a full ground fault (120 volts), checks for reverse polarity, low dock voltage, verifies the AC and DC ground systems are connected, and continuously monitors AC ground currents flowing in the green grounding wire and for drops in dock voltage.
At minimum, you should test for continuity of the AC green wire with a "three-prong receptacle tester," available at most hardware stores. The LED lights will indicate reverse polarity, but also whether the green wire is broken somewhere from the receptacle to the electric panel on shore. Keep in mind, however, that even one strand of wire will show continuity, but will fail to conduct a significant load of current. It helps to inspect the green wire where possible for damage and corrosion. Also be aware that three-prong testers are not fool-proof; salt water can have enough conductivity to pass AC current to ground and indicate a ground connection on the LED, yet that ground connection will be gone if the boat moves up river into fresh water. Such false readings can be avoided by first testing the dock receptacle with a three-prong tester, if you have an adapter for the shore-power plug. Given these limitations, the status monitoring devices are a good investment. Overall, the effects of galvanic corrosion are fairly easy to identify, but there may be instances that defy logic, such as when a more noble metal becomes severely corroded, leaving a less noble metal unharmed. This is likely the effects of DC "stray current" corrosion, which can drive normally cathodic metals to become anodic, which will be covered in a future article.
Suggested reading: For those wishing to learn more about testing galvanic isolators with a multimeter, Nigel Calder's Boatowner's Mechanical and Electrical Manual is an excellent resource, available through BoatU.S.
Taken from Seaworthy, BoatUS Marine Insurance Report, April 2001, "Damage Avoidance".
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