When you plug in the yellow cord, do you really know what you’re hooking into? Dockside alternating current can suffer from fluctuating voltage, power spikes, poor grounding, and other maladies. Even worse, connecting to shore power also opens the door to increased galvanic corrosion caused by neighboring yachts.
Along with the current-carrying wires in the AC circuitry is a green wire; in case of a short or other malfunction, the green wire provides a low-resistance path to ground. Without the green wire, that path could be provided by somebody touching an appliance or tool affected by the short circuit. (Ouch!) If the green wire becomes energized, the Ground Fault Interrupter (GFI) aboard the yacht or at dockside opens instantly and stops the current flow. (If you don’t have GFIs protecting each AC circuit aboard your boat, stop reading now and install them. Not every marina has GFIs.)
But the green wire can cause problems, too. Both the U.S. Coast Guard and the American Boat and Yacht Council (ABYC) recommend connecting it to the boat’s direct current ground. This protects against short circuits between the AC and DC systems – if the grounds were not connected, a short could energize the DC wiring at AC voltage without tripping the GFI, or even pump 120 volts into bonded metal parts. (Past practice was to keep the AC and DC circuits isolated from each other, and some boats are still wired this way.) If there’s a genset or inverter, it must be grounded to provide “green wire” protection when the boat is away from shore power.
Typically, the ground connection for both electrical systems is on the engine or transmission, connecting to “earth” (in our case, sea) through the propshaft. But back at the marina, when the yellow cord is plugged in, the green wire on Vessel A is connected to both dockside ground, through the cord, and vessel ground, through the shaft. Vessel B in the next slip, also plugged in, is connected to Vessel A through both the common dockside ground and its own ship’s ground, using the sea water as a conductor. If Vessel B has weird electrical stuff going on, it can cause rapid galvanic corrosion of Vessel A’s zinc anodes. Since the same connection exists between Vessel A and plugged-in Vessels C, D, E, and F through Z, the chance for galvanic disaster increases exponentially.
What to do? Galvanic corrosion is caused by extremely low-voltage current flowing between underwater metal that’s connected above water – in this case, by the green wires. Adding a galvanic isolator – essentially a resistor – in the green-wire circuit blocks low-voltage current, opens the galvanic circuit, and prevents corrosion; if line voltage is applied, the isolator closes the circuit and the GFI or breaker trips. Galvanic isolators are relatively inexpensive ($400 to $500) and easy to install; be sure to choose one approved by ABYC.
A better solution is an isolation transformer ($500 to $5,000+, depending on capacity and sophistication). An isolation transformer connects the vessel to shore power magnetically rather than electrically; since there’s no connection, there’s no corrosion! (Transformers use alternating current flowing through one set of coils to induce current to flow in adjacent coils.) Some isolation transformers have an electronic brain that compensates for fluctuating dockside voltage by stepping-up output – this is ideal for yachts visiting foreign ports where power is dubious, and/or for those with lots of sophisticated electronics demanding precise power inputs.
If your boat spends much of its life plugged in, you need to isolate it electrically from its neighbors. Either a galvanic isolator or isolation transformer will do the trick – and think of the money you’ll save on zincs.
Courtesy of our friends at www.boattest.com