Bird on a Wire Page 2
The most serious problem with marina shore-power systems is grounding. Consider how modern buildings have ground rods driven into the earth and hard-wired to both the neutral and backup grounding lines running out to all the outlets, thus giving any short in the system two ways to flow back and trip a circuit breaker. Especially dangerous locations like bathrooms are further protected by active ground fault circuit interrupters (GFCIs). That’s why shore-side electrical fires and shock accidents are largely a thing of the past. By contrast, a marina’s ground is way off by a main panel somewhere, and those big yellow wires running to the docked boats are more often like a nasty tangle of Christmastime extension cords than a well-wired building. Junction boxes and outlets corrode, and if either path to ground isn’t up to snuff, a short or fault may fail to push enough juice back up the line to trip a breaker.
Instead the fault area will heat up, possibly causing a fire or, worse, the current flowing into the boat may seek another route back home, i.e. out through the boat’s D.C. ground (to which the A.C. ground is properly bonded) and through the adjacent water toward land. It doesn’t take much of a leak like this, especially in less-conductive fresh water, to seriously impair a human being in that water. Hence the tragic stories I heard from Budd and read about later in ABYC publications and the reason for the “No Swimming” signs seen in many marinas.
Now, this shore power issue is much more complex than I can go into here—and be assured that the ABYC and others are concerned and working on fixes (super-size GFCIs are a possibility)—but at the moment, the safest move appears to be the isolation provided by a transformer. You’re just transforming 120 A.C. into 120 A.C. but in the process the shore ground terminates to a shield between the transformer’s windings, separated from the yacht’s A.C. ground. “It makes a boat into a bird on a wire,” explains Budd, referring to the electrical isolation that lets a flock happily line up on a high-voltage wire.
There are further advantages to being isolated from a marina and all the other boats in it. One is that your A.C. will be cleaner, unaffected by, say, a neighbor’s electrically noisy battery charger, which might otherwise be spreading interference through your circuits, too (i.e. buzz in the VHF). An isolation transformer also prevents galvanic action between your vessel and others, eliminating the need for a dedicated galvanic isolator. It’s also possible to design these transformers so that the shore power voltage is low—a fairly common problem resulting in dim lights, overheated motors, and slow stoves—they can give it a boost, though Ohm’s Law dictates a commensurate cut in available amperage.
And I learned that there are differences between isolation transformers. Charles’ are unique for having a full current-carrying shield, which means they can and have taken a lightning hit without adverse affect on the boat’s electrical system. Budd and St. Romain proudly described seeing proof positive of the shield’s effectiveness, which meant using a Jaws of Life tool to slice into a zapped transformer—necessary because they are also fully encapsulated in a solid block of sand and varnish. Talk about heavy duty.
I’m still bullish about what microelectronics can do for all those power components down in an engine room. I picture a graphic helm screen like the one on Prius hybrids able to show in real time just where the juice is flowing and whether each element in the system is okay. But Lake Pontchartrain was where I learned a new respect for certain boxes that don’t appear to be doing a darn thing—and to never, ever swim in a marina.
This article originally appeared in the April 2007 issue of Power & Motoryacht magazine.