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Checking and changing your galvanic isolator

Galvanic Guardian


1 I removed the old galvanic isolator and discovered it was not working.

Now and again, I serendipitously stumble across an onboard problem well before it snowballs into something expensive. For example, I was recently talking with my industrial-electrician brother and the subject of multimeters came up. “You should have one on your boat at all times,” Mike said, “and don’t go chintzy—spend at least $70 and get something decent.” Of course, I took this last bit of advice with a grain of salt—I, like many, have been struggling with boat-related budgetary concerns of late—and purchased something slightly less expensive.

My new Ancor seven-function digital multimeter ($45 at West Marine) was an intriguing device nevertheless, and I simply had to give it a try as soon as I got back aboard Betty Jane. So, in accordance with a test procedure Mike outlined during a subsequent call I made from Betty’s saloon, I shut down my shore power dockside, disconnected the shore power cord from Betty’s transom-mounted power inlet for good measure and then using wire cutters, removed the el cheapo galvanic isolator (boat-related budgetary concerns again) I’d installed in the lazarette two years before. The plan was to give it a checkup while parenthetically checking out the new multimeter.


2 Installing a new Guest isolator was easy thanks to straightforward directions.

Galvanic isolators are humble but useful devices, by the way. A good one properly installed will virtually nix chances that stray D.C. current will sneak aboard through your shore power cord’s ground wire, thereby closing the circuit on the battery-like relationship your boat shares with other boats nearby and seriously damaging metal parts (through-hulls, props, etc.) via galvanic corrosion. A galvanic isolator does all this with diodes, which are electronic gizmos that stop marauding D.C. current from riding a shore power cord while maintaining the cord’s ability to return A.C. current ashore should a dangerous short circuit somehow occur onboard.

The test went quickly. After setting my new multimeter on “diode mode,” I measured the voltage across the el cheapo’s two terminals, first one way and then the other. Neither way showed an appropriate voltage of between 0.7 and 0.9 D.C. “Who’da thunk?” I marveled during my final call to you-know-who. “The isolator’s shot!”


3 The job entailed mounting the unit and splicing it into the green ground wire.

Installing a new one—a fail-safe model from Guest (Defender price, $253)—was pie-easy, though. With the defunct unit excised (see photos on the opposite page), I simply added its replacement, making sure that the studs for “Shore Ground” and “Boat Ground” on top were correctly spliced into the ground wire coming from the back of Betty’s shore power inlet fitting. I also had to add a mounting block of marine ply and secure it to the aft bulkhead in the lazarette—the heftiness of the new Guest unit seemed to warrant extra support. But, with a little help from a table saw, a tube of 3M 4200 adhesive, wire strippers, ring terminals, and a few screws and lock washers, I finished the job in four hours.

Fail-safe? From what the engineers at Guest tell me, the diodes in Betty’s new 50/60-amp galvanic isolator are so big and reliable that the status lights and panels you sometimes see on high-end isolators are simply unnecessary. Considering the five-year warranty that accompanies this excellent news, I’m convinced that any budgetary concerns arising from this project later on will hardly be worth a whiff of worry.

Guest (800) 307-6702.

This article originally appeared in the December 2010 issue of Power & Motoryacht magazine.