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Posted by on Sep 22, 2011 in Safety and Seamanship, UK, US |

Boat Wiring: Shorepower Mystery Box

The white box connected to the shorepower grounding wire is a galvanic insulator, Ed Sherman explains.

Question: I’ve begun cleaning out the lockers on my J/109 in preparation for winter lay-up. I noticed something the other day that was a bit troubling and I’m wondering if you can help. I’ve enclosed a picture of a box that I found connecting into the green wire on my shore power system, and I can’t imagine what it might be. I know the green wire is the safety ground for my boat’s electrical system, but this box is a mystery. What’s it for?

The “fail safe” galvanic insulator on this J/109 will keep the boat's anodes insulated from other boats in your marina.

The “fail safe” galvanic insulator on this J/109 will keep the boat's anodes insulated from other boats in the marina.

Answer: The white box in the photo is a “galvanic isolator”.  For any boat plugged into shore power on a regular basis, this device is considered essential by those of us who work in corrosion analysis.

Its purpose in life is to create just enough voltage drop between your boat and your dock mates to stop the flow of galvanic level current from one boat to another. Galvanic level current is defined as a low voltage typically less than 1.5 volts DC. The galvanic isolator allows the flow of any AC fault current that may occur on your boat, but it blocks the low level DC current that occurs naturally when dissimilar metals are connected together (via the green wire in a docked situation) and submerged in an electrolyte (the water the boats are floating in).

Galvanic isolators are the only device that standards groups allow to be spliced into the grounding wire on a boat. Even then there are very specific requirements; amperage ratings must match the shore power rating, and push-on friction connectors must never be used.

In your case, you have one of the new “fail safe” devices made by Professional Mariner. The fail safe rating ensures that no matter what, this unit will not fail in a way that opens the circuit in your boat’s grounding system, which would cause a very unsafe situation.

This piece of equipment will go a long way toward ensuring that the anodes on your boat are not sacrificing themselves to protect other boats at the dock.

Ed Sherman