Your boat needs galvanic isolation! part 2
4/12/2021

Most boats experience corrosion of some sort, some a lot more than others. Recognizing the need for professional marine techs to better understand what causes corrosion and how to prevent it, boat electrics guru Ed Sherman wrote an article for Marine Electronics Journal focusing on one pesky type of corrosion, galvanic corrosion.

Ed says the best solution for smaller boats is a device called a galvanic isolator and explains how it works and why proper installation is essential. Our goal in presenting the information (which is a bit techy in spots) is not to turn you into an expert but to give some background to help you understand the problem and what can be done about it when talking to a pro.

Last week in part 1, Ed talked about corrosion basics, shore power and isolation transformers.

By Ed Sherman


The galvanic isolator

Enter the galvanic isolator, which is a simple electronic device that utilizes some diodes and a capacitor to effectively block galvanic level currents. Therein lies the only major weakness with these devices, in my view. Galvanic level voltages are quite low, less than approximately 1.5 VDC. So, if for example there is a battery level voltage leak into the boat’s grounding system, the galvanic isolator will only block about 1.5 volts of it. Although rare, this can and does occasionally happen.

The primary cause is a boat owner that uses their bonding system as a DC negative conductor. The appliance will work fine, but every time the device is running the bonding system, which is ultimately connected to the boat’s grounding system, will be conducting 12 volts plus back to the DC power source. Some of that voltage and current will also attempt to go thought the galvanic isolator (all but 1.5 volts) in search of a pathway back to its source. Symptomatically, this will show up as extreme and rapidly occurring corrosion.

Of course, the other major concern over galvanic isolators is the fact that it is installed in series with the all-important safety wire, the green (or green with a yellow stripe) grounding conductor on board the boat. Any failure of the isolator that causes an open circuit will eliminate the ground fault (read shock hazard) protection on the boat. It was due to this worry among ABYC (American Boat & Yacht Council) electrical committee members that led to acceptance of a new generation of isolators with a rating of "failsafe,” which essentially means it cannot fail in an open circuit mode. It may fail to block galvanic level currents, but it will not fail in an open-circuit mode. Things known to cause failure are lightning strikes that may enter any dock wiring network.

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