What is a galvanic isolator and why should my shore power system have one?

A galvanic isolator is a device used to block low voltage DC currents coming on board your boat on the shore power ground wire.  These currents could cause corrosion to your underwater metals; through hulls, propeller, shaft etc.

Boats in a marina plugged into shore power all act as a giant battery.  They are all connected together by the green shore power ground wire, which is (or should be) connected to their DC grounds, engine block, and bonded underwater metals.  If the boats are in salt water then that forms an electrolyte and the dissimilar metals connected together act as a battery, causing corrosion.

The galvanic isolator has two pairs of diodes set up so that a voltage of about 1.2 volts is required to cause them to conduct.  As most DC voltages caused by galvanic action will be less than this, they are blocked. Good quality isolators also contain a capacitor, which only conducts AC current, as a backup.

Normally no AC current is carried on the shore power ground wire, but it has to be able to carry the full load of the circuit in the event of a fault.  Therefore it is important to have a good quality unit that will not overheat when required to carry the rated load.  Some heat will be generated by the voltage drop and the unit must be able to withstand this.

As the galvanic isolator fulfills such a key function in the AC circuit it is only prudent to use the best quality unit available. 









7 Comments

Bill maas

Date 4/14/2019

Once the isolator is in place is there a gauge to let you know that it is working, and if no gauge how can you tell it is working?

Peter Kennedy

Date 4/15/2019

This is not a self testing isolator. There were a number of self testing models available for a number of years before the ABYC allowed failsafe ones to be used in lieu of self testing. The self testing ones were expensive and difficult to install and so were not very popular This isolator comes with detailed testing instructions that is suggests you do before installation and again annually.

Steve

Date 7/12/2019

Can a GI cause a battery charger powered from the AC distribution panel to malfunction?

Peter Kennedy

Date 7/13/2019

No, I don't see any way that could happen. The galvanic isolator is on the incoming ground wire from shore. No AC current flows in the ground wire unless there is a fault. The purpose of the galvanic isolator is to block small but unwanted low voltage DC currents.

Marilyn Pierce

Date 9/5/2019

New codes in effect for new docks being built require a GFCI to be installed at the main breaker, which trips everytime we plug in our shore power. Can the main hull bonding wire be disconnected on our boat and still operate normally?

Peter Kennedy

Date 9/6/2019

John Bierer

Date 9/23/2019

should you use a Galvanic isolator in fresh water? I have a houseboat on a lake that is hooked up to shore power and I have also installed solar when out one the lake.

Peter Kennedy

Date 9/24/2019

Yes. You still use zincs in fresh water so you can protect them with a galvanic isolator.

Hugh MacCallum

Date 11/11/2019

Is a GI required in a boat which was never electrically "bonded" ?

Peter Kennedy

Date 11/12/2019

It is an ABYC safety requirement to have the AC ground connected to the engine block. The simple explanation I tell my customers is that if you sit on the engine and reach over to touch the air conditioner you want the two of them to already be electrically connected so you are not the one making the circuit. If you choose to omit this to save a few bucks on a galvanic isolator it might not seem such a wise choice if someone gets electrocuted.

Vincent F Rakstis

Date 3/29/2020

Peter is wrong about saving zink's in fresh water. They will save themselves. Ion's will not flow in fresh water. It requires an electrolite. Water must have a salt content or a very high tannin (acid) content to have an electrolitic action. Capt. Vince Rakstis, Ret. MS

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