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

Posted by Peter Kennedy on 1/21/2026 to Shore Power

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 a Failsafe model. To comply with ABYC standards the galvanic isolator must be FAILSAFE which means that in the event it fails it will continue to conduct, which is the primary purpose of the safety ground. In the event of failure the isolation feature is lost but the safety feature is retained. Isolators should be tested annually and at other times as needed. (Failure might occur in the event of a lightning strike.)

ProMariner 22034 ProSafe FAILSAFE 30amp Galvanic Isolator

Installation is super easy and just involves interrupting the green wire of the shore power connection to insert the isolator. This needs to happen before the green wire connects to anything else so it normally gets installed right next to the shore power inlet, or before the first circuit breaker.

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?

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?

Date 9/6/2019

The AC ground is an essential safety component of your electrical system and disconnecting it could lead to injury or death for which you would be found liable, so the answer to your question is no.

You might like to read my Blog Post How to deal with a boat that is tripping the dock Ground Fault circuit breaker

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.

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" ?

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

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

7 Comments

Bill maas

Peter Kennedy

Steve

Peter Kennedy

Marilyn Pierce

Peter Kennedy

John Bierer

Peter Kennedy

Hugh MacCallum

Peter Kennedy

Vincent F Rakstis

Add Comment

Categories

Recent Posts

Archives