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Peter Kennedy has been in  business since 1991 designing, installing and servicing marine electrical systems. The purpose of this blog is to offer support to both professional installers and  do-it-yourself boat owners who wish to undertake this work themselves.

Peter Kennedy has been in  business since 1991 designing, installing and servicing marine electrical systems. The purpose of this blog is to offer support to both professional installers and  do-it-yourself boat owners who wish to undertake this work themselves.

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

Posted by Liam Kennedy on 1/21/2019 to Corrosion
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. 

Comments

3 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
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.

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