Isolation Transformer or Galvanic Isolator - which is preferable?

Posted by Peter Kennedy on 10/24/2014 to Corrosion
Isolation transformers and galvanic isolators represent two very different methods of achieving the same effect: isolating the boat from low voltage DC galvanic currents that can cause corrosion to underwater metals and excessive wasting of sacrificial anodes.

The galvanic isolator uses semi-conductors to isolate the boat from low voltage DC currents carried on board on the AC ground wire.  A galvanic isolator is a relatively small lightweight device that  contains diodes and a capacitor arranged in such as way that they will not allow currents to pass when the voltage is less than about 1.2 volts. As the currents that cause corrosion are almost always less than 1 volt the protection is achieved, while the diodes still allow higher voltage current to pass in the event of a fault. While not exactly cheap these galvanic isolators are less expensive than their alternative, the isolation transformer.

Isolation transformers are big, heavy and expensive.  They achieve isolation by providing an electromagnetic connection between the boat and the dock. There is no wire directly from the dock on board the boat, shore power goes to the primary windings of the transformer when it generates an electromagnetic field that is picked up by the secondary windings and serves the boat.  No actual wire connects the primary and secondary windings, it is all done my magnetism.  The shore ground wire stops at the transformer primary.  A new ground wire is made at the secondary windings.

So these two very different ways both achieve the same result.  How can we compare and contrast them and say which is the best?

Size weight and cost:  

Transformers are big heavy and expensive so on that basis they lose.


Reliability:  

Because galvanic isolators contain semiconductors they are more prone to damage from power surges and lighting strikes.  The transformer is more resilient to surges and lightning, it could still be damaged by a direct hit but is less likely to be damaged by a power surge due to lightning down the line.  Modern galvanic isolators are almost always of the "fail safe" kind.  This doesn't mean they can't fail, it just means that if they do they will fail in the closed position and will maintain the safety ground even if they do not maintain isolation.  If a transformer is damaged in a lightning strike it probably won't function at all.  So on the question of reliability the transformer wins.  

Other uses:  

Transformers have other uses, and galvanic isolators do not, so the "other uses" usually ends up being the deciding factor.  Transformers can easily accommodate boats that travel the world and need to plug into different voltages. Transformers can also be used to adjust or condition power, for example many docks in the US have 208 volt power supplies and transformers can be used to boost it to 240 volts for use on board.  There are other specialized applications for transformers that I wont go into here.

Summary:

In summary the decision to use an isolation transformer or a galvanic isolator boils down to availability of space, weight carrying ability, expense and whether the other uses of the transformer are considered desirable for the boat in question.

Additional information:


Feel free to add your comments below..... 


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6 Comments

Paul

Date 6/19/2018
Reply

"As the currents that cause corrosion are almost always less than 1 volt " That's an interesting measurement unit for current. -P-

Lou Reynolds

Date 6/20/2018
Reply

While I agree with everything you said in your description. You should point out that isolation transformers block both AC and DC stray currents. While it's a given that DC stray currents are the worst offender when it comes to corrosion, AC stray currents cause corrosion as well. But the main point of a isolation transformer is to prevent electrical shock hazards and the Galvanic Isolator is not designed for that purpose. The only thing a galvanic isolator does is block DC stray current.

Simonosteopath

Date 2/24/2019
Reply

"As the currents that cause corrosion are almost always less than 1 volt the protection is achieved, while the diodes still allow higher voltage current to pass in the event of a fault. " Shore earth and neutral are often several Volts difference and shore earth often many Volts difference to the water itself. Surely this presents a problem for a galvanic isolator?

Peter Kennedy

Date 2/25/2019
Reply

The shore ground and shore neutral are connected to each other at the power source (the transformer on the dock) They are not supposed to be at a different potential to each other but it is possible that if there are bad connections in the system or if you are at the end of a long dock on a hot day when all the air conditioners are running there will be a difference between them due to voltage drop in the neutral. You are correct that when this happens the galvanic isolator will conduct for part of each cycle.

Ann-Marie Foster

Date 12/6/2019
Reply

Metal boats, aluminum in particular, have a built in voltage to salt water as high as 0.9 volts. This leaves only 0.3 volts protection remaining. The ground connection in docks can be 10 times this amount, typically due to mis-wired boats using the ground connection instead of the neutral. AC leakage can easily exceed the traditional protection this so on each 1/2 cycle DC will be passed through the isolator and cause electrolysis. You should use a 2.5 volt isolator. Search Google for "2.5 volt galvanic isolator".

Peter Kennedy

Date 12/6/2019

That's a very interesting comment, thank you for bringing that up. Most aluminum boats I have come across have had an isolation transformer. I took a look at some of the Galvanic Isolators on offer for aluminum boats and it seems that none met the ABYC requirements of either being 1) Self Testing or 2) Fail Safe

Kevin McNeill

Date 4/4/2021
Reply

I'm in the process of rewiring my boat starting with the AC system. I'm going to use the Nigel Calder suggestion for isolating the systems using the shore power connection to only the battery charger and using an inverter to power the AC side. Should I install an ELCI between the battery charger and the shore power input and if so before or after the 30 A main breaker? Secondly, should the onboard AC ground be connected to the shore power ground and will doing so negate the isolation feature? I've just added your blog to my DIY section under good ideas, thanks.

Peter Kennedy

Date 4/5/2021

The ABYC requires you to connect your AC Ground to the engine block. The simple explanation is that if you sit on the engine and reach over and touch the air conditioner you want the two to already be connected so you are not the one completing the circuit. As a result of this connection your engine and propellor ends up being connected to everyone else in the marina's engine and propellor so you are all one giant battery floating in conductive salt water. That's why you need either a galvanic isolator or an isolation transformer. Galvanic currents are DC Currents that arise because of different metals being immersed in an electrolyte. The purpose of the ELCI main breaker is different. As well as being a main breaker it is also checking for fault current. This is AC Current that can electrocute a swimmer in the water. The level of fault trip is set at 30 mA. The ELCI part and the breaker part are combined in one unit and this needs to be within 10 feet of where shore power comes on board the boat.

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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.

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