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Old 12-30-2012
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Testing Your Galvanic Isolator

A 30A Galvanic Isolator
Ok so just what is a Galvanic Isolator (GI), why do I need one or have one & why on Earth would I need to test one?

Why do I need one? A GI is a device that is inserted, in series, into the green grounding wire (safety ground) of your shore power feed to help minimize galvanic current from flowing into your vessel. While blocking stray DC galvanic current it also has to allow for the passage of AC fault current. GI's are a simple hook up and installation. If plugging in at a marina a GI is the bare minimum level of protection a boater should have. I do prefer an isolation transformer (IT) to a GI but the conversation of a galvanic isolator vs. isolation transformer is a whole other discussion & topic.

To install a GI all one needs to do is to break the green wire after the shore power inlet but before the AC panel. Cut the green wire in two, crimp on two terminals and connect them to each stud. Very simple. The purpose of a GI is to block low voltage stray DC current while still allowing any fault current to pass through the green wire to ground and allow it to activate fault protection devices..

This blockage of low voltage stray DC current is achieved by using two diodes in each direction. Each diode drops approximately .6V or requires more than .6V to open and "Flow". Two of them in series results in approximately a 1.0V - 1.2V threshold for blocking stray DC current. They normally have two diodes in each direction so the green wire is not "check valved" and acts just like a wire would. The only difference is it acts as one that won't pass current at voltages below 1.0V - 1.2V. Simple and pretty effective at blocking stray DC current.

What is a diode? Think of a diode as an electrical check valve. It allows current to flow in only one direction but not in the other direction. One of the inherent traits of diodes is the voltage drop associated with them, which is usually around 0.6V. In a galvanic isolator application however they have used this often assumed bad trait of a diode to an advantage. By wiring two diodes in series you now have a device that can block any stray current below 1.0V - 1.2V from flowing into or out of your vessel.. This essentially blocks any DC currents caused by dissimilar metals being connected in the marina when you "plug in".
A 30A Galvanic Isolator



A Simple Device
As I said above these are simple devices. When opened up you can see just how simple. Keep in mind that this particular model no longer meets the current ABYC safety standards and I will get to that in a moment. On the back wall we can see the two wires going to the gold plated studs. This is where your green grounding wire would be cut and connected to. Inside we can see the two diodes which allow current to flow in both directions but at the same time blocking voltages below 1.0 - 1.2V.

So why do I need to test my GI? Remember in the last paragraph when I mentioned that this particular GI no longer meets the current ABYC safety standards? Well,there is a very good reason for that.

Think back to when I described where a GI gets installed? It goes in the GREEN SAFETY GROUND WIRE! So now what happens if one or both of those diodes get blown? NO SAFETY GROUND....! There were far to many incidences of boats with GI's that completely lost their safety ground connection to shore due to failed diodes. A single lightning strike on a boat a few docks away could take out half the GI's in the whole marina and folks rarely even noticed.. Couple this with the fact that many more boats are not wired, on-board, to current safety standards where the AC green/grounding wire is bonded to the ships DC grounding bus and this means metallic parts of AC devices could become "hot" in the event of a fault. Hot cases and metallic parts of your boat spell the potential for electrocution or death to swimmers by electric shock drowning. The problem is that without the safety ground the fault protection devices will not trip as they should. This = BAD !

Below is a direct quote from Captain David Rifkin. He is perhaps the leading expert in marine electrical shock drowning deaths, bonding and corrosion.

"In the many boats I have tested with galvanic isolators, approximately 5% tested open circuited (meaning the boat did not have a connection to the marina grounding system) and the operators were completely unaware."

So put another way 5% of the boats with GI's have an open circuited ground! This may not sound like much but consider that with the millions of boats in the water there are potentially hundreds of thousands of GI's installed on boats. How many boats in your marina? 100? So if 5 of those boats are completely lacking a green safety ground wire do you really want to go swimming in your marina or touching metal objects???

So just what does meet the current standards for GI's? The ABYC standards for galvanic isolators have changed a few times. First it required "active monitoring" and remote idiot lights so an owner would know they had a working GI and still had their safety ground intact. This "active monitoring" added significantly to the cost of GI's and was a total PITA in terms of parasitic loads because it had to be "wired in" to more than just the green wire.

A number of years ago a company called Dairyland Electrical Industries, or DEI for short, invented/brought the "fail safe" galvanic isolator to the marine market. This advancement brought the simplicity back to the GI, did away with the idiot lights and related circuitry, but added a major new safety feature, "fail safe".

Fail safe means that these devices fail closed instead of open as a normal diode would. By failing closed you only lose galvanic protection but not your SAFETY GROUND wire....! According to Henry T., one of the owners of DEI, they have not had a single failure of a DEI Fail Safe galvanic isolator where it failed "open", even to lightning strikes. A pretty impressive feat considering the number of failures that occured with the older technology, like the one you see here.

Today companies such as Guest, ProMariner & DEI all make ABYC compliant galvanic isolators with the fail safe technology. Yandina also makes a GI but it does not meet the current ABYC safety standards. I do mention them though because they are at least 100% honest that the product does not technically meet the standards. Kudos to Yandina for being honest in a world of BS marketing hype.. If you are a mooring sailor who rarely stays at a dock the Yandina GI can be a good value. If adding a GI, for regular dock side use, I would advise going with a "fail safe" product so you don't lose your safety ground wire.
A Simple Device



How To Test
If you have an older non-fail safe isolator how do you test it? Perhaps the easiest method is to use a DVM like the one pictured set to the diode test function. On this particular meter the diode test shares a dial position with the Ω feature. Here it is set to test Ω.
How To Test



Switching To Diode Test
With this meter pressing the yellow button moves it from the Ω test to the diode test feature. The diode symbol can be seen on the left hand side of the meters screen.
Switching To Diode Test




Connect Your Test Leads
First UNPLUG YOUR VESSEL FROM AC POWER !! With the AC power to the vessel OFF disconnect both green wires from the GI and connect your DVM leads to the studs.

If the diodes are working you should see a reading of .7 to 1.0. here the reading is .973..

If the diodes are bad you will not get a reading or will get the OL message (open leads) if using a Fluke. Older style galvanic isolators like this usually fail "open"...
Connect Your Test Leads




Now Reverse The Leads
Remember the current needs to flow in both directions and it is possible for one side to be blown and the other side still be fine.

Here the reading is .968 and well within range. You are allowed roughly a 10% variance from one side to the other so .968 and .973 are well within that spec at a 0.5% variance.

Despite being a relic, this isolator is still operable. If however you have an old isolator like this, and you keep your boat at a marina, I would strongly advise upgrading it to newer ABYC compliant unit. The other alternative is to regularly test your GI to ensure its operation.

My personal preference for GI's are the DEI isolators. I feel they are the most robustly built GI's on the market and you don't pay any more for them than you would another "fail safe" brand.. Again DEI has had ZERO GI failures that were not failed "safe" and this includes lighting strikes...
Now Reverse The Leads


You Don't Need A Fancy Meter

I love my Fluke equipment, and you'd need to pry it from my dead fingers, but for work like this even an inexpensive meter will do, provided it has a diode test function, and many "cheapies" do.

In this photo each meter has been set to the diode test function.
You Don't Need A Fancy Meter



The Diode Symbol
The dial of this meter is pointing to the diode symbol. If a meter has the diode symbol it offers the diode test function.
The Diode Symbol




Set To Diode Test
On this meter the diode test function shares a dial position with the "audible" continuity test. This meter was about $20.00 at Wal*Mart. I bought it one day when I forgot my meter bag.....
Set To Diode Test



Diode Test
On the Fluke 179 you simply toggle between the diode test and Ω with the yellow button.
Diode Test



Alternative Test
While I prefer to use the diode test functionality of a DVM you can do a quick and dirty test with a 9V battery and a 12V light bulb. I grabbed an LED bulb but I won't guarantee all LED bulbs will fire at sub 9V. You will get a dimly lit 12V bulb with a 9V battery but the test works. I only used the LED because it showed up better with the picture.

If you first connect your bulb direct to the 9V battery then test it through the isolator you will notice a change in brightness. This is caused by the voltage drop across the diodes and is 100% normal and should be expected..

Wire the 9V battery and light bulb as shown and the bulb should light if the diodes are good..
Alternative Test



Reverse The Leads
Again, just like with the diode test function on the DVM you'll need to reverse the leads to check the flow in the opposite direction. The bulb should light.

Galvanic isolators are in the path of the safety ground for your vessel so it is critical that you know your GI is working. Newer GI's have either idiot lights that tell you it is on or are fail safe so there is much less need to worry about losing the safety ground.

This test is for older style GI's like the one pictured. There are still perhaps hundreds of thousands of boats with this style GI installed on them...
Reverse The Leads



Blocking Current @ 1.1V
This is a test most won't be able to conduct unless you have an adjustable bench top DC power supply or a 1.5V battery and some diodes to drop the voltage below 1.1V.

In this photo I have set the power supply to 1.1V DC and am measuring the current with the Fluke meter. As you can see, based on the Fluke meter, the diodes in the Galvanic Isolator are doing what they are intended to do, block DC current when the voltage is below 1.1V - 1.2V.. The Fluke reads 0.00A DC and this is how it should perform.
Blocking Current @ 1.1V



Current Flowing @ 1.3V
In this final image I turned up the DC voltage to 1.3V and we can now see the diodes beginning to allow current to flow. The Fluke is measuring 0.03A or three hundredths of an amp at 1.3V across the diodes. Galvanic voltages from connected dissimilar metals will rarely if ever approach 1.0V thus stray DC current is blocked.

This GI is doing what it should and will block dangerous DC galvanic current and allow AC fault current to flow when needed..
Current Flowing @ 1.3V


Hope this helps..
jrd22, gtod25, sailordave and 6 others like this.
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Last edited by Maine Sail; 01-05-2013 at 10:23 AM.
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Old 12-30-2012
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Re: Testing Your Galvanic Isolator

Why not use a DVM to verify that a GI is doing it's job. You could simply look for "+" or "-" ˝ VDC across the two connectors on the GI...

As long as there is a potential difference, the GI is working.

Thoughts?
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Re: Testing Your Galvanic Isolator

Quote:
Originally Posted by eherlihy View Post
Why not use a DVM to verify that a GI is doing it's job. You could simply look for "+" or "-" ˝ VDC across the two connectors on the GI...

As long as there is a potential difference, the GI is working.

Thoughts?
Because there may not always be a voltage present and this can trick your readings and could be inclusive. It can be a good quick test to see if galvanic voltages are present though. It won't however tell you if the GI is blocking or working in both directions.. The GI only blocks these stray currents if there are stray currents to block, no current/voltage no reading. These currents and voltages are not a "given" and in an ideal world there should be zero voltage / current on the green wire...

The diode test applies a known quantity so you can test it reliably and compare side to side to make sure all four diodes are "balanced". You really need to test it both ways and measuring voltage won't tell you if it is working in both directions only that there is voltage across the terminals or green wire. You could disconnect the GI and test across the green wires and get the same reading without the GI even in the loop because you are really testing the voltage on the green wire not the functioning of the GI...

In short voltage across the terminals/green wire does not tell you whether or not the GI is working only that there is voltage on the green wire..
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Old 12-31-2012
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Re: Testing Your Galvanic Isolator

I know my GI doesn't work. It's still in the box!

I bought it a few years ago, when at a marina that had serious stray current issues. They kept swearing they didn't, but zincs would disappear like ice under rock salt. I ended up changing marinas right after buying the isolator and haven't gotten around to installing it. This past season, I never changed the zincs all season and there was approx 25% remaining after 8 months in the water. Pushed a bit too far, but still.

Any home made simple test for stay current itself?
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Re: Testing Your Galvanic Isolator

Part of the reason that I disconnect the shorepower at the dockbox whenever I leave the boat is to assure that I will not be connected to a "hot" marina.

A simple test would be to connect to the dockbox, and shut off the shore power at the dock box, and onboard. Then disconnect the main green (ground) wire after the shorepower connector. Starting at 100VAC, and working your way down to 1VAC, use a DVM to check for voltage between the green wire, and the connector that you just removed the wire from. Take all necessary precautions to NOT electrocute yourself.

The problem with this test is that the issue at the marina could originate with a single vessel that is leaking current. You would have to catch the problem when the offending vessel is connected to the marina, and near enough your boat to have an effect on your boat.

Much better to install the GI, and unless you need to be connected to shorepower, unplug from the dockbox whenever you are away.
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Re: Testing Your Galvanic Isolator

As always, excellent article M.S. !
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Re: Testing Your Galvanic Isolator

Quote:
Originally Posted by eherlihy View Post
Part of the reason that I disconnect the shorepower at the dockbox whenever I leave the boat is to assure that I will not be connected to a "hot" marina.

A simple test would be to connect to the dockbox, and shut off the shore power at the dock box, and onboard. Then disconnect the main green (ground) wire after the shorepower connector. Starting at 100VAC, and working your way down to 1VAC, use a DVM to check for voltage between the green wire, and the connector that you just removed the wire from. Take all necessary precautions to NOT electrocute yourself.

The problem with this test is that the issue at the marina could originate with a single vessel that is leaking current. You would have to catch the problem when the offending vessel is connected to the marina, and near enough your boat to have an effect on your boat.

Much better to install the GI, and unless you need to be connected to shorepower, unplug from the dockbox whenever you are away.
We should keep in mind that "galvanic isolators" are only intended to block the voltages created by connecting two or more dissimilar underwater metals in an electrolyte. Without a GI you've just connected your underwater metals to everyone else in the marina thus creating a "galvanic cell". With this galvanic cell comes the anode/cathode relationship between the dissimilar metals now all connected by "plugging in"... This can result in your zincs being used to protect the rest of the marina, or your metals becoming the anodic ones....

Galvanic corrosion is perhaps the most common corrosion in marinas but certainly not the only dangerous corrosion that can occur. DC voltage leaks caused by improper wiring, non-marine battery chargers etc. etc. can all result in damaging DC currents that exceed the blocking ability of a GI. GI's are only for limiting current created by connecting underwater dissimilar metals. They do nothing to prevent corrosion from live DC leaks such as a poorly sealed & energized bilge pump wire sitting in bilge water...

These dissimilar metals galvanic voltages can approach 1V but rarely go over that, thus the GI protects. GI's will not block DC or AC current when voltages go over 1.2V. This is why an isolation transformer is a better device for truly isolating your boat...
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Re: Testing Your Galvanic Isolator

Quote:
Originally Posted by Maine Sail View Post
.....GI's are only for limiting current created by connecting underwater metals. They do nothing to prevent corrosion from live DC leaks...

These dissimilar metals galvanic voltages can approach 1V but rarely go over that, thus the GI protects. GI's will not block DC or AC current when voltages go over 1.2V. This is why an isolation transformer is a better device for truly isolating your boat...
Have you written anything on isolation transformers? Since I haven't installed the GI yet, perhaps I should change tracks, or does one need both?

As mentioned, my current marina doesn't seem to be a problem, but you never know what the future holds. In my last, I not only lost zincs like melting snow, there were plumes in the bottom paint surrounding my grounding plates and anything metal. Suspect it was more than a galvanic problem. Still, the marina denied everything. Went away when I moved.
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Re: Testing Your Galvanic Isolator

Quote:
Originally Posted by Minnewaska View Post
Have you written anything on isolation transformers? Since I haven't installed the GI yet, perhaps I should change tracks, or does one need both?

As mentioned, my current marina doesn't seem to be a problem, but you never know what the future holds. In my last, I not only lost zincs like melting snow, there were plumes in the bottom paint surrounding my grounding plates and anything metal. Suspect it was more than a galvanic problem. Still, the marina denied everything. Went away when I moved.
I've not written much on IT's as getting people to even invest $270.00 on a GI is next to impossible. Introduce a four figure device that hums and even less will bite.. I should do a write up though...

As for your zinc and haloing, this is usually indicative of "over protection" or too much zinc. You can still lose zinc at a rapid rate and be over protected at the same time. A fiberglass boat should be somewhere between 550 to 1100 mV DC using a Ag/AgCl reference cell..

If you have an aluminum leg sail drive then you want to be between -850mV and -1050mV anything less than -850mV and you are under protected for the aluminum and anything over -1050mV and you are over protected. Without a Ag/AgCl reference cell installing zincs is a crap shoot. I know our boat takes two streamlined shaft zincs but I have many customers boats who are over protected with just one or under protected with two etc.. It all amounts to how much underwater metal you are protecting as to how much anode protection you need.

If you are over -1100mV DC you have too much zinc. You are over protected and can see haloing or galvanic burning of the copper bottom paint around thru-hulls etc... Other things that cause haloing are improper wiring or the use of "bonding" circuits as return paths for 12V DC appliances. Non marine battery chargers can also cause some serious "burning/haloing" as they are poorly "isolated"......

People always assume that with zincs more or bigger is better and it is not...
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Re: Testing Your Galvanic Isolator

Fascinating, MS. Thanks. In fact, I only use one zinc on the shaft, but admit to adding two occasionally. That practice just ended. Isn't it also curious that the haloing stopped when I changed marinas? Your explanation sounded more like my ship would be the likely suspect.

I look forward to reading about the IT, although, humming would drive me nuts.
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