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Old 03-18-2016, 02:53 PM   #1
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Galvanic Isolator

Zincs on my boat are degrading faster than I anticipated. I am relatively certain there are stray currents in my salt water marina.

I understand the whole subject can be complicated, so I am limiting this initial inquiry to one single element of the overall puzzle:

If my boat is the one leaking AC (or DC) current to the water, will installing a galvanic isolator on my own boat be of value?
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Old 03-18-2016, 03:04 PM   #2
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Short answer, no.


A galvanic isolator shunts any DC to ground that is present on the incoming shore power cable. If the problem is inside your boat, a galvanic isolator will have no effect. Come to think of it, it will block your DC from getting back to the marina shore power system and affecting someone else's boat ;-).


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Old 03-18-2016, 03:18 PM   #3
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We had a corrosion analysis performed by an ABYC Certified Technician having the same worries a few years ago. I worked with the tech and it took one hour on site. He looked at the AC, DC, neutrals, grounds, etc. on Hobo and on shore. His primary tool was a portable silver/silver-chloride electrode. All was good and it helped with my piece of mind.
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Old 03-18-2016, 04:21 PM   #4
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I have a clamp on digital meter which measures both AC and DC current.
Not as sensitive as this Fluke meter, which here describes testing for current leaking.
Leakage current measurement basics

Another test is use a GFCI which are rated at 5 milliamp, meaning if the current exceeds a 5 milliamp leak, it will trip.

I have all my outlets on GFCI, and I have an extension cord with GFCI. When I run the entire boat on that GFCI extension cord, it does not trip. But also the devices your testing need to be switched on.

So is 5 milliamp low enough leakage test for AC ?
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Old 03-18-2016, 04:30 PM   #5
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Pretty sure a galvanic isolator is really nothing more than a big diode that prevents stray current from your shore power ground from coming aboard. The could be more complicated electronically, but that's what they do.

So while it may help, it will not affect stray currents already in the boat.

Galvanic Isolator Explained
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Old 03-18-2016, 04:44 PM   #6
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Quote:
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Zincs on my boat are degrading faster than I anticipated.
How long did you anticipate? How much underwater metal are they protecting? With no GI you are probably protecting your neighbors boats with your anodes.

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If my boat is the one leaking AC (or DC) current to the water, will installing a galvanic isolator on my own boat be of value?
Because our boats AC grounding wire (green) is supposed to be bonded to the DC grounding system, for human safety, this means that when you plug into shore power your underwater metals are connected to your neighbors underwater metals via the green AC grounding wire.

A galvanic isolator is a device that is inserted, in series, into the green grounding wire (safety ground) of your shore power feed to help minimize or reduce the effects of galvanic current from flowing between your vessel & your neighbors. While blocking galvanic level current it also has to allow for the passage of AC fault current. This is why any galvanic isolator should be of the "fail safe" type or have idiot lights that tell you you still have an AC grounding connection.

This blockage of low voltage galvanic current is achieved by using two diodes in-series 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 DC galvanic level voltage & current.

GI's normally have two diodes in each direction so the AC green wire is not "check valved" and acts just like a wire normally would. The only difference is it acts as one that won't pass voltages below 1.0V - 1.2V. Simple and pretty effective at blocking galvanic level current. GI's do not however block stray current that exceeds 1.2V...

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 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 galvanic level voltages below 1.0V - 1.2V from flowing into or out of your vessel.

If we understand the voltage potential spreads, between underwater metals, which is usually below a 1.0V differential, it becomes easy to see how a GI works.

Because the dissimilar metals connected together in the electrolyte can't really create more than 1.2V the GI stops your vessels anodes from protecting your neighbors underwater metals.

If plugging in at a marina a GI is the absolute bare minimum level of protection that every boater should have. On personal level you'd never catch me plugged into any marina without an isolation transformer (IT). A simple GI won't cut it for me. Of course the conversation of true isolation versus galvanic isolation only takes on another life and is a whole other discussion & topic.

At a bare minimum you should have a fail safe galvanic isolator. If anode life does not improve after that then you will need to start with corrosion testing..
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Old 03-19-2016, 07:07 PM   #7
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Thanks for all the responses. I have anodes from boatzincs.com mounted on both shafts, both rudders, and the transom. The shaft anodes were the heavy duty version. They all needed replacement at 4 months.

I'll start the process of testing aboard my own boat first. If mine is leak free, then I'll take the next steps.
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Old 03-19-2016, 07:21 PM   #8
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I have had zincs last from 2 months to over a year depending on boat and/or location. Less than 4 months and it could be a problem. 6 months is normal in some places and some boats....

Usually, boats around you can give you a feel for what is and isn't normal for location. As per boat...all you can do is an electrical survey for stray currents and enough zinc protection.
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Old 03-20-2016, 10:14 AM   #9
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How long did you anticipate? How much underwater metal are they protecting? With no GI you are probably protecting your neighbors boats with your anodes.



Because our boats AC grounding wire (green) is supposed to be bonded to the DC grounding system, for human safety, this means that when you plug into shore power your underwater metals are connected to your neighbors underwater metals via the green AC grounding wire.

A galvanic isolator is a device that is inserted, in series, into the green grounding wire (safety ground) of your shore power feed to help minimize or reduce the effects of galvanic current from flowing between your vessel & your neighbors. While blocking galvanic level current it also has to allow for the passage of AC fault current. This is why any galvanic isolator should be of the "fail safe" type or have idiot lights that tell you you still have an AC grounding connection.

This blockage of low voltage galvanic current is achieved by using two diodes in-series 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 DC galvanic level voltage & current.

GI's normally have two diodes in each direction so the AC green wire is not "check valved" and acts just like a wire normally would. The only difference is it acts as one that won't pass voltages below 1.0V - 1.2V. Simple and pretty effective at blocking galvanic level current. GI's do not however block stray current that exceeds 1.2V...

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 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 galvanic level voltages below 1.0V - 1.2V from flowing into or out of your vessel.

If we understand the voltage potential spreads, between underwater metals, which is usually below a 1.0V differential, it becomes easy to see how a GI works.

Because the dissimilar metals connected together in the electrolyte can't really create more than 1.2V the GI stops your vessels anodes from protecting your neighbors underwater metals.

If plugging in at a marina a GI is the absolute bare minimum level of protection that every boater should have. On personal level you'd never catch me plugged into any marina without an isolation transformer (IT). A simple GI won't cut it for me. Of course the conversation of true isolation versus galvanic isolation only takes on another life and is a whole other discussion & topic.

At a bare minimum you should have a fail safe galvanic isolator. If anode life does not improve after that then you will need to start with corrosion testing..
That is the correct answer and a very good explanation of how a galvanic isolator works.

And yes, regardless of any conditions you may or may not be experiencing you should have a galvanic isolator installed on your boat if it has a shorepower connection.
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Old 03-25-2016, 07:41 AM   #10
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Is this a unit we can buy and install ourselves? If so, can you show a picture of what we're looking for? Thanks.


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Old 03-25-2016, 07:45 AM   #11
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Is this a unit we can buy and install ourselves? If so, can you show a picture of what we're looking for? Thanks.


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If you can cut wire and crimp on terminal ends...yes, installing one is easy.
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Old 03-25-2016, 08:36 AM   #12
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The term "stray current" is misleading. Electrical current doesnt just float around randomly attacking anodes. Its moving from one place to another via the easiest path. If the current is using your boat as a path in its flow it would normally go in at the anode and exit on the green wire. Normally this doesnt cause a problem as the anodes dont lose material. If your anodes are used up that quickly you probably have something going on with your boat causing it.
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Old 03-25-2016, 10:58 AM   #13
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Is this a unit we can buy and install ourselves? If so, can you show a picture of what we're looking for? Thanks.


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http://www.defender.com/pdf/204577.pdf
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Old 03-25-2016, 01:08 PM   #14
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[QUOTE=sdowney717;425121]I have a clamp on digital meter which measures both AC and DC current.
Not as sensitive as this Fluke meter, which here describes testing for current leaking.
Leakage current measurement basics

Another test is use a GFCI which are rated at 5 milliamp, meaning if the current exceeds a 5 milliamp leak, it will trip.

I have all my outlets on GFCI, and I have an extension cord with GFCI. When I run the entire boat on that GFCI extension cord, it does not trip. But also the devices your testing need to be switched on.QUOTE]

All of the above refers to leakage on the AC side, not the DC side. Rapid failure of your zincs is caused by DC leakage. AC leakage is important to control because of the shock potential which is why GFCI's disconnect the circuit in question when leakage to ground is detected. 12vdc is not a danger to life, while 120vac is. Most new or rewired marinas are now installing GFCI's on the shore power breakers (per code). Boats without an isolation transformer often times will not be able to use shore power due to small AC leakage currents, mine included. The only solution is to install a transformer, or cut the ground wire which is NOT recomended. Doing so is a large hazzard to those aboard, and can cause swimmers to drown when the are swimming near the boat. So, when troubleshooting excessive zinc usage, you are looking for problems on the DC side. The isolation diodes prevent that DC current from flowing between your boat and others in the marina, but do nothing for currents circulating within your own boat.
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Old 03-25-2016, 02:52 PM   #15
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Is this a unit we can buy and install ourselves? If so, can you show a picture of what we're looking for? Thanks.


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You will have to evaluate your own electrical abilities. It's really just cutting the green (in the USA) conductor from the shore power inlet to the electrical panel and attaching the cut ends to the device.

In reality, it can be a little more complicated that it would seem and you might already have one and not know it.

If in doubt, hire a marine electrician and sleep well at night. It should take less than an hour.
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Old 03-25-2016, 06:25 PM   #16
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If in doubt, hire a marine electrician and sleep well at night. It should take less than an hour.
I would say that is really good advice. Reason being is if you do it yourself and its not right, YOU wont know. And a hole in your rudder or prop is a serious lesson best passed on learning the hard way.
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Old 03-25-2016, 07:12 PM   #17
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My zinc was disappearing way to fast for my liking. It was my battery charger over charging the battery's. Now the zink seams to be lasting a year. Go figure~~
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Old 03-25-2016, 10:26 PM   #18
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That is generally the case. A bad charger gone bonzeye, or a shorted battery in the bank.
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Old 03-26-2016, 06:33 AM   #19
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I've found battery chargers to be the most common cause of current leakage.

- disconnect shore power
- turn off inverter and or generator
- turn on all breakers
- set meter to ohms and put across neutral and ground at inlet.
- if any continuity shows on meter, turn off breakers one at a time.
- when you see continuity disappear, that is the problem circuit.
....... 9 out of 10 its the battery charger circuit.

Some chargers (Xantrex) have a very small neutral/ground leak right out of the box, usually in the range of 27-30 kilohms. This is a design issue and not really a problem. if it it gets down to around 10kilohms it requires attention.
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Old 03-26-2016, 08:52 AM   #20
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[QUOTE=Arctic Traveller;427288]
Quote:
Originally Posted by sdowney717 View Post
I have a clamp on digital meter which measures both AC and DC current.
Not as sensitive as this Fluke meter, which here describes testing for current leaking.
Leakage current measurement basics

Another test is use a GFCI which are rated at 5 milliamp, meaning if the current exceeds a 5 milliamp leak, it will trip.

I have all my outlets on GFCI, and I have an extension cord with GFCI. When I run the entire boat on that GFCI extension cord, it does not trip. But also the devices your testing need to be switched on.QUOTE]

All of the above refers to leakage on the AC side, not the DC side. Rapid failure of your zincs is caused by DC leakage. AC leakage is important to control because of the shock potential which is why GFCI's disconnect the circuit in question when leakage to ground is detected. 12vdc is not a danger to life, while 120vac is. Most new or rewired marinas are now installing GFCI's on the shore power breakers (per code). Boats without an isolation transformer often times will not be able to use shore power due to small AC leakage currents, mine included. The only solution is to install a transformer, or cut the ground wire which is NOT recomended. Doing so is a large hazzard to those aboard, and can cause swimmers to drown when the are swimming near the boat. So, when troubleshooting excessive zinc usage, you are looking for problems on the DC side. The isolation diodes prevent that DC current from flowing between your boat and others in the marina, but do nothing for currents circulating within your own boat.
Spot on and well said. For marinas installing leakage protected shore inlets, these are referred to as ELCIs, equipment leakage current interrupter, they are differentiated from conventional GFI receptacles, which trip at 5-7 mA, by their significantly higher trip threshold, which is 30 mA.

In many cases those plugging into a shore ELCI for the first time find it trips. When that happens the source of the leakage must be found and it isn't always easy. Common culprits are fluorescent fixtures, water heaters, electric stoves, washing machines and other household appliances. Or, an outright, intentional but incorrect connection between neutral and ground at the panel.

An alternative is to install a shore power transformer, which will essentially isolate the vessel's electrical system from the shore. That approach works in that it prevents the tripping, however, I remain uncomfortable with it unless all efforts have first been made to find the source of the leakage. If there's a genuine fault it could still pose a fire or electrocution risk to those aboard.

ELCI's are now mandated aboard boats where ABYC compliance is sought (it's not the law, there are virtually no US laws governing wiring on diesel-power recreational vessels). The installation requirements, and exceptions and how they interface with vessels using transformers, and those that don't, can be a bit complex, it's work that should only be carried out by an ABYC certified tech, or very savvy DIYer with access to the ABYC Standards . New builders that claim ABYC electrical compliance will have an ELCI main breaker aboard.
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