Galvanic Isolator

The friendliest place on the web for anyone who enjoys boating.
If you have answers, please help by responding to the unanswered posts.
See posts with no answers
C

Catbird

Veteran Member
Joined
Dec 16, 2012
Messages
85
Location
USA
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?
 
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 ;-).


David
 
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.
 
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 ?
 
Last edited:
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
 
Catbird said:
Zincs on my boat are degrading faster than I anticipated.
Click to expand...

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.

Catbird said:
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?
Click to expand...

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..
 
Last edited:
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.
 
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.
 
CMS said:
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..
Click to expand...

That is the correct answer and a very good explanation of how a galvanic isolator works. :thumb::thumb::thumb:

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.
 
Is this a unit we can buy and install ourselves? If so, can you show a picture of what we're looking for? Thanks.


Sent from my iPad using Trawler Forum
 
Oceania said:
Is this a unit we can buy and install ourselves? If so, can you show a picture of what we're looking for? Thanks.


Sent from my iPad using Trawler Forum
Click to expand...
If you can cut wire and crimp on terminal ends...yes, installing one is easy.
 
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.
 
sdowney717 said:
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.
Click to expand...
 
Oceania said:
Is this a unit we can buy and install ourselves? If so, can you show a picture of what we're looking for? Thanks.


Sent from my iPad using Trawler Forum
Click to expand...

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.
 
WesK said:
If in doubt, hire a marine electrician and sleep well at night. It should take less than an hour.
Click to expand...

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.
 
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~~:blush:
 
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.
 
Arctic Traveller said:
sdowney717 said:
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.
Click to expand...

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.
Click to expand...
 
when the zincs die the metal on the boat builds a "large" dc potential. That potential closes it's loop with other boats via the earth ground to ships dc ground connection.

Isolators break that path, and prevent other boats from eating yours.

Right?
 
Steve DAntonio said:
Arctic Traveller said:
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. .
Click to expand...

Steve, Thrilled that you replied, and thanks for the kind words. I first became aware of the new Equipment Leakage protected inlets in Ketchikan Ak when I couldn't plug in with out tripping the breaker. At the time I suspected that they had used conventional GFI's with a low trip threshold causing nuisance tripping. I saw what looked like a standard GFI in the power tower, but I now wonder if it really was a ELCI instead. Can you tell the difference by looking at the breaker without taking off the cover? Are ELCI's the only way to meet the new code requirements for new or rewired marinas? I suspected my leakage to be under 10mA, but now I wonder if it's higher, time to do some investigation I guess. Last question, any publication date yet for your book?.............
Click to expand...
 
My thoughts have been that since my boat is all run on DC other than the battery charger, so there is no need for a galvanic isolator.
The CITEK charger has no AC ground connection to marina power.

Am I correct in this assumption?
 
Arctic Traveller said:
Steve DAntonio said:
Steve, Thrilled that you replied, and thanks for the kind words. I first became aware of the new Equipment Leakage protected inlets in Ketchikan Ak when I couldn't plug in with out tripping the breaker. At the time I suspected that they had used conventional GFI's with a low trip threshold causing nuisance tripping. I saw what looked like a standard GFI in the power tower, but I now wonder if it really was a ELCI instead. Can you tell the difference by looking at the breaker without taking off the cover? Are ELCI's the only way to meet the new code requirements for new or rewired marinas? I suspected my leakage to be under 10mA, but now I wonder if it's higher, time to do some investigation I guess. Last question, any publication date yet for your book?.............
Click to expand...

I can't say for sure (although I was at that marina last year I don't recall the breaker arrangement) what they are using. Typically, a GFI is in a common receptacle form, like you'd have in your galley or bathroom at home. Those are often used on power pedestals, however, they are isolated, meaning they protect only what's plugged into them, not the shore cord, and they are limited to 15 and sometimes 20 amps. Trip threshold is 5-7 mA. Again I couldn't be sure without seeing the pedestal.

An ELCI looks like an actual circuit breaker, with an added component which monitors for an imbalance on the supply to the shore power inlet and opens the breaker at a designated trip threshold. Most boats have enough collective faults aboard, damp insulation, fluorescent ballasts etc, to trip a common GFI if you actually could plug the whole boat into this wall receptacle-like plug, whose threshold is just 5-7 mA, but there's no way to know for sure what your leakage is without measuring it, so that's an assumption that shouldn't be made, 8 mA may trip a GFI, but you may have 800 mA of leakage. Again worth a test. The cord leakage test must be carried out with power supplied to the boat, and again with the shore cord plugged in but main dock breaker off, to determine if the source of the leak is your boat or one nearby. This PowerPoint walks you through the process. http://www.boatus.com/seaworthy/assets/pdf/marina-dock-safety.pdf

ELCI breakers look different, in addition to the normal breaker portion, they will also have a test button right next to the breaker handle, and some will also include status lights. ELCI incidentally is the term used for these devices when installed aboard a boat, and they are set to trip at 30 mA for ABYC compliance. When installed ashore, at the pedestal for instance, or elsewhere on the dock, they are referred to as Ground Fault Equipment Protective Devices (GF-EPDs), and some other similar names. Per NEC, the trip threshold at the marina or dock main supply breaker may be as high as 100 mA, but typically, if they are at the pedestal (that's preferred because if that breaker trips you know the source of the trouble, the boat that's plugged in to it) they are 30 mA. If that's the case in Ketchikan, and your boat is causing the pedestal breaker to trip, you should investigate and find the cause.

Probably more than you wanted to know.
Click to expand...
 
AusCan said:
My thoughts have been that since my boat is all run on DC other than the battery charger, so there is no need for a galvanic isolator.
The CITEK charger has no AC ground connection to marina power.

Am I correct in this assumption?
Click to expand...

Once you plug into shore power, regardless of the loads aboard, your vessel's underwater bonded metals are (or should be) now connected to AC safety ground wire in the shore cord, which is connected to the dock AC safety ground, which is then connected to the shore cords and AC safety grounds of all other boats that are plugged into shore power as well. In short, you are all connected and your zinc anodes may be protecting the underwater metals of the boats adjacent to you (provided they too have no galvanic isolator), regardless of what AC power you are using aboard. In fact, if you plug into shore power and never turn on the dock pedestal breaker, you are still connected in this fashion because the AC safety ground is never (supposed) to be switched or disconnected as long as you are plugged in.

Therefore, if you want to inhibit potentially harmful galvanic voltage flow, and corrosion, you should have a galvanic isolator. Every vessel's shore power system should have one.

Now on to your comment about he charger not being grounded. If I understand you correctly, if the charger has no AC safety ground connection, its plug has only two prongs? If so, it is double insulated and needs no third ground prong on the plug. (The CTEK charger appears to be designed for automotive use). Regardless, the vessel shore ground/corrosion connection issue remains as long as you are plugged into shore power, one that would be mitigated by a galvanic isolator.
 
Steve DAntonio said:
Probably more than you wanted to know.
Click to expand...

No, that's exactly what I wanted to know, thanks and sorry for running this thread off the rails. Back to premature zinc failure.............
 
Last edited by a moderator:
Catbird said:
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?
Click to expand...

It is very unlikely that the issue is stray current unless the anodes are disappearing in a matter of days or maybe weeks. Stray current is a very quick process while galvanic corrosion is much slower.
 
Last edited:
boatpoker said:
It is very unlikely that the issue is stray current unless the anodes are disappearing in a matter of days or maybe weeks. Stray current is a very quick process while galvanic corrosion is much slower.
Click to expand...

Well, that's true sometimes, but it totally depends on the AMOUNT of current. Any amount of current will degrade your zincs, it's why they are called sacrificial anodes. If they become insulated from the metal they protect, they will last forever, but provide no protection. the higher the current is, ithe faster the zincs are wasted. The corrosion rate is dictated by the current that is produced.
 
You must log in or register to reply here.

Similar threads

D
Isolation transformer reverse inputs
Replies
5
Views
774
twistedtree
twistedtree
Serene
Adding Inverter to Existing System?
2
Replies
40
Views
2K
Serene
Serene
P
Sorting through AC electrical system.
Replies
16
Views
880
PierreR
P
J
Aluminum/Zinc Anodes
Replies
20
Views
2K
fstbttms
fstbttms
Mischief Managed
50 amp 250V isolation transformer options
Replies
25
Views
5K
Mischief Managed
Mischief Managed

Latest posts

Back
Top Bottom