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Old 26-10-2009, 04:40   #16
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Originally Posted by s/v Jedi View Post
... for Barnie: it doesn't matter if your AC and DC are separated. That does not protect your under water metals from an shore AC problem...
Nick.
EXACTLY !!!

BTW: Your AC Shore Power Ground is NOT isolated (or should not be) from your DC ground. Only the AC Neutral is isolated (on the boat side).
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Old 26-10-2009, 08:01   #17
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Originally Posted by GordMay View Post
BTW: Your AC Shore Power Ground is NOT isolated (or should not be) from your DC ground. Only the AC Neutral is isolated (on the boat side).
That needs refinement:

On the perfect installation, the ground from shore power enters the boat inside the shore power cable and is terminated within the isolation transformer, as a secondary shield around the primary winding of the transformer. The transformer housing and all the rest inside the boat is connected to the boat ground, using under water ground plate(s). So, basically, the shore ground IS isolated from the boat.
This is true wherever in the world the boat is built.

In the US, most (if not all) boat builders follow the guidelines from ABYC. These state that the boat's AC ground should be connected to the boat's DC negative using exactly one wire.

In the EU and many other places, boat builders follow the guidelines from another organization (CE I think but all changed with that EU thing) and they state that there should be NO connection between AC ground and DC negative.

Everywhere in the world, the AC neutral is connected to AC ground at the AC power source or converter (generator, transformer, inverter). Your inverter has that internal connection. When your inverter has an internal transfer-switch (like all inverter-chargers have, with one or more AC inputs), a relay is used instead, which is closed in inverter mode and opened in transfer-mode.
BUT... if you have an isolation transformer you can decide not to do that and keep your AC fully floating. Both wires will be hot (when referenced to ground) and double-pole breakers must be used, but it is the safest approach for boat and crew.

The info above isn't complete because in some setup's a ground-fault interrupt device must be installed while it is optional in the other options. Never just hook something up; the installation must be designed by an electrical engineer.

cheers,
Nick.
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Old 26-10-2009, 11:30   #18
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"In the EU and many other places, boat builders follow the guidelines from another organization (CE I think but all changed with that EU thing) and they state that there should be NO connection between AC ground and DC negative."

Can you please show me this regulation you are talking about? I have worked for several major boat builders as a engineer, consultant and in house surveyor. These companies have shipped 100's of boats overseas all with CE certification and not one has had the AC and DC grounds NOT connected.

This is a inherently dangerous set up. If you loose your shore ground for what ever reason you need a path to earth ground. The best path is thru your DC ground to the engine and then to the water. All AC appliances need to be grounded not just the outlets. There is a reason ABYC recommends this it is to save lives. If there is a fault onboard and you touch a AC appliance and a grounded or DC applicance the fault will find ground thru you. There have been many deaths from this. I personally have been shocked more than once from this type of setup. Ac power will find a path to ground even thru you.

AC power can be a very dangerous thing on a poorly wired boat. I would like to see this "EU" recommendation you talk about if I am wrong about this then I will humbly admit it, if you are wrong somebody could die.

Wayne Canning, AMS
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Old 26-10-2009, 11:55   #19
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I think the European standards are contained (partly) in ISO 13297:2000(E) & ISO 10133:2000(E)

AC ➥ http://www.sychut.com/nav/el/ISO10133.pdf

DC ➥ http://www.sychut.com/nav/el/ISO10133.pdf

Excerpted from 13297:

"4.2 The protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as practicable to the battery (d.c.) negative terminal.
NOTE If an RCD (whole-craft residual current device) or an isolation transformer is installed in the main supply circuit of the
a.c. system (see 8.2), the negative ground terminal of the d.c. system need not be connected to the a.c. shore ground
(protective conductor)."

"4.8 The neutral conductor shall be grounded (earthed) only at the source of power, i.e. at the onboard generator, the secondary of the isolation or polarization transformer, or the shore-power connection. The shore-power neutral shall be grounded through the shore-power cable and shall not be grounded on board the craft.
"

These seem to match the ABYC Standards used in N.A.
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Old 26-10-2009, 11:57   #20
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Ok back to zincs.

What causes galvanic (and to stray currant) corrosion is the flow of electrons from one metal object to another. The electrons need a electrolyte to flow thru such as sea water. This is just like a battery with the metals being the plates and the electrolyte the acid. When the electrons flow they take bits of the metal with them and deposit those bits on what ever they are flowing to. This could be on your boat or another boat next to you.

A proper bonding and electrical system will not allow the electrons to flow in the first place. That green wire or copper strap connecting all you under metal is there to provide a path for any currant to flow thru stopping it from flowing thru the water. The Zinc is on there only for protection in the event something is out of balance or not properly connected.

So if your bonding system is all intact with no resistance at any connection life is good. If however you get resistance at anyone connection that part is subject to currant flow thru the water. If the zinc is being wasted it is because there is a flow of electrons from the metal it is protecting to something else. This is why i said at the beginning if you are going thru zincs then that is a sign you have other problems.

Now i will admit it is almost impossible to completely prevent some currant flow you should not have so much that your zincs are completely wasted in a season. If it is you either have too little zinc to protect the amount of underwater metal or you have a problem.

By adding another zinc hanging over the side you are adding just one more path for currant to flow on. This is sort of a band aid approach to the problem is all i am saying. Sure use it if it makes you feel better but if you have to use use it then find out why.

This is a bit of a simplified explanation but i just think people should understand that throwing more zinc at it is not always the best solution.

Wayne Canning, AMS
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Old 26-10-2009, 12:01   #21
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Gord

YES thats it exactly what I was saying. Thank you for that info that clears this up. It bothers me to think someone would misunderstand and end up with a dangerous boat. And this is no different than the ABYC standards

Thanks for clarifying that.

Wayne Canning, AMS
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Old 26-10-2009, 14:25   #22
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This is a bit of a simplified explanation but i just think people should understand that throwing more zinc at it is not always the best solution.
Wayne,

Everything you wrote is correct but it all assumes your boat is the only man-made thing on Earth. As soon as you anchor in a busy anchorage or enter a marina, things change.

The under water metal parts and the boat's grounding system will interact with those of other boats and their AC systems plus the marina AC system. It can get much worse around boat yards where welding on boats/ships in the water is done.

The marina we are moored now has transformers within 100' of the dock. But on my power-post, I measure 40V AC between ground and neutral. And this is a modern marina, the kind with floating concrete docks etc.

But I agree with your standpoint that it is better to change something else to correct the problem. An isolation transformer is that solution.

cheers,
Nick.
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Old 26-10-2009, 14:50   #23
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It bothers me to think someone would misunderstand and end up with a dangerous boat. And this is no different than the ABYC standards
I am not convinced. Gord has the right documents and I quote from them:

Quote:
NOTE If an RCD (whole-craft residual current device) or an isolation transformer is installed in the main supply circuit of the a.c. system (see 8.2), the negative ground terminal of the d.c. system need not be connected to the a.c. shore ground (protective conductor)."
All modern EU sailboats I've seen have the RCD and do not have the AC ground to DC negative connection. The RCD is prominently visible on the main switch panel, while I have never seen one on a US panel. This is clearly allowed in the ISO standard. Now, may be ABYC changed to allow that too, but as said, I am not convinced (connection must be there regardless of RCD is what I understand).

When EU boats are built for the US market, they might do it differently, I am not sure about that.

I clearly mentioned the requirement for the RCD with some setup's and a warning to use an electrical engineer for system design in my post.

cheers,
Nick.
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Old 26-10-2009, 15:33   #24
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Wayne,
The marina we are moored now has transformers within 100' of the dock. But on my power-post, I measure 40V AC between ground and neutral. And this is a modern marina, the kind with floating concrete docks etc.
Nick Wow I would not plug into that source at all I would insist they fix it or move. No wonder you need more zinc. But this is what I am talking about when I say fix the source of the problem. And yes I know there are times it cannot be avoided so I agree 100% about the isolation transformer that really is the way to go. Here in the US that could be reported to local authorities an they would force a repair. That type of thing could kill a swimmer in the water. But of course this forum covers many areas. So yes for any boat cruising unknown ports the isolation transformer is really the best solution.

And you are right about the RCD ABYC has just adopted this as of July 1st this year so you will see US boats with it. I just do not want readers without these items and with older boats which I would guess is the majority of the readers to think it is safe to not have the 2 grounds connected.

I do not think a properly wired boat is at much risk moored next to another boat with problems as long as they are not on the same power source meaning the shore power. A boat at anchor is not likely to have problems.

Part of the problem is we are really talking about 2 different things here stray currant corrosion and galvanic corrosion. Hard to separate them at times. But they are different.

I just think safety should come first then the protection of the vessels underwater metal. It is the responsibility of those of us who have a better understanding of this to help those who do not. Best to err on the safe side.

Good discussion hope we are all learning from this I know I am as I am not as familiar with the EU methods.

Wayne Canning, AMS
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Old 26-10-2009, 23:07   #25
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Wayne-I do not want to offend, but there are some technical flaws in some of your earlier posting that need clarification:

Quote:
What causes galvanic (and to stray currant) corrosion is the flow of electrons from one metal object to another. The electrons need a electrolyte to flow thru such as sea water. This is just like a battery with the metals being the plates and the electrolyte the acid. When the electrons flow they take bits of the metal with them and deposit those bits on what ever they are flowing to. This could be on your boat or another boat next to you.
Sorry, but this just is not technically correct. And stray current is another subject altogether!! Electrons flow from the underwater metal through the bonding system conductor to the sacrificial anode. Ions flow from the underwater metal to the sacrificial anode.

Quote:
A proper bonding and electrical system will not allow the electrons to flow in the first place. That green wire or copper strap connecting all you under metal is there to provide a path for any currant to flow thru stopping it from flowing thru the water. The Zinc is on there only for protection in the event something is out of balance or not properly connected.
Not really the way cathodic protection works. The bonding system ties all the underwater metal parts together electrically with each connection < 1 ohm of resistance. The sacrificial anode is electrically connected to the bonding system and depresses the potential of all of the underwater metal by -200 mVDC referenced to a AgAgCl reference cell. This voltage depression will arrest corrosion of the underwater metal that is bonded together.

Quote:
So if your bonding system is all intact with no resistance at any connection life is good. If however you get resistance at anyone connection that part is subject to currant flow thru the water.
Yes, life is good if your bonding system is intact with low resistance electrical connections. High resistance at a fitting simply removes that fitting from the bonding circuit and it becomes susceptible to corrosion depending on its chemical and metallurgical properties. If it is made of marine grade material; e.g., bronze, its corrosion rate will be so low that it may not be damaging at all.

Quote:
If the zinc is being wasted it is because there is a flow of electrons from the metal it is protecting to something else. This is why i said at the beginning if you are going thru zincs then that is a sign you have other problems.
Calculations are done to determine the number of pounds of sacrificial anode are required to protect the square feet of underwater metal on a vessel. If the anode weight is insufficient, it may be because the electrons are leaving our vessel and traveling along the safety green wire to protect other, nearby, vessels, submerged metal on the dock or on shore. This is an experiential situation: if your anodes were not wasting at a rapid rate last season at the same dock but your diver is putting them on every month this season; something has changed galvanically.

Quote:
Now i will admit it is almost impossible to completely prevent some currant flow you should not have so much that your zincs are completely wasted in a season. If it is you either have too little zinc to protect the amount of underwater metal or you have a problem.
There is no rule of thumb that says anodes have to "last a season." What is important to note is: has the anode wastage rate changed and, if so, why?

Quote:
By adding another zinc hanging over the side you are adding just one more path for currant to flow on.
Not really true. If you have accelerated anode wastage (which implies that you had acceptable anode wastage previously) than hanging an auxiliary anode over the side and connecting it to your bonding system makes very good sense.

Quote:
This is sort of a band aid approach to the problem is all i am saying. Sure use it if it makes you feel better but if you have to use use it then find out why.
It also may be the easiest way to increase your anode poundage to counter the galvanic situation that your vessel is existing in.

Again, I really do not like to nit-pick, but there is a lot of bad information out there on the subjects of galvanic corrosion and cathodic protection.

Best regards,
Charlie
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Old 27-10-2009, 05:06   #26
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Charlie yes you are right and I suppose I was over simplifying, just trying to help folks understand. But your post is good and I hope this thread has been helpful to some. For something that is basically simple it is very complicated. I suppose my point really is that if your zincs are wasting so fast that they do not last the season you have a problem that needs correction. Just adding more zinc is not always the best solution.

Thanks
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