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Old 29-06-2007, 23:12   #1
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Shore Power

I have 240vac shore power on my fibre glass yacht. At the moment the green Earth wire on the shore power is floating and NOT connected to the engine block.
Is there any advantage / disadvantage in connecting the green Earth wire to the engine block ?
All GPO's have an earth connection and the circuit is RCD protected. The earth is connected back to shore. I am concerned about electrolyses.
The DC negative is connected to the engine block which in turn is connected to the shore earth through the sea water.
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Old 30-06-2007, 00:44   #2
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The AC system, including it earth is NOT conected to the boat electrical system in anyway. The AC earth is there for protection within the AC environment. If you connect AC ground to the DC ground, then stray AC currents will want to flow from the metal underwater fittings back to shore AC ground.
The DC system is also a seperate system. Even though it is connected to the metal underwater fittings, because it is isolated from the AC system, current can not flow from the DC ground to AC ground.
When such a situation arises that there is a grounding issue, often an Galavnic Isolator is fitted to "lift" or isolate the AC ground.
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Old 30-06-2007, 01:07   #3
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Alan,

I agree whith what you are saying and this is what I have done, but I have also read a lot of articles saying you should connect the AC earth to the DC negative. (engine block) See the article below for comment. Maybe I should have the AC earth connected to the engine block, but through a galvonic isolator ? Do you know where I can purchase one of these in Australia ? I have not been able to locate one ?

Grounding
The green "grounding" wire in AC circuits provides a low resistance path to ground should any of the various metal cases enclosing your AC system become energized. But what if the leak is into the DC wiring, caused, for example, by crossed wires or a short in a battery charger, inverter, or other dual-voltage appliance? AC leaking into the DC system will seek ground, meaning it will automatically travel through the wiring to the ground connection on the engine and down the prop shaft to the water. This is essentially the same as dropping a hot wire into the water. In fresh water, this poses a real risk of electrocution for anyone in the water nearby. Electrocution is less likely in saltwater, but the current field can be enough to paralyze muscles and cause a swimmer to drown.

Connecting the green wire to the ground terminal on the engine offers AC leakage into the DC system a lower-resistance path to ground--through the grounding wire. This eliminates the risk to swimmers as long as the grounding wire connection to ground is sound. However, if corrosion on the ground prong of your dock cord or some other fault breaks the ground connection, all ground-fault current, not just AC to DC leakage, will flow into the water. It is essential to test the ground connection at the dock and to maintain cords and plugs in good condition.
In your breaker box at home the neutral wires and grounding wires all connect to the same terminal strip (or bus bar), but on a boat the AC grounding wire is connected to the DC ground. Also connecting the neutral wire to it makes underwater hardware a current-carrying path to ground, potentially lethal for anyone in the water nearby. On a boat the neutral (white) conductor and the grounding (green) conductor MUST NEVER BE DIRECTLY CONNECTED.
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Old 30-06-2007, 01:16   #4
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Crak, check out this article in Professional Boatbuilder. It explains it all very well.

Professional BoatBuilder - April 2006/May 2006

Click on "shore power safety tests".

I would call your attention to the subheading "Breaking the Galvanic Circuit" on about the third page.

I'll let Nigel Calder and Wheels fight it out...
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Old 30-06-2007, 11:17   #5
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Don't confuse our Aussie friends with North American Grounding Info. The power distribution systems are very different. In North America we run a single wire highline with a neutral which is referenced to ground. It is then transformed to a centre tapped 240 volts where the centre tap is grounded on the secondary. Therefore we get two 120 volt circuits that have a common neutral which is grounded. If the marina doesn't use isolating transformers you end up with a common path to ground and we use the third wire ground to protect against floating neutrals. The English, european and I believe the Australian systems don't really need the third wire because the distribution is not referenced to earth ground. I might be wrong about the Australian system, I am assuming they are following the british System.
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Old 30-06-2007, 14:06   #6
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I am not sure either now. Here in NZ we have a system called MEN. Multiple Earthed neutral. The earth and Neutral are tied together at multiple points along the distribution system. All electrical equipment obtains it's power via connection to Phase and Neutral and the Earth is a return path for fault currents back to the distribution panel. I think this is the same for all systems, but the differences are found in the way the earths are used and "connected" to what woudl be called "earth". We have dedicated earth spikes driven in the ground. We DO NOT allow the system Earth to be connected to water pipes and such to provide Earth, but metal objects may be connected to system Earth if they are deemed as needing protection. eg. you may have a SST bench top and an electricval appliance on the bench top. If a fault develops that the appliance becomes live and "it's" earth has failed in some way, the SST bench would now be live, unless it was also connected to the system Earth. So SST bench tops are Electricaly Earthed.
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Old 30-06-2007, 15:29   #7
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Make sure your earth & neutral busses in the (boat's) switchboard are not tied together. As long as you have good seperation between 240V outlets/appliances and any conductive material on the boat then simply connect the earth to the outlet terminals and nothing else on the boat's structure.
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Old 30-06-2007, 16:35   #8
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That system of tying the two together went many many years ago now Pete. If you still have such a connection onboartd, there will be one of two things that accurr. Firstly, you will trip the GFI and secondly, it means you do not have a current EWOF.
Just so as we keep this all on the main point, and it is this point I too am still a little unsure about, is the relation to the AC Earth and the Boats DC negative ground system.
It was interesting reading the link to Nigel Calders discussion. I agree totaly with all that is stated. And my original comment was made in relation to what can happen in the event of a fault and how the fault current will want to flow back to shore via the sea water. He made a brief comment about Galvanic Isolator, same as I, but he did not go anyfurther into that. So my question is, do you tie the AC earth to the DC neg via a Galv. isolator???
On my boats sytem, the two systems of AC and DC are very seperate and Isolated. It passed the EWOF this way, but what surprised me the most, was the inspector was askign me many of the questions as he wasn't entirely sure. Typical eh.
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Old 30-06-2007, 16:48   #9
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The reason I mentioned the buss bar tie was because the earth had been left "floating", possibly indicating a fault that hadn't been found. A lot of people fit pre-made domestic switchboards (me included) that have the earth & neutral tied and this tie needs to be removed for marine use.
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Old 30-06-2007, 18:07   #10
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Using the terminology or the referenced article, the neutral is the "ground" and the ground is the "grounding" wire. As Wheels mentioned it is a safety tied to the case or chassis of electrical equipment and tied to the ground (neutral) only at the main distribution feed. Nigel Calder mentions that the point where they are tied together differs when using the electrical mains, generator or invertor.

Wheels, I don't think you read far enough. The article goes into some depth on where the galvanic isolators and isolation transformers are installed in the cct. They basically are the first devices after the input breakers.

The tie between the AC and DC "grounds" is also discussed and the safety issues that this connection addresses, although there is a difference between ABYC and EU specs. I believe he (Nigel Calder) says the EU can leave off this connection, something Calder disagrees with.

It is an interesting article that for me at least requires re-reading periodically.

Cheers.
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Old 30-06-2007, 19:16   #11
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To confuse you even more please read this article:
http://www.xantrex.com/web/id/268/docserve.asp
In any case, the conclusion for us boat users is simple: for AC system HOT & NEUTRAL condutors are to deliver power - EARTH (GREEN) is to protect us. The galvanic isolator is always installed at the AC "intlet". It is to protect against our boat to become a battery if right conditions appear and at the same time not to "disable" or divert electric shock hazard protection.
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Old 30-06-2007, 22:27   #12
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Thanks DeepFrz. I re-read it again and saw where I missed the comment. I am just getting over a mild bought of Pnuemonia and it seems it must have affected my eye sight :-)
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Old 30-06-2007, 23:01   #13
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When I was in Wangarei, New Zealand, I inspected a Catana catamaran that had severe electrolysis damage to the rudder posts - to the point that they could have snapped off if you gave them a hard kick with your foot. The stainless steel rudder posts were pitted so badly that they looked like the seratted edges of a postage stamp where the rudders entered the hull. They had to send new rudders from France to repair the damage.

The catamaran had saildrives, and when I touched one of the saildrives in the boat yard I could feel the tingle of electricity in my fingers. So I got my voltmeter and stuck one electrode in the dirt and the other against the saildrive, and the voltmeter registered 80 volts ac between the saildrive and the dirt/earth in the boatyard.

I don't know exactly what was going on inside the yacht, but it was obvious that there was some massive electrical problem with leakage of AC current into the saildrives. There was also sign of extensive electrolysis in the saildrives as well.

I suspect that the AC and DC systems were interconnected in some dangerous way, and that this boat was leaking massive amounts of current in the water. I never found out what was causing the problem, but I'm sure that if they didn't solve it, the new rudders wouldn't last long after they put the boat back in the water.
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Old 01-07-2007, 16:58   #14
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Thank you for the reply's .
It looks as if I have created an interesting discussion but I have still not found the answer to my conundrum.
Should I connect the AC protective earth to the DC -ve ground ? (Should I connect the AC protective earth to the engine block ?)
There is a safety advantage in doing this, as it will provide protection in the case of AC coming in contact with the DC system due to chafing of wires etc.
My only concern is, will this cause a path for electrolysis ?
If by doing this will not increase the chance of causing electrolysis, then I can see a safety advantage and will go ahead and do it.
The authorized method of installation for Australia / New Zealand is covered in AS 3004, but unfortunately you have to pay for these standards.
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Old 01-07-2007, 18:46   #15
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I think you have created interesting dillema: what is more important: electrolysis or your health. Your choice. However, you should follow recommendations as stated in the standard of the country you are in. 60 AUD is not a fortune but it does not replace knowledge of the subject. The subject is more complicated than it sounds. It requires thorugh knowledge of electrical systems and one solution may not be applicable to all configurations.
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