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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #1  
Old 05-18-2009
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galvanic isolators and the green wire

A friend with a Spencer 35 and I (CS27) were discussing whether to bond through hulls or not. The concensus seems to be not to and I agree. In our research we came across info on galvanic isolators. After some reading we found that the green ground wire of an ac system on board should be connected to the boat's negative ground - negative bus and ultimately the engine ground. His boat doesn't have this connection and neither does mine. The ac system on my boat was by all accounts professionally installed with Blue Seas ac panel and tinned marine wire etc. All ac outlets are gcfi - 3 with a 15 amp breaker for each one. The shore charger is on its own breaker. The main breaker is the familiar double to break both hot and neutral and show reverse polarity. My friends Spencer 35 has a more basic system with a household box and no main double breaker but he does have gcfi outlets. Everything that we read explains ABYC regulation of grounding the ac green and goes on to explain the need for the galvanic isolator. My question is what is wrong with not grounding the green to the engine block and using gcfi outlets. It obviously works and has for many years. In my reading it seems that by connecting ac ground to the engine block and therefore to the dc ground you are inviting more electrolysis issues. What does everybody else think of this?
Brian
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Old 05-18-2009
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First, just because someone is a professional, it does not mean that they are competent or know what they're doing. All it really means is that the IRS knows they get paid money for doing it.

Second, not all outlets need to be GFCIs. As long as the first outlet in the circuit is a GFCI, the entire circuit is protected by the first outlet. If the electrician installed GFCI outlets at every location, he's an idiot and spending a lot of extra money where it isn't necessary.

Third, there are some who don't like connecting the AC ground to the DC ground. However, IMHO, it really should be done. From the Blue Seas website:

Quote:
Without a good connection between DC negative and AC safety ground, stray AC current may enter the DC ground system. When this happens, AC current may enter the water around a boat and injure or kill swimmers near the boat.

The green wire is the safety ground wire that connects the DC negative ground block to the AC safety ground bus. The purpose of this wire is to provide a lowest-resistance path to ground for any stray AC current that finds its way onto the DC ground system. There have been cases of AC current entering the water around a boat through the engine shaft and killing swimmers near the boat.


There is a down side to this green wire connection. This safety ground can also provide a path for galvanic current if the boat is not adequately protected with galvanic isolators. However, most marine industry organizations and professionals now consider it standard practice to install this wire. Safety requires providing the grounding wire, either directly or through a galvanic isolator, or using a properly installed marine isolation transformer. Some people have left off the ground wire in a mistaken notion that they are providing galvanic protection, but forget that they are compromising safety for those on the boat, on the dock, and in the water. Electrically induced drowning is now recognized as a previously undocumented cause of death. The Coast Guard is funding a study to isolate and investigate this hazard.


The green wire can be tested and indicate continuity but be unable to safely carry enough current to trip a circuit breaker during a fault. There are ways to check the quality of the connection.


An Ohm meter test may show very little resistance in a green wire installation, yet the wire may be incapable of carrying 30 amperes or the higher currents needed to trip a circuit breaker during a fault. The minimum resistance reading of an Ohm meter will not necessarily indicate if a connection is compromised, such as a connection making to only a single strand of wire. There are specialized ground resistance testers that apply significant current, but they are uncommon. Careful visual inspection of the grounding connections helps, but even a careful surveyor may have a hard time finding all connections and tracing the wiring path.


One way to test the green wire connection quality is to connect a spot light or other heavy 12V load, positive to the boat's battery, and the negative to the safety ground pin of the shore cord. In a properly wired boat, the safety ground pin should return to the battery negative after first connecting at the AC panel. If the light burns bright and steady, there probably is a good grounding system. This is a good check to perform if a boat has an unknown maintenance history, has been rewired, or is being repaired after damage.
Fourth, your friend really needs to get a proper marine AC shorepower panel. They're not all that expensive or difficult to install. I have recommended this one for a lot of small boat setups:


Photo courtesy of Sailorsolutions.com, click to see product there.

The reason for the double breaker is to protect the boat from having a hot 110 VAC connection when you're not expecting it. If the shore power post has reversed polarity, a single breaker setup will not interrupt the 110 AC coming in on the neutral line and a serious risk of electrocution then exists.

While many boats don't need a galvanic isolator, the double AC breaker and the AC ground to the DC ground are both NECESSARY.


Quote:
Originally Posted by mitiempo View Post
A friend with a Spencer 35 and I (CS27) were discussing whether to bond through hulls or not. The concensus seems to be not to and I agree. In our research we came across info on galvanic isolators. After some reading we found that the green ground wire of an ac system on board should be connected to the boat's negative ground - negative bus and ultimately the engine ground. His boat doesn't have this connection and neither does mine. The ac system on my boat was by all accounts professionally installed with Blue Seas ac panel and tinned marine wire etc. All ac outlets are gcfi - 3 with a 15 amp breaker for each one. The shore charger is on its own breaker. The main breaker is the familiar double to break both hot and neutral and show reverse polarity. My friends Spencer 35 has a more basic system with a household box and no main double breaker but he does have gcfi outlets. Everything that we read explains ABYC regulation of grounding the ac green and goes on to explain the need for the galvanic isolator. My question is what is wrong with not grounding the green to the engine block and using gcfi outlets. It obviously works and has for many years. In my reading it seems that by connecting ac ground to the engine block and therefore to the dc ground you are inviting more electrolysis issues. What does everybody else think of this?
Brian
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Last edited by sailingdog; 05-18-2009 at 01:03 PM.
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Old 05-18-2009
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I had a case, albeit in a house, where the guy installed a gfi breaker and multiple gfi plugs on the circuit. He also wired one backwards which is interesting to figure out when the circuit breaks. It was very interesting to figure out what was going on since the plug that blew as the last in line in the kitchen and the ones not working were in the dining room and it took be the better part of a day to discover what circuit they were one. Since the one in the kitchen was wired backwards it was live and those downstream were off.

Since the guy who wired yours obviously didn't know what he was doing, I would check to make sure each of the plugs is wired correctly. If they are wrong, line and load reversed, it won't protect against a short. It might also be good idea to remove the redundant plugs.
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Old 05-18-2009
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Thanks for the replies and advice. The gcfis are not redundant as each ac outlet is on its own breaker and therefore one will not protect the other. I installed them myself when I bought the boat as there were none. They are installed correctly. One outlet=one breaker. I agree my friend needs a proper ac panel - I have the Blue Seas with the double main w/polarity indicator and 6 additional breaker positions - three for the three ac outlets, one for shore charger and two spares.
As far as the green ac ground to dc negative, I'm not sure. It seems to invite issues as well as solving one. How does stray ac current enter the dc system if there is no connection between the two? Other than a bad shore charger I can't think of any as there is no other connection to my knowledge.
Still looking into this.
Brian
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Old 06-03-2009
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"ABYC E11.17.1.4: The shore-grounding (green) connector is connected, without interposing switches or overcurrent protection devices from the shore power inlet to -an optional galvanic isolator, and then to -all non-current carrying parts of the boat's AC electrical system, including -the engine negative terminal or its bus."
That's what any surveyor or insurance inspector will be looking for.

The engine is assumed to be electrically connected to the propeller shaft and therefore grounded to the water, which provides the safety ground.

What I often find when working on an AC system is that although the AC green wire is connected to the engine negative terminal, the owner has recently installed a propeller shaft flexible coupling made of plastic or rubber (ie PYI, Globe, Vetus, etc) , without also installing the optional grounding connection through the coupling. Therefore, although the AC green ground wire is connected to the engine, the system is still dangerous because the engine is not grounded to the water through the propeller shaft and therefore the AC safety ground is not actually functional.
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Old 06-06-2009
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Further to my last post, here's a good explanation from the Blueseas website regarding the need for the green wire connection to the engine DC ground for safety and for minimizing galvanic corrosion (as I suggested above, it assumes that the engine is electrically connected to the propeller shaft)

Ten Deadly Conditions to Check for in Your Boat's Electrical System - Part 1 - Resources - Blue Sea Systems
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Old 06-08-2009
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My understanding..

The idea behind the green wire in an AC system is that anything which you could get shocked on (such as a metal chassis or the DC ground system in the boat) is electrically connected to "green" earth ground. If somehow, the AC hot wire gets accidentally connected to something which you could get shocked on, and the green earth ground is implemented, a lot of current will immediately flow and the fuse supplying the current will blow and remove the hot AC. Without the green wire protection, if the hot AC wire accidently got connected to for example your boat DC ground, no fuse would blow and you would have a safety risk and possibly significant because your near water. You may not even know everything on the boat is "hot" since things still seems to work fine (except maybe if your prop is connected to the ground then you might notice the electrolysis boiling action going on under the boat until the prop soon turned to dust)

GFCI is different but also needs to have the green ground wire to work. Normally an AC load has all the current flowing in the hot and neutral (black and white for 110) wire. The GFCI is checking to make sure the currents are equal in the hot and neutral. If for some reason even a small amount of the hot current is diverted to Earth (green) ground, the GFCI will very quickly trip. An example of how this works would be if your barefoot standing on very wet lawn and are using an AC weedwacker - possibly a beer in the other hand. If the AC cable was frayed and you touched the hot wire, some of the hot current would be diverted to the wet ground your standing in though your body. Since the hot and nuetral currents are now not equal, the GFCI outlet would trip removing the hot AC. Seems to me lawyers are partly responsible for GFCI being needed in a boat as the only time I can see GFCI doing anything is if you have AC shore power plugged in, your boat is sinking so filled with water and you standing in the water touching the AC hot wire at the same time. Why is the boat sinking - maybe you just took a direct lightning strike just before you decided to grab the hot AC wire. As correctly stated already, GFCI outlets have a input (line) and a load outlet. Any AC outlet or device connected to the load line is protected by the single GFCI outlet.

At the marina, the green earth ground is likely connected to the neutral line back at some breaker box and also, the green wire is actually at earth potential because its connected to some sort of underground metal structure. So the green wire at the breaker box is likely also at the same potential as the water.

If everyone wired boats using the marina AC perfectly, all the AC current would be carried on only the hot and neutral wires - there would be no current on the earth ground wire and it would always be at the same potential as earth and water. But, you can easily use the green earth ground wire to carry current similar to the neutral wire since it is likely connected to the neutral wire back at the marina switch. So its an easy mistake to make as everything seems to work fine and you still actually get the safety protection of the earth ground wire.

But.. since the green earth wire is now incorrectly carrying AC current back to the marina fuse box, there will be a voltage drop on the green wire "net" because the wire has some resistance (ie, current and resistance = voltage drop). If you have a slip that lives somewhere on this net, because the current flowing in the green wire and the induced voltages, your outlet green wire will NOT be at the same potential as the water. So if you connect the earth green wire to structures on the boat which are in contact with the water, you have created an electrolysis problem because the green earth and the water are not at the same potential.

The simple solution is to put in the diode isolator which allow up to about 1.4 volts difference between the boat DC ground and the AC earth ground and you "hope" that your slip AC Earth ground has less than 1.4 volts difference to actual earth potential. But you still get the same safety protection because if you accidently short the AC hot wire to the boat ground, the diode isolator becomes forward biased (i.e., turn on), lots of current flows and a breaker trips.
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