AC DC grounding

[gonzo]

One possible issue is the potential between the neutral on the AC and the DC ground. Since they are not connected, there can be a significant voltage between them. I think that sacrificial anodes are disposable items. If they are wearing out it means there was a problem and they did their job. Saving anodes, in my opinion, is less important than protecting your life.

 

[fallguy]

One possible issue is the potential between the neutral on the AC and the DC ground. Since they are not connected, there can be a significant voltage between them. I think that sacrificial anodes are disposable items. If they are wearing out it means there was a problem and they did their job. Saving anodes, in my opinion, is less important than protecting your life.

I keep the engines out of the water, ftmp.

But the issue of potential between the grounds is surely the reason for the connection.

 

[rangebowdrie]

I keep the engines out of the water, ftmp.

But the issue of potential between the grounds is surely the reason for the connection.

The idea behind connecting the DC ground bus to the bonding is to bring everything to zero potential,
and then to let anodes take care of any stray currant.
It's basically a "save the metals" scheme, given that today's boats are so electrically active and generally have a wide range of metals at all places on the nobility scale.
Sort of trying to bring a "pea soup" of potentials together and let the anodes "take the fall".
Connecting AC and DC grounds together is to protect people.
However, if AC and DC conductors are bundled together, with the AC conductors carrying currant there can
be a voltage present on the DC conductors due to "transformer effect", the DC wires acting as the secondary winding in a transformer.
It's small, but can be detected, similar to the "phantom power" in some amplifier circuits.

 

[fallguy]

The idea behind connecting the DC ground bus to the bonding is to bring everything to zero potential,
and then to let anodes take care of any stray currant.
It's basically a "save the metals" scheme, given that today's boats are so electrically active and generally have a wide range of metals at all places on the nobility scale.
Sort of trying to bring a "pea soup" of potentials together and let the anodes "take the fall".
Connecting AC and DC grounds together is to protect people.
However, if AC and DC conductors are bundled together, with the AC conductors carrying currant there can
be a voltage present on the DC conductors due to "transformer effect", the DC wires acting as the secondary winding in a transformer.
It's small, but can be detected, similar to the "phantom power" in some amplifier circuits.

If I keep the engines out, but the raw water intake is exposed; should I add a wired anode?

 

[fallguy]

The GFCI only works if the AC and DC have a common ground. That is the reason houses have grounding rods that are connected to the AC ground.

Is this valid for certain? It seems like probably not..

 

[rangebowdrie]

Is this valid for certain? It seems like probably not..

NO, it's not valid

 

[missinginaction]

I am looking for a definitive answer on grounding and bonding and the AC and DC system.

My boat is a twin outboard powercat. I have one raw water bronze intake below water.

The main question is whether or not I am required to tie the AC and the DC ground bus. I was told by surveyor it was required, and so I did, but I got some flack recently on a Facebook forum for saying so.

My boat has a galvanic isolater and iirc, the ground off the iso heads to the AC distribution box and then I tied that node to the DC ground bus as per surveyor. Someone said the risk is that I could run AC short to ground back through the DC ground bus.

On a side note, I tied the bonding system to the DC ground buss as well.

Did I do anything wrong; despite the surveyor requirements?

I do have @Ike handbook, and plan to read it again..later today.

I love these discussions. There are so many variables and differing standards. European ISO standards differ from what the ABYC requires for American boats. I find it fascinating. We take electricity largely for granted. Yet, some of the best minds in the business differ considerably in what they consider safe.

To answer your original question.....

1. In the United States you are required by ABYC standards to connect the AC grounding wire(s) (green) and the DC grounding wire(s) (generally yellow) to the common grounding bus onboard your boat.

2. As for the galvanic isolator, the advice that the surveyor gave you sounds correct. For clarity's sake the GI should be "connected in series with the shore power cable AC grounding conductor in a manner that no other ground conductor will bypass the isolator back to the shore power ground". The quotes are from the ABYC. All this means is that the GI should be installed on the incoming AC grounding wire directly downstream from the shore power inlet. Nothing should be connected to the shore power side of the isolator except the shore power grounding wire. Simply put GI's are just 4 diodes. They were (and still are) designed to block DC current that can cause corrosion but allow AC current to pass through the grounding cable in the event of a short to ground. The person who told you that your GI could allow an AC short to pass through your DC bonding cabling is right in theory. This was addressed back in 2002 and the rules strengthened over the years through ABYC standard A-28. Standard A-28 requires specific properties that must be incorporated into any GI meeting the A-28 standards. While too lengthy to get into here, GI's meeting A-28 standards do a good job at safely blocking galvanic currents while allowing AC fault currents to pass.

To sum up, I don't think you did anything wrong and your surveyor was correct in the recommendations he made based on what I'm reading here.

Like many things electrical there are differences between ABYC standards and European ISO rules. That's a story for another day though.

MIA

 

[fallguy]

NO, it's not valid

All good, I was hoping

I love these discussions. There are so many variables and differing standards. European ISO standards differ from what the ABYC requires for American boats. I find it fascinating. We take electricity largely for granted. Yet, some of the best minds in the business differ considerably in what they consider safe.

To answer your original question.....

1. In the United States you are required by ABYC standards to connect the AC grounding wire(s) (green) and the DC grounding wire(s) (generally yellow) to the common grounding bus onboard your boat.

2. As for the galvanic isolator, the advice that the surveyor gave you sounds correct. For clarity's sake the GI should be "connected in series with the shore power cable AC grounding conductor in a manner that no other ground conductor will bypass the isolator back to the shore power ground". The quotes are from the ABYC. All this means is that the GI should be installed on the incoming AC grounding wire directly downstream from the shore power inlet. Nothing should be connected to the shore power side of the isolator except the shore power grounding wire. Simply put GI's are just 4 diodes. They were (and still are) designed to block DC current that can cause corrosion but allow AC current to pass through the grounding cable in the event of a short to ground. The person who told you that your GI could allow an AC short to pass through your DC bonding cabling is right in theory. This was addressed back in 2002 and the rules strengthened over the years through ABYC standard A-28. Standard A-28 requires specific properties that must be incorporated into any GI meeting the A-28 standards. While too lengthy to get into here, GI's meeting A-28 standards do a good job at safely blocking galvanic currents while allowing AC fault currents to pass.

To sum up, I don't think you did anything wrong and your surveyor was correct in the recommendations he made based on what I'm reading here.

Like many things electrical there are differences between ABYC standards and European ISO rules. That's a story for another day though.

MIA

Thanks to you and all commenters on this thread. I am starting to understand the arguments.

 

[Ike]

Persons who are not familiar with marine electrical systems often get confused by the terms "Ground" and "Grounding Wire".
In a three wire AC system the term Ground is talking about the white wire which is also called the neutral wire, and it is the normal path for current to return to the source. The term Grounding wire refers to the green wire, the safety wire, which normally has no current flowing in it. It is only there to provide an alternate path of least resistance back to the source of power if there is a ground fault in the neutral.

If the green is connected to the boats ground point (usually the engine block) there is the potential for DC to leak into the green wire. That's what Galvanic Isolators are for. They pass AC but block DC. Or if you prefer, an Isolation Transformer accomplishes the same thing.

 

[missinginaction]

I'm not so sure that a "Definitive answer" exists.
Various answers exist depending upon each boats construction, wood/fiberglass/metal, (what metal).
As well as what metals are underwater.

"The main question is whether or not I am required to tie the AC and the DC ground bus."

Your surveyor is most likely basing their judgement on the ABYC recommendations.
That recommendation is one designed to save people from being electrocuted, (you are laying across a defective water heater and grasp a seacock).
I went a different route.
EVERY device that uses AC in the boat is wired thru a GFCI outlet, (breaker to line/load to device,) and also GFCI protection on the dock power.
I believe my system is superior at protecting life and limb.
But anything I say will not appease your surveyor.

This is IMO the best way to go Rangebowdrie and I'm glad that you brought it up.

Back in 2007, one of the first things I did as I began the restoration of my little cruiser was remove and trash the entire DC and AC electrical system. I kept the engine harness, but that was it. Back then I had only a rudimentary knowledge of marine electrical systems. Even so, I cringed when I saw the Square D box and the household AC breaker. The DC grounding bus consisted of a machine screw with 20 to 30 14/16 AWG wires stacked on it and maybe a 10 gauge wire running back to the engine block. No wonder insurance companies are balking at writing policies on these older boats. There were a few outlets that someone had scattered around. These were wired with Romex of course.

So early on, as I was doing stringer replacement, fiberglass and woodworking I studied at night. This was the textbook that I primarily used.

https://www.amazon.com/Boatowners-M...t=&hvlocphy=9004606&hvtargid=pla-333901114316

In his book (page 78) Mr. Calder discusses what he calls "An inverter based (DC based) boat". Studying his book, eventually this is what I built for my small cruiser.

Shore power comes onboard through a standard 30 amp main breaker (Blueseas #8077). I also use a portable inverter/generator, a Honda EU1000i which supplies AC power by the same route.

Shore power travels a couple of feet from the main breaker and runs through this: https://www.gordonelectricsupply.co...i8S2hb6XUhAtWxpHT65MCj40hw_hT_jcaAhukEALw_wcB

A Xantrex TrueCharge2 40 amp charger is connected a few inches downstream of this GFI switch/outlet combo. The charger is protected by the GFCI via the switch which turns it on or off as is the outlet.
That's where the shore power cabling ends.

Since the TruCharge2/40 is an isolation transformer based charger, there is no physical connection between the primary and secondary windings in the charger.

DC cabling exits the charger to a Blueseas disconnect and then on to the batteries. I do use marine rated battery terminal fuses where the charger cables connect to the batteries.

2/0 cables connect the batteries to a Xantrex Pro 1800 watt modified sine wave inverter.

The AC inverter output is entirely contained on the boat. There are eight good quality duplex outlets, all protected by the GFCI that is built into the inverter. Since there is no physical connection to the shore power circuit, all AC wiring on the boat originates and terminates at the inverter. It doesn't matter that the batteries are working on their own or are being charged by shore power, or are being charged by power from the Honda generator. The inverter is the power source for the boat. The shore power circuit or generator supplies power to the battery charger only (via it's isolation transformer) and is not connected to the boats wiring in any physical way. I'll post a photo of the system below that was taken as I was laying the components out. The only change is to the switch that controls the charger.



The ABYC says that I must ground the battery charger to the main DC grounding bus. But there is no physical connection from the AC side of the charger to the DC side. Since the charger is directly downstream from a GFCI and wired in series with it, I can't figure out how grounding the case protects me in this instance.

The inverter is also supposed to be grounded. But since all outlets on the boat are GFCI protected, I can't think of a situation where the inverter case to the DC grounding bus would offer anymore protection than the GFCI alone.

The other aspect of all this is that while the ABYC requires the charger case and inverter case be connected to the main grounding bus, in Europe the International Organization of Standards (ISO) does not require these AC to DC grounding connections to be made. This is provided that the boat be protected by a RCD (residual current device) typically an ELCI (equipment leakage current interrupter).

GFCI's are also a type of RCD. I look at GFCI's as much more protective than ELCI's as the GFCI will trip much quicker at a much lower leakage. The reason ELCI's were developed is that many boats had too many issues with devices with low leakage thresholds (GFCI's) nuisance tripping.

For now my charger case and inverter are connected to the common grounding bus. I haven't noticed any issues with galvanic corrosion as a result. I may decide to disconnect those AC to DC grounding wires in the future. We'll just have to see.

By removing those AC to DC grounding wires I'll pretty much eliminate the possibility of any galvanic corrosion issues when connected to shore power while not compromising safety in any way.

FWIW I agree completely with Rangebowdrie's statement.

Regards,

MIA

 

[fallguy]

Case grounding is done so if the charger has DC potential to the case via any internal fault not sensed by the AC connection (the mystery) it will short before a human hand touches it, afaik. But that wasn't my question.

I got into a few Facebook and forum arguments with people claiming it is a mistake to combine the bonding, the AC, and the DC grounds.

Once upon a time I bought an outboard in Canada. My brother told me make sure you don't tell them at the border. I got home and applied for my refunds of gst and the sheet said 'attach export documents'. Of course, I had none, so called the border and they asked me why I didn't declare the purchase. I told them my brother told me not to and they laughed and said my brother cost me $300 because the export documents were not and could not be done anymore. This issue reminds me of that somehow. One person says do and another says don't. My brother was right, but illegal, for a motor transacted between persons, but wrong for a dealer purchase where 15% taxes were paid and 7% was refundable.

I'm gonna leave the AC-DC grounds connected.

 

[Nidza]

Safety is not a joke! There is a sub-field of engineering regarding safety. It is present in aviation, automotive, medical, etc. industries to prevent all sorts of issues (e.g. from being killed by touching AC wall outlet, to being killed because "smart" car suddenly aplly full braking force at full speed on the highway because it "saw" a non-existent barrier on the road ahead).

There are two issues today:

1. An average guy with access to a screwdriver is often self-proclaimed electrician to overcome financial issues. I do not say that he cannot learn anything, but in this case lives could be or are at risk due to his phase of gaining experience (mind that accident can happen much later). In case of boats - most boats will go to someone elses hands at some point in time, the potential danger could be waiting behind the corner, maybe a new buyer, maybe your son/daughter.
2. The people hiring (self-procalimed) "experts" managing to persuade them to do something not safe to save money (e.g. using incorrect cable insulation in very specific environment conditions, etc.)

As experienced electrical engineer, I do not like giving installation topic advices to people outside of the profession, rather advise them hiring expert(s) due to responsibility and liability. (Expert=professional known with the field of application, not the most expensive option "per se"). And I have seen at least a few very risky "installations" on boats. The reasoning of people continue doing it and using it is: "Well, it works (and is cheap)".

Regarding the original question, I can say just the following - connecting of AC "earth" and DC "ground" is for safety, even though it is potential bonding issue (and this issue could be detected by expert research or omitted by expert design). Imagine just a few of the following safety issue scenarios on the boat that come to my mind:

• DC and AC cables are routed together and
  • Due to a incorrect choice of insulation, insulation becomes brittle and breaks and DC and AC wire(s) make a contact.
  • Even if cable insulation was selected correct, due to a vibration cables are rubbing aginst other (sharp) surface/edge, AC and DC wire(s) make a contact
  • etc.
• DC and AC cable are routed separately, but
  • Due to a vibration, AC cable is rubbing against other (sharp) surface/edge, AC wire(s) make a contact with any metalic object in proximity which is connected to DC system (ground or positive) or even metallic object not connected to anything, but accessible to a human body, e.g. hand-rails
  • Due to a vibration, AC cable is rubbing against other (sharp) surface/edge, does not make a physical contact to any object in proximity, but it is located in a humid (salty) area which is conductive, e.g. making contact to bilge water and from there you have many options - bilge pumps connected to DC, seacocks, etc.
    
  • etc.
• DC and AC instalation done "perfect", but fault happens to be inside equipment, e.g. an inverter
  • Inside an inverter, AC makes a contact with inverter casing/housing. If "earth" conected to casing, the circuit breaker will pop-out, otherwise, AC is present on casing. But if casing is connected to DC ground and not to AC earth, AC is then present on any DC ground surface on the boat.
  • Inside an inverter, AC makes a contact with DC ground circuitry. If AC earth connected to DC ground, circuit breaker will pop out, otherwise, AC is then present on any DC ground surface on the boat
  • Inside an inverter, AC makes a contact with DC positive ground circuitry. If AC earth connected to DC ground, circuit brekaer will probably pop out after any DC load is turned on. So, in this case, it depends of the whole installation/"system" what happens if you touch positive DC side, but again, it is much higher probability of touching DC ground which is accesible by human on many places cmopared to DC positive.
  • Initially, it was installed very expensive inverter with covered all safety features, and the whole boat was designed by professionals/boat builders, passing all possible tests for the "purpose of certification" = to be safe for the user. Now the boat is second/third/... hand and inverter died and is exchanged with cheap version which was never required to pass specific safety test for the specific environment.
  • etc
• etc.

So, in general, if some rule is given for safety and no matter if you understand it or not, and you are not an expert and still want to do it yourself, I advise for following the safety rule. Even though you maybe come to a bonding issue then. By solving introduced problems, maybe you also solve some life threatening issue which would have been overseen otherwise.

By the way GFCI and RCD is basically the same thing, just in EU common name is RCD, while in US common name is GFCI. It is better to have different names from the safety perspective because each one is optimized for the specific AC network 230V/50Hz vs 120V/60Hz. See, the safety could be hidden just in an acronym, too.

And well, remember, boat is just a boat, but life is life.

 

[missinginaction]

We're all in agreement regarding the AC to DC bonding issue onboard the boat but I do it reluctantly. Here is my issue with all this.

We have two main governing bodies that set the rules. The ABYC and the ISO. One based in the US and one by definition an international body.

Regarding the AC to DC grounding onboard we have two contradictory rules.

The ABYC says that regardless of how many safety devices are installed on the boat, the cases of AC equipment onboard must be connected to the main DC grounding bus.

The ISO says that so long as there is a RCD built into the system, typically an ELCI but it could be another device such as a GFCI as Nidza points out, the AC case grounding connection is not required.

In addition the ABYC is the representative of the United States at the ISO.

Does the ISO value life less than the ABYC? I don't think so. Then why the difference in rules? We need to all be on the same page.

That's my point.

Have a great day!

MI

 

[Nidza]

OK, I see your point, I missed that one in the your message since it was very long post, I apologize (it was mixed message about standards and description of your system).

Well, safety is about calculating the percentage of risk, and as you know, it can never be zero. If it would have to be zero, e.g. cars would not be manufactured because they can kill humans (you cannot say there is 0% risk that car will kill a human). Therefore, the logic is what is the highest percantage of killed humans by car to be acceptable by the society. It sounds awful, but that is how the metrics can be given to safety risk calculation when you design the specific system. Now how low number is acceptable is not always the same for everyone/every society (different apporaches/politics/laws/cultures).

So, one can add the safety feature to the system (i.e. boat AC electrical installation), e.g. to prevent human death in case system (AC boat electrical installation) fails. For some that is sufficient. For others, they probably wanted to also add additional safety in case primary safety mechanism fails (they wanted to increase the safety or i.e. to decrease the safety risk probability).

So primary safety feature for the boat electrical system, in case AC system fails is ELCI/GFCI/RCD. Additional safety mechanism can be added to the system in case ELCI/GFCI/RCD fails (and it is plausable on boats, due to unfriendly environment, especially if houshold equipment is used, which often is in case of AC systems on boats => often without required IP protection for the given conditions).

Theoretically, if you have RCD, as soon as current difference between hot AC wires i.e. leakage is detected, it will trip and prevent the presence of AC on the affected component and protect your life. In case RCD fails, then at least, the circuit breaker/fuse will react and prevent fire in addition (fuse most often has too high current rating to protect human life). Now, where you could go into philosophy is if RCD fails, is it the higher probability that circuit breaker/fuse will be activated first or human will be killed first. And the question can extend do you want to protect the person or also the asset? Then you come to the discussions about the law, insurance, etc. By reading different materials, I have a feeling that USA is much more stringent than EU regarding insurance and asset protection in general and that could also be the reason for more stringent rule(s).

Mind that adding more and more features is rising the costs, not only by adding equipment, but also by doing the math about these probabilities. Of course, in this case where safety feature is adding simple connection between AC and DC, I personally am for more conservative US approach (also I do consider that it is higher probability that fuse will trip before I would be killed - most often when you arrive on the boat, you first enable something/whatever by the means of an insulated switch). But, if you ask manufacturer who needs to manufacture e.g. milion pieces of something, this additional task is really adding to the costs (the labor, the materials, etc.), therefore, manufacturers would probably select ISO if possible.

Oh, and let's not forget the interpretation of sentences in the standards, because they can have legal consequences - e.g. in case of ABYC it is required so if you do not add it you are legally responsible if the second mechanism was lacking or fails, while in case of ISO it says not required, but you are allowed to add it and you will not be legally responsible even if the second machanism fails, but still your reputation could be on the stake. It could be a million dollar question for some or one hour of easy work for one private person, do you agree? So, what to do becomes a very specific question for the person who is asking that question.

And yes, have a great day!

 

[rangebowdrie]

Perhaps this is not essentially germane to the discussion, but to address some context.
The ABYC dates from 1954, when the vast majority of boats were still of wooden construction and in sheer numbers the production was quite low.
In those days the entire electrical system of, say a 30'>40' boat might consist of some nav lights, a few cabin lights, a bilge pump, and if the owner was financially able, perhaps a depth sounder and some sort of radio.
We also need to remember that almost without exception all of the underwater metals and much of the interior metals were of high nobility on the galvanic scale, ie. bronze/copper/Monel.
Having an AC system on board was quite rare except for a few outlets and a battery charger, and many boats just used an extension cord brought in thru a portlight from the dock to charge the batteries.
With rudimentary systems and little electrical activity on board there was no need for complicated grounding/bonding schemes, and all the underwater metals existed quite comfortably with each other, (being all the same,)
As electrical systems proliferated, it was inevitable that ground-loops/radio grounds/reverse polarity issues started popping up.
Then the "bright idea" of bonding all the underwater metals would solve everything, WRONG.
What happened next was the mysterious signs of de-lignification of the wood around the seacocks and other hull penetrations.
The answer from the "experts", was to add zincs, and more zincs, WRONG again.
The addition of zinc only acerbated the destruction of the wood and OVER PROTECTED the bronze which if left alone DOES NOT need any protection.
Then as non-copper-based alloys started being used underwater on a larger scale, again the use of zincs increased, (they needed anodes to protect the inferior metals).
Fast forward to widespread AC use and people getting shocked.
The answer, connect the AC grounds to the DC ground, (engine>shaft>water).
But that wasn't enough, what if someone touched a seacock?
The answer, bring back the bonding, connect everything to the water.
Then there is the issue of ESD, which took years to even recognize, (it's much more common in fresh water).
So, now we have the means to prevent onboard shock hazard and "in water" shock hazard.
It's the RCD/GFCI devices, as well as isolation transformers, and used properly not only can they eliminate the AC ground to DC ground connection, but the bonding scheme becomes superfluous, and with it the episodes of stray currant corrosion.
At that point individual anodes can be used to protect ONLY what is necessary, (bronze prop on stainless shaft is an example).
Yes, the ABYC is a good org, but they are hidebound and behind the curve on some issues.