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Bato & Co avatar image
Bato & Co asked

Boat grounding and mass

Hello there !

Forgive my "french" english but I'm hoping to find an answer here.

I am working on a 1908 Steel boat, doing everything. I plan on using a Multiplus II 24v, a Smart Solar 150/60, Orion Tr Smart to charge the service battery via the alternator, galvanic isolator, with all the fuses and protection needed, and a Cerbo GX.


My questions are : Do I need to use the hull of the boat as a mass for all of my DC current? To share an equipotential point or is it better to isolate the service (lights pumps etc) and having the motor side connected to the hull (connected to the - of the starter motor so the hull).

I know that in the US it is mandatory to connect the hull with the negatives of the batteries but it isn't in France and I wonder why...


About the AC current, with the shore power, my earth/ground is provided by the shore, I get it. But when sailing, where is driven the current if a fault occurs? I know that the multiplus has a Ground wire, is that it? And so, this Ground wire has to be connected to the hull or to my negative Busbar? I read about the ground relay that switch to the neutral, might be a stupid question but should I connect a neutral wire to the hull/negative busbar?


I tried to find a solution on the internet but I read something and then the reverse 3 min later... I plan to live on this boat with my wife and my son and I want it to be as safe as possible.


Thanks for your time, I am really worried about this.

Baptiste

Multiplus-IIGroundingbusbar
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Trevor Bird avatar image Trevor Bird commented ·
@Bato and Co just a quick question. Will the boat be predominantly plugged into shore power at a marina or will the boat be mainly at anchor or on a swing mooring? Will you be able to live only on solar power? If you can exist only on solar power and shore power is not necessary, your design will of course be simpler. For instance on my boat we can live using only solar if needed.
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kevgermany avatar image
kevgermany answered ·

Multiplus automatically connects earth and neutral when off grid. The casing earth is for safety and must be connected to the same earth as AC and the DC system.

I can't answer why US regs require a hull connection. But seems safer as it limits potential build up between electrics and hull.

Boats where I am will run no current through hull. But do have good anodes. Single connection from electrical system to hull. This avoids earth loops.

Would be a good idea to have a way of checking the polarity of your shore power and a reversing cable for correcting faults.

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Bato & Co avatar image Bato & Co commented ·
@kevgermany Thank you for your answer. So my ground in the boat has to be connected to the hull somewhere to allow the multiplus to use the relay when offgrid right?

What do you mean by "single connection from electrical system to hull, this avoids earth loop"?

Yes, shore power in some marinas are bad or old, many times neutral is wired on the left but you're never sure

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kevgermany avatar image kevgermany ♦♦ Bato & Co commented ·

The ground relay in the multiplus will always work off grid, whether or not it is connected to the hull.

Ground loop is a circular connection in the earth circuits. Any AC or intermittent current will cause a current in it. Best to bring the earth from each device to a single point (busbar) without any interconnections, then to battery negative, then to ground/hull. If you're familiar with computer networks, think of it as a star topology.

Best to have house battery and starter battery connected through the negative terminals. Keeps both batteries at the same potential.


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Bato & Co avatar image Bato & Co kevgermany ♦♦ commented ·
Alright, I get it. You've answered my questions, I've checked in some boats around me (some with multiplus and other without) and they've all connected their ground to the hull.


And as I thought, better to have same potential with the batteries. Thank you very much, you've helped me twice !

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Paul B avatar image
Paul B answered ·

There is no one answer here, and the below is not a victron comment or suggestion

steel/ alloy have plenty of corrosion issues, basically as far as I am aware its best NOT to have the battery negative connected to the hull,

now a AC earth thats a different matter, and you must comply with the rules of the country.

A isolation transformer is by far a better solution than a galvanic isolator as its physically issolated by air gap.

i would have a read of this site as a reference point https://www.seabis.com/

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Bato & Co avatar image Bato & Co commented ·
Thanks for your answer, I get it but the motor battery is connected to the negative of the starter, which is automatically linked to the hull, the question is if the motor batteries are connected to the hull, should the service also be connected?


Isolation transformer cost much more, I was hoping that the galvanic isolator is able to do the job.

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Paul B avatar image Paul B Bato & Co commented ·
again please read the link, on a steel and alloy hull even the motor battery should not be connected to the hull.

even though it currently is this does not mean its correctly done.


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kevgermany avatar image kevgermany ♦♦ Paul B commented ·

@Paul B

That's a very amateur web site designed to sell a current detector. It set my snake oil detector on high alert. It does not say 'do not have a safety connection to the hull'. It is also factually incorrect in places, for instance effectively denying corrosion in one boat being due to faults in other boats connected to the same shore power source.

The pictures should be a warning to you. 12 months in the water with no fouling?





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Trevor Bird avatar image
Trevor Bird answered ·

@Bato and Co The reason you see different versions of what is “right” is because it is a complex subject and more than one answer can be correct. The difference is the compromise for one or the other.

It is like asking if the underwater metal of a fibreglass boat should be bonded or left isolated. Many are bonded but I own a Jeanneau 42DS that is isolated. I can argue passionately for either as each has advantages and disadvantages. That conversation is beyond the scope of a forum reply however.

If I owned a steel or aluminium hulled boat I would isolate the hull from all electrical potentials. You need to understand the pros and cons but I would research the Alubat Ovni as a boat that in my humble view sets best practice in this area.

It is hard to provide full guidance in the space that is reasonable on a forum reply but please understand that strangely more than one answer can be right in this instance.

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Bato & Co avatar image Bato & Co commented ·
@Trevor Bird Thanks for your answer. Earthing in boat is strangely a difficult concept.

In my case, about DC current, the motor batteries are linked to the starter motor and the motor is linked to the hull. Changing it would be a big work. I can still isolate the house batteries and wiring from the hull but I can't for the motor batteries.

As for the AC current, while sailing and producing 230v with the inverter, if I don't connect the green cable to the hull, where would go the current if a fault occurs?

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Trevor Bird avatar image Trevor Bird Bato & Co commented ·
@Bato and Co . Inverters have a ground neutral link relay. This is engaged when the inverter go into “inverter mode” as opposed to simply passing the voltage through from the dock. When the inverter has the ground neutral relay engaged, fault current flows back through the ground wire to the inverter. When you are away from the dock, no actual real “ground” reference exists unless you create it. The hull of the boat has no reference whatsoever to the output of the inverter so if you choose to keep it that way that is fine. Simply treat the AC loads as “appliances” and do not allow them to touch the hull.

This is important - install an RCD on the output of your inverter so you know if you have any leakage back through your ground wire to the inverter. If you do that any ground leakage will cause the RCD to trip. That is the way I would handle it.

By the way, many production boats separate mains earth from DC earth. By way of example, Jeanneau yachts ( European Boat) do not connect mains ground to DC ground at all.

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Bato & Co avatar image
Bato & Co answered ·

I have made my own schematic using Victron's schematic (thanks a lot btw) ! Which I modify along the way.

First one is house batteries isolated from the hull. And motor batteries and earth connected to the hull

circuit electrique olse 7.0 house isolé.png

Second one is house batteries and motor batteries connected to the hull and earth connected to the hull

circuit electrique olse 7.0 non isolé.png

I'm still not sure what to do with these connections...


circuit-electrique-olse-70-house-isole.png (1.0 MiB)
circuit-electrique-olse-70-non-isole.png (1.0 MiB)
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kevgermany avatar image kevgermany ♦♦ commented ·
Quick comment. I'm not going to argue hull connections, except to say that my previous comment was about inland boats in fresh water with steel hulls.I bow (pun intended) to @Trevor Bird experience. Worries me as my boat is powered by an AC motor connected directly to the hull.


You show 120V shore power going into a 240V system, guess this is a typo.

If I understand correctly your concerns are the multiplus case earth and the AC earth. AC earth will go to each socket/appliance and to the boat side of the isolator. Optionally to hull and there isn't a clear answer. But it seems better to omit the hull connection unless there's a legal requirement. Then there's the case earth. This is a safety earth, must be connected and an answer is outstanding. The only option is to the AC earth circuit.


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Trevor Bird avatar image
Trevor Bird answered ·

@Bato and Co ...nice drawings. All boaties need to be concerned about corrosion. Plenty of information exists about all different types of corrosion but for us we really need to be concerned about two types, Galvanic corrosion and Leakage current (so called electrolysis) corrosion.


Galvanic corrosion occurs when two metals of different types are immersed in an electrolyte (salt water). Your steel hull if not protected will corrode. It is a natural process, it perfectly normal, the currents involved are low but persistant and it is well understood. That is why we use sacrifical anodes so that expensive metal bits do not eat away. The cheaper sacrificial anodes eat way and we periodically replace them so they can do their job again. The electrical potential causing the current comes from the dissimilar metals in the electrolyte. They have a natural electrical potential with respect to one another as can be seen on the Galvanic Scale. The electrical potential is relatively small, the damage relatively slow because the current generated is quite low.

The other mechanism, leakage current corrosion (Electrolysis), occurs because of the battery bank you have shown in your drawing. The battery bank is powerful, the electrical potential is high, the leakage currents can be high and the potential for damage occuring very quickly is extreme.

Everyone prepares and counters the effects of galavanic corrosion. That is a given.

The ability to reduce the risk of leakage current corrosion rests on your design and that risk is multiplied many fold by having a metal hull. You are right to be concerned but do your homwork or engage an expert in this area to assist.

By the way, I know you are showing AGM batteries but for a new refit please consider Lithium batteries. They will change your life, particularly as a liveaboard




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Bato & Co avatar image
Bato & Co answered ·

@kevgermany my boat is also inland in fresh water. I corrected my schematic, it was a typo issue, it is 230v coming from the shore lol...

Here's what the legal requirement says :

legislation.png

source : https://www.plaisance-pratique.com/230-volts-a-bord-la-terre-du-quai

In theory, the multiplus is replacing the shore power while sailing so this schematic won't change because of an inverter in the system, right?


@Trevor Bird You might know that but in the boat community, everyone says they're expert but everyone's opinion is different, especially with this concept ahahah... I would love to use Lithium batteries but my lady is hard to convince, not about efficiency but about the price of Lithium batteries + the whole setup.

I'll isolate the DC current from the hull. And for the motor batteries, I'll install a power switch to positive and negative so that when the engine is not running, no DC current is connected to the hull via the motor batteries.


legislation.png (48.0 KiB)
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kevgermany avatar image kevgermany ♦♦ commented ·
Nice diagram. My wiring is going to be similar. You'll be putting the shore power into the multi, then from there into the boat. There's a marine electrician on site. I'll bounce it off him, when I did it before I got a very maybe answer. Very much in line with @Trevor Bird saying there are different opinions.
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kevgermany avatar image
kevgermany answered ·

Update. Based on what I was told. So may be an error.

UK boat safety certification for steel hulls requires

DC negs to be earthed to hull. Reason is to ensure correct operation of fuses etc. Hull should not be used to carry current.

AC must have earth connection to hull. Full RCD protection. Neutral connected to earth unless on shore power. No AC current through hull as per DC.

Shore power should have galvanic isolator on earth. Connection earth to hull on boat side of isolator. This can be same connection. Advisable to add RCD between shore power and multiplus etc. Risk of circular connection.

Batteries must have negative connection to hull, again no current carried here.

I'd welcome corrections/comments. But this is my understanding from the guy who will be doing my electrical certification.


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pwfarnell avatar image pwfarnell commented ·
I have 2 year old UK canal boat and this summary is correct and is how my boat is wired. The only difference is I chose an isolation transformer not a galvanic isolator for shore power.
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kevgermany avatar image kevgermany ♦♦ commented ·
Missed the case of the multi. This should be earthed to hull using the earth terminal. Safety. No current flows.
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Bato & Co avatar image
Bato & Co answered ·

@kevgermany Pretty much what I was going to do, in France you can isolate DC current but I don't get why a hull connection with negs battery is better for fuse operation?

Boat's law should be international, same for everyone with a clear way of doing things...

Negs to hull doesn't mean there's current flowing through the hull right? Kinda like "earth in AC"

With a shunt before negs battery it won't change the infos readen by the BMV if negs connected to hull ?

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kevgermany avatar image kevgermany ♦♦ commented ·
Fuses. Agree, but the explanation/reason I got was vague so I didn't and won't repeat it. Regulations were the real reason, explanation was to try and justify.

With negs to hull there's potentially some current due to induced voltages from other wires, especially AC, running close to the DC. Also from appliances switching on and off.

As for universal regs... Good luck. They won't harmonise them for cars, trucks, trains.... Why should water craft be any different?


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kevgermany avatar image kevgermany ♦♦ commented ·
Sorry, missed the shunt question. Batts should earth through the shunt. Might need to also earth the battery case, depending on construction of battery.
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Bato & Co avatar image Bato & Co kevgermany ♦♦ commented ·
I get it, but messing the fuse operation that's a funny answer ahah anyway

Yes that's a utopic dream indeed

well I've learned a lot, thank you very much !!

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Related Resources

MultiPlus-II Product page 

MultiPlus-II Manual

Multiplus-II GX Online Manual

MultiPlus-II 230V Datasheet 

VE.Bus Error codes

Ground, earth and electrical safety


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Additional resources still need to be added for this topic