AC green to DC negative buss ?

[genki]

I've been trying to figure this one out for years now, and have read a lot of threads on here that appear contradictory.

So... without wanting to start world war 4... let me ask this question.

First details of my AC/DC system.
I have shore power AC coming into the boat with the next hop being a Yandina Galvanic Isolator (yes I know it's probably not a fail safe)

Then the GREEN AC wire goes to the Negative DC buss. The GREEN AC wire terminates there. My Sterling Power battery charger has a "ground wire" and that also connects to the negative DC buss.

Then there is a DC black negative wire to the Alternator of the Beta Marine engine, from there the engine has DC connectivity to the prop (I verified this with a meter) and then the prop zinc.

Question
Should I move the AC Green wires onto their own and totally separate buss bar, with no connections to the negative DC buss bar to isolate them?
If so, can somebody explain why?

Thanks!

[Hartleyg]

Yes, you should separate them. The AC "Green wire" ground is a safety conductor that is there to allow fault current in the AC system a path that does NOT include the operator! While it is externally connected to an "earth" ground via the shore power connection, this does you little good in the event of something like a lightning strike, and if you connect it to the grounding electrode of your vessel, your vessel becomes part of the grounding system for the marina - something that does you no good at all, both from the standpoint of hazard reduction and galvanic corrosion - in fact, it makes you a magnet for galvanic corrosion induced by other boats around you.
The DC negative buss should include ONLY DC negative conductors - however, because the starter and alternator tend to connect the DC negative to the engine block, and the block tends to get connected to the seawater via the prop and shaft, confusion reigns! In addition, the stream of salt water flowing through the engine/heat exchanger/etc. also connects the block to the boats "grounding electrode".


The boat's grounding is there for three reasons - 1. galvanic protection ; 2. Lightning protection; and 3. radio (SSB primarily) antenna counterpoise.
1. Galvanic protection is why all the metal bits on your boat are connected together, and should include enough zinc (or aluminum depending on your water) to safely prevent electrolysis to these metals.
2. Lightning protection is there so that conducted or induced currents from lightning are allowed to go into the water without involving your sensitive electric/electronic stuff or your crew! Things like masts, shrouds and other large metal structures (railings, hull oarts, etc.) should be connected to this ground.
3. SSB radios need a "counterpoise" to the whip or insulated backstay that forms their antenna - a ground connection works well for this.


This all sounds neat and easy - BUT - things like battery chargers and inverters that connect their AC protection ground (case) to the DC negative can make your life difficult. This is where experienced marine electricians can help - the range of possible fixes eludes any general rule, unfortunately. Galvanic isolators are a good start, but there are other tools available.
In the case of your Sterling Charger, you may be relying on your isolator to protect you from potentially damaging galvanic currents - but I don't think that the charger actually connects the DC negative and the grounding terminal - you should check. I would definitely remove the green wire connection directly to the DC negative buss!


Hartley
S/V Atsa

[sailingunity]

Does it make sense to have the green wire on a switch then, so that it can be disconnected while on shore power, and reconnected while out and about?

[Hartleyg]

No, that wouldn't help - in your situation, the primary problem is that you might be providing "grounding service" to your marina, with potential for galvanic damage, or maybe just eating up your zincs in a hurry. I would definitely remove the unneeded direct connection from AC green wire to DC negative, however.



Your galvanic isolator might be all you need - the way to find out is to obtain a meter that will measure AC and DC milliamps (most good multimeters should do it), then open up the connection from your isolator to either the shore power cord, or the vessel, and measure the current (if any) looking at both DC and AC. If the current is a milliamp or two (or less), no problem - more than that, I would first check to see what the voltage drop across the isolator is (should be 600-800 millivolts AC). If its zero, your isolator is popped, and needs replacement!

Any current flowing on that lead (AC or DC) is affecting your underwater metals - eating your zincs or, worse, your expensive stuff like prop and shaft!


If the current is zero or close, no problem, you can live with it, though it's worth checking from time to time to make sure all is well.


Hartley
S/V Atsa

[genki]

Hi,

Thank you Hartleyg your comments make sense to me. I can see why I would not want the AC to share the DC negative buss.

But it's gets more confusing when I checked my boat owners manual.

The online version is hard to make out but it does seem to show that the boat was originally wired with the AC negative going to the "GND Buss" and then onto the engine "ground terminal"

Since I have owned the boat there has only ever been a single negative buss bar.

So am I to conclude that Sabre had originally wired the green to a shared DC negative

The main reason I ask these questions is because I am getting blistering of the lead keel. Also once when plugged into shore power and on the hard, I experienced small tingling shocks from the propeller at that time I could see that the "reverse polarity" light was lit on the AC panel. We deduced that it was bad wiring in the boatyard.

However I think that separating the AC green wire out would ensure that AC has no path via the engine block to the prop...

Let me see if I can attach a pic of the Sabre 30 AC diagram

[Bowdrie]

The green wire AC "ground" is most commonly connected to the DC ground in order to provide a low resistance path to the water via the engine>prop shaft.
It's to protect people, not the boat or anything in/on it from galvanic issues or stray currants.
That is the ABYC official narrative, or "rule" if you will.
However, all of the above is just a simplistic overview, it can and does get more complicated as various electrical devices are installed.

Paging S/V Jedi.

[Pete7]

Originally Posted by genki

I've been trying to figure this one out for years now, and have read a lot of threads on here that appear contradictory.

So... without wanting to start world war 4... let me ask this question.

Confusing isn't it. Our yacht like a lot of European yachts, has the AC earth connected to the DC negative circuit and from there to a hull anode. Wait until you add an inverter into the mix, then it gets really interesting.

Good luck.

Pete

[Hartleyg]

As several have mentioned, it is common in older boats to connect the AC "Green" to the "Ground" in a boat, and this is the DC negative because the engine block (plus prop and shaft) are DC negative.
That doesn't make it right, however.


If you had the "reverse polarity" light on, that means that your marina had voltage on the neutral AC line relative to ground - but this could come from either side -- if their miswiring put voltage on the "Green" wire of your shore power, then you would get exactly what you describe - a "hot" prop and shaft, and the reverse polarity light on.
It would also produce severe galvanic corrosion and the keel problems you mention.


It strikes me that your boat may well NOT be the problem here, but rather your marina, and getting an expert involved might be the appropriate move. If you don't have the ability to move to a better place, things like an isolation transformer may well be indicated.


Hartley
S/V Atsa

[boatpoker]

Originally Posted by genki

I've been trying to figure this one out for years now, and have read a lot of threads on here that appear contradictory.

So... without wanting to start world war 4... let me ask this question.

First details of my AC/DC system.
I have shore power AC coming into the boat with the next hop being a Yandina Galvanic Isolator (yes I know it's probably not a fail safe)

Then the GREEN AC wire goes to the Negative DC buss. The GREEN AC wire terminates there. My Sterling Power battery charger has a "ground wire" and that also connects to the negative DC buss.

Then there is a DC black negative wire to the Alternator of the Beta Marine engine, from there the engine has DC connectivity to the prop (I verified this with a meter) and then the prop zinc.

Question
Should I move the AC Green wires onto their own and totally separate buss bar, with no connections to the negative DC buss bar to isolate them?
If so, can somebody explain why?

Thanks!

You are talking about what is commonly referred to as "bonding" and it is a complex and controversial subject.

Be wary of corrosion or electrical advice from anyone using the word "electrolysis" on boats.

Electrolysis is an induced process found in industry and laboratories in which the anode is positive and the cathode is negative whereas on your boat it’s the anode that is negative and the cathode which is positive.

Sometimes these people mean “galvanic corrosion” and sometimes :”stray current corrosion” and some are just confused.

Consult a Certified Marine Corrosion tech on the issue to consider the pro's and con's. I will comment no further as I get tired of the dog fights on the issue.

[genki]

Thank you for all of the responses guys. This is really helpful.

I am considering installing a separate small Blue Seas buss bar to serve ONLY as the ending termination point for the AC green wiring and keeping that completely separate from the DC system.

I will need to check the internal wiring for the Stirling Power 30a battery charger that I am using and if that Green wire could in anyway be linked back to AC or if it's purely a DC ground.

Same with the small Victron inverter that I intend to install this spring (but I do not intend to tie this into the boat AC outlets, the inverter would stand-alone)

I am also going to need to check that Galvanic Isolator and perhaps purchase a better fail-safe one.

I looked up purchasing a half-cell to check potentials at the spatan bronze seacocks around the boat, the prop etc.

I must admit that I still do not really understand this green wire and marina AC pedestal stuff, any other insight is most welcome.

Thanks

[s/v Jedi]

I’m being paged

So I also have a Yandina isolator and you should be good with that. Use a multimeter to see that the AC ground you have aboard is not connected to:

- shore power inlet AC ground and that side of the isolator
- DC negative
- metal parts of the boat

Also use the multimeter to test that it -is- connected to:

- metal housings of inverters, chargers etc.
- ground pins of outlets.
- the inboard side of the isolator.

Here is a basic diagram showing the ground connections. It has an isolation transformer which is better but costs more and is large. That AC ground busbar also gets the green conductors from the outlets aboard, but nothing else

[smac999]

Is the charger the only ac load? Do you have an ac panel or is it just shore inlet to isolator to charger? All the ac ground wires should have their own bus, at the ac panel. But that bus does require a connection to dc neg under abyc guidelines. But it would normally be bus to bus at the ac and dc panels. Then people can see and understand what’s going on. Instead of random wires all over.

[genki]

Hi smac999,

No the charger is not the only load, there is a legacy AC system on the boat, with several outlets, and a couple of GFCI in the path also. These circuits have their own AC buss behind the circuit panel. The AC Green wire extends from there to my DC negative buss at the moment.

After the comments here I was intending to break that AC Green from the DC negative buss onto it's own totally separate buss. I would not touch anything on the main distribution panel at the nav station.

I have seen several people also say that ABYC wants the AC Green on the DC negative buss, and thus onto the engine block and that's why I opened this thread.. I was getting hot prop tingles from a badly wired marina and so I wanted to sanity check my wiring.

Thanks

[genki]

Hi s/v Jedi,

Thank you very much for your diagram.

So does that AC ships ground buss bar just sit there isolated, without a connection to anything else other than the collections of AC green wires?

Thank you

[Fishspearit]

The reason why ABYC says you should have the green AC grounding conductor connected to the DC negative bus is for safety. For example your battery charger shorts the ungrounded (hot, positive, whatever you want to call it) conductor to its housing. Then when you, a guest, or a worker is leaning their leg against your engine and touches the charger they get electrocuted as they are completing the circuit. This would not have happened if the AC grounding conductor were already connected to the DC negative bus as the ABYC recommends, and that is why they recommend it. The ONLY drawback to connecting the AC grounding conductor to the DC negative bus is that it can lead to galvanic corrosion because your grounding conductor is connected to other boats grounding conductors. But that's why you have a galvanic isolator. With the galvanic isolator in place and working there is NO drawback, and yet there is a BIG safety advantage to connecting the AC grounding conductor to the DC negative bus.

I'll repeat, with a working galvanic isolator you gain NOTHING by disconnecting the AC grounding conductor from the DC bus, but you create the potential for electrocution if you have a fault. Jedi has an isolation transformer, the source of his AC current is the transformer not the shore. You don't have an isolation transformer, your source of AC current is the shore and providing the return path to shore through the dc negative and shaft is important.

One of the SAMS regional directors was recently telling me that his preferred method of verifying that a boat is properly bonded/wired is to check the resistance between the shore power grounding connection on the boat and the fuel fill. Less than one ohm tells you that both the AC grounding conductor to DC negative is properly connected and the fuel fill is properly grounded. Many surveyors will check this, and if you've disconnected that AC grounding conductor from the DC negative bus it will be a high priority finding because it's a matter of safety and protecting crew from electrocution.

Now cue the people to tell you that ABYC is wrong. I won't be responding.