Marine AC - Why Tie Neutral and Earth?

[dbraymer]

Good and highly technical discussion. People are however, glossing over the very obvious stuff. As the previous owner of a large wooden boat, (all of whose wood was not completely dry), I feel it very important to point out that there may be saltwater in the bilge, and it might complete a circuit to a crew member through wet wood, if the water ground is not the same as the local earth ground.

Walking around barefoot in my old schooner I twice picked up a trickle of current from wooden structure members.

My solution was to give all components a ground, and while at dock, connect the grounds at exactly one spot, the primary AC cable. My current vessel has independent AC outlets. White outlets are connected to shore power, and red outlets are connected to the inverter. This isolates 110 coming from the batteries and grounded per the vessel plate.

This only works perfectly (without destroying zincs) if the local ground potential is very close to the salt water that your vessel's grounding plate is sitting in, so the situation actually has no uniform, exactly correct answer for a vessel that travels about, which is the whole reason that we own them.

[Crazy Norwegian]

Let me provide an example from DC powered subway trains.

The rails the train runs at and the exterior shell of the train are at DC Neutral. DC Neutral is tied to earth ground at the DC power stations.

I have seen instances where a person was in contact with earth ground and touched a rail car. There was over a 150 volt difference at that point between DC Neutral and AC Ground. The person received such a shock that they fell and were severely injured by the fall.

[Thumbs Up]

In an isolated power supply (IT) derived from an isolation transformer on a boat, dedicated GFCI protected power outlets should be used. The safety ground connection on these outlets should:
A. not be connected to anything
B. interconnect only
C. interconnect and terminate at a chassis ground point.
D. interconnect and terminate at L1 or L2 of the transformer outlet.
E. C and D combined.
F. interconnect and terminate at a center tap on the transformer.

Wotname said B, Jedi said C, Jammer (I think) said D. Edit: Jammer says F
One advantage of D, E, or F is that almost any current on this wire could trigger an RCD circuit breaker in the distribution panel.
One problem with F is that most isolation transformers don't have a center tap.

So we are right on point with this discussion but we have to consider:
MOVs which are found on surge protectors https://electronics.howstuffworks.co...protector1.htm
Lightning,
transient voltage spikes,
static electricity,
capacitance of the environment,
and anything else that I am missing.

At this point I don't have an opinion but am hoping that we can hash out the different fault scenarios and at least narrow it down and be able to know the advantage and disadvantage of each option. We know that hospitals that use these (IT) systems use expensive fault monitoring devices but that they prioritize uninterruptability.

[s/v Jedi]

Originally Posted by dbraymer

Good and highly technical discussion. People are however, glossing over the very obvious stuff. As the previous owner of a large wooden boat, (all of whose wood was not completely dry), I feel it very important to point out that there may be saltwater in the bilge, and it might complete a circuit to a crew member through wet wood, if the water ground is not the same as the local earth ground.

Walking around barefoot in my old schooner I twice picked up a trickle of current from wooden structure members.

My solution was to give all components a ground, and while at dock, connect the grounds at exactly one spot, the primary AC cable. My current vessel has independent AC outlets. White outlets are connected to shore power, and red outlets are connected to the inverter. This isolates 110 coming from the batteries and grounded per the vessel plate.

This only works perfectly (without destroying zincs) if the local ground potential is very close to the salt water that your vessel's grounding plate is sitting in, so the situation actually has no uniform, exactly correct answer for a vessel that travels about, which is the whole reason that we own them.

You would have been much safer with an isolation transformer. Eliminating a threat is much, much better than coping with it.

[Jammer]

Originally Posted by Thumbs Up

In an isolated power supply (IT) derived from an isolation transformer on a boat, dedicated GFCI protected power outlets should be used. The safety ground connection on these outlets should:
A. not be connected to anything
B. interconnect only
C. interconnect and terminate at a chassis ground point.
D. interconnect and terminate at L1 or L2 of the transformer outlet.
E. C and D combined.
F. interconnect and terminate at a center tap on the transformer.
[...]
Jammer (I think) said D. Edit: Jammer says F
[...]

I am a fan of D, which of course makes the system so that it no longer isolated -- it is a TN system rather than an IT system, to use the IEC terminology.


I think F is an interesting thought exercise that could provide the safety benefits that are claimed upthread for an IT system, without the risk of high voltage between the power conductors and ground. I don't necessarily recommend it for actual use.

[s/v Jedi]

Originally Posted by Jammer

This is not my experience in the many designs I have reviewed where a ground connection is present and MOV surge protection is used. There is always a neutral-to-ground MOV and a hot-to-ground MOV. On smaller electronics that don't have a ground connection to the mains plug then yes, the MOVs will be placed across the two lines.

No surprise there, and that's fine if it's your boat. I don't believe it's prudent to advocate that others follow your lead, though.

Where we disagree is that I don't believe it's wise to second-guess decades of field practice and replace it with personal theory or the piecemeal borrowing of highly specialized practices from other, very different applications.

I'm not smart enough to figure out lightning behavior from first principles. Maybe you are.

But you’re wrong. I attached some diagrams to support that. The primary MOV is between phase and neutral. Next, cor grounded appliances, there is an additional MOV between phase and ground and a third one between neutral and ground.

Even when MOV’s would only be connected to ground, then this still works 100% effective in my diagram. The use of MOV’s and surge suppression is 100% compatible with floating power distribution.

Your statement that my personal opinion goes against decades of experience for safe systems is a bit shocking and maybe I even feel insulted a bit by that. Wow.

This is exactly what they mean when they write:

Let us start initially with a clear commitment:
the IT system ("unearthed system") is a type of system infrequently used com- pared to the TN or TT system ("earthed system") – but it would often be the better alternative.
So why is the worse alternative accepted in prac- tice? The answer is probably: habit, convenience, ignorance. Many electrical planners are unfamiliar with the IT system. It is barely touched upon in uni- versities and training centres. The earthed system has therefore become increasingly widespread.

It’s hard to believe that ignorance becomes a reason not just for doing things less than optimal, but even for attacking superior methods

[Jammer]

Originally Posted by s/v Jedi

Your statement that my personal opinion goes against decades of experience for safe systems is a bit shocking and maybe I even feel insulted a bit by that. Wow.

I believe you're putting words in my mouth, because that's not what I wrote. I respect your posts and generally find them useful. In this case I believe you're dispensing potentially dangerous advice which is why I'm pushing so hard.


I've sat in meetings with UL representatives for hours on end discussing the minutiae of electrical safety. I've toured the local labs where UL testing is conducted for equipment designed by my colleagues. I have other, more hands-on electrical experience. I've read accident reports regarding electric shock injuries and deaths in various industries. I've sat in endless discussions of surge protection alternatives and their tradeoffs and costs.



In short, I'm not just some guy on the internet when it comes to this stuff.


I don't believe that anyone, no matter how smart or how knowledgeable they are, can design a grounding system that is safer as an armchair exercise. This stuff has to go through peer review and be tested and proven in the field no matter how good of an idea it seems to be on paper. The burden of showing that a new approach is safer is on those who propose it.

[caffel]

Keeping it basic, if the boat (or house) panel doesn't tie neutral to local earth then stray (or truly faulty) voltages from the source/dock and cables will continue on and be present on your boat.
Many of the three wire appliances and power tools on a boat were created with the neutral tied to ground convention in mind.
If you allow for bare hands or damp feet, in the mix then you're always safer avoiding stray voltages in as many ways as possible.

[Ozmirage]

IF anyone recalls the movie, "The Hunt for Red October," remember the scene where Jack Ryan is being winched from the hovering helicopter to the submarine. Did you note what happened to the seaman who didn't "ground" the helicopter hardware, via a conducting pole, to the boat?
>>KABAAAAAM<<
Big electric shock.
Though there might not be any high voltage equipment on the helicopter, by being ungrounded, its potential could be thousands of volts ABOVE ground reference.
. . .
So it's not just a matter of controlling currents and short circuits, but to eliminate the hazard of a voltage source having a huge differential to the ground state... which can happen with a "floating" ground.
. . .
Therefore, please ground / earth circuits as suggested by the codes.

[tdgaffney]

Green Wire – Where does it go
I’m a simple man reconditioning an older boat. Esoteric discussions put me to sleep, but I did read eleven pages, 158 posts, and appreciate everyone’s interests and ideas. What I really need is a simple understanding of what to do with that damn Green Wire!


Coopec43 in post #41 provided the URL for a West Marine article (thank you coopec43) – this article speaks only to shore power and, amongst other things, says the Green wire should go to the Engine Ground.


Comparing the article to what I have … my shore power system includes a Galvanic Isolator and my 120vac Elec Panel includes a polarity indicator and an ELCI. My 120vac outlets (every damn one of them) is a GFCI outlet. Coming back from these outlets, all the Black wires collate at the 120 Hot Bus, the Whites at the 120 Return Bus and the Greens at the 120 Ground Bus. These marry to the incoming shore power, the 120 Black-White-Green wires. So Far, so good.


Additionally, the Green Bus connects to my Engine Block (which it should, according to the West Marine article) by connecting to my 12vdc Ground Bus (which has all those Yellow 12vdc incoming wires), which is connected to my Engine Block. So Far, so good? Well, maybe not …


Boatpoker in post #91 says, “If your DC negative and AC ground are bonded as per ABYC, you will send AC into your DC system. This is what causes ESD in fresh water.” Damn!


And then Jedi, in post #81, says, “So the ground wire (green/yellow, earth, bare copper [green in my case]) on the outlets all go to the ground busbar as shown in my diagram. This is where the ground wires from transformers, inverter/chargers, boat-side of galvanic isolator and isolation transformer etc. go. It is not connected to DC negative and not to AC Neutral. Its only job is to connect all external metal surfaces of appliances together.”


I buy the No Connection to AC Neutral, but West Marine says “connect the Green Wire to Engine Ground,” which is what the DC Negative is connected to. Boatpoker doesn’t like that!


So maybe I should do as Jedi instructs, just join all the outlet box Green Wires to the Green Bus and let them sit there until shore power is connected. (i.e. Where does the Green Wire go?)


[My girlfriend (Ex) once said, as she observed me kinking the garden hose to shut off the water … “Don’t do that! Where does the water go?” (true story).]


My particular concern has been in the marriage of my new inverter/charger with this 120vac shore power system. The inverter’s only acknowledgement of a green wire is the chassis screw for its own connection to the loop. Nothing about grounding. And so, again, where does the Water go?

[Thumbs Up]

Originally Posted by caffel

Keeping it basic, if the boat (or house) panel doesn't tie neutral to local earth then stray (or truly faulty) voltages from the source/dock and cables will continue on and be present on your boat.
Many of the three wire appliances and power tools on a boat were created with the neutral tied to ground convention in mind.
If you allow for bare hands or damp feet, in the mix then you're always safer avoiding stray voltages in as many ways as possible.

Keep in mind that this discussion is about isolated power systems connected through isolation transformers.
The current in these systems has no affinity whatsoever to local earth.
I think that you might know already but to be clear, in a conventional system, when you are plugged into a dock pedestal the neutral/ground connection is made in a breaker panel on shore and there should never be a neutral to ground connection anywhere on the boat!

[Thumbs Up]

Originally Posted by tdgaffney

Green Wire – Where does it go
I’m a simple man reconditioning an older boat. Esoteric discussions put me to sleep, but I did read eleven pages, 158 posts, and appreciate everyone’s interests and ideas. What I really need is a simple understanding of what to do with that damn Green Wire!


Coopec43 in post #41 provided the URL for a West Marine article (thank you coopec43) – this article speaks only to shore power and, amongst other things, says the Green wire should go to the Engine Ground.


Comparing the article to what I have … my shore power system includes a Galvanic Isolator and my 120vac Elec Panel includes a polarity indicator and an ELCI. My 120vac outlets (every damn one of them) is a GFCI outlet. Coming back from these outlets, all the Black wires collate at the 120 Hot Bus, the Whites at the 120 Return Bus and the Greens at the 120 Ground Bus. These marry to the incoming shore power, the 120 Black-White-Green wires. So Far, so good.


Additionally, the Green Bus connects to my Engine Block (which it should, according to the West Marine article) by connecting to my 12vdc Ground Bus (which has all those Yellow 12vdc incoming wires), which is connected to my Engine Block. So Far, so good? Well, maybe not …


Boatpoker in post #91 says, “If your DC negative and AC ground are bonded as per ABYC, you will send AC into your DC system. This is what causes ESD in fresh water.” Damn!


And then Jedi, in post #81, says, “So the ground wire (green/yellow, earth, bare copper [green in my case]) on the outlets all go to the ground busbar as shown in my diagram. This is where the ground wires from transformers, inverter/chargers, boat-side of galvanic isolator and isolation transformer etc. go. It is not connected to DC negative and not to AC Neutral. Its only job is to connect all external metal surfaces of appliances together.”


I buy the No Connection to AC Neutral, but West Marine says “connect the Green Wire to Engine Ground,” which is what the DC Negative is connected to. Boatpoker doesn’t like that!


So maybe I should do as Jedi instructs, just join all the outlet box Green Wires to the Green Bus and let them sit there until shore power is connected. (i.e. Where does the Green Wire go?)


[My girlfriend (Ex) once said, as she observed me kinking the garden hose to shut off the water … “Don’t do that! Where does the water go?” (true story).]


My particular concern has been in the marriage of my new inverter/charger with this 120vac shore power system. The inverter’s only acknowledgement of a green wire is the chassis screw for its own connection to the loop. Nothing about grounding. And so, again, where does the Water go?

Your system is AYBC correct. The contention between AYBC and Euro (ISO) is that ISO allows you to eliminate the green wire to water connection but requires an RCD. The new AYBC also allows this. If your ELCI (RCD) is the proper recommended type, you should be able to eliminate the green wire to engine ground connection. Most of what has been said in this thread only applies to isolated power systems which you do not have (you would need an isolation transformer)Will someone please double check this advice I have given?
As for your invertor, it would help to know the model.

[john manning]

In the age of RCD's the neutral earth link has quite limited application. Where it is still a valid safety device is in the case of a line voltage short circuit to the case of a device where the short occurs prior to the power switch.

In that case the only conductive path which allows the fuse to trip is via the earth circuit.

On shore based power systems a genuine conductive earth exists which will easily carry the current required to trip a fuse. In mobile power systems such as yachts and caravans that true earth situation does not exist so the neutral earth link is actually more important there. It also requires that all AC devices are bonded together. ie All earths are connected.

[Wotname]

Originally Posted by s/v Jedi

The ground wiring also need to connect to metal parts of appliances not connected through an outlet, like the inverter housing, the aluminum breaker panel etc.

................

Agreed!

[Wotname]

Originally Posted by Thumbs Up

In an isolated power supply (IT) derived from an isolation transformer on a boat, dedicated GFCI protected power outlets should be used. The safety ground connection on these outlets should:
A. not be connected to anything
B. interconnect only
C. interconnect and terminate at a chassis ground point.
D. interconnect and terminate at L1 or L2 of the transformer outlet.
E. C and D combined.
F. interconnect and terminate at a center tap on the transformer.

Wotname said B,...........
..........

Wottie might have said B but he really meant C (ish) because Wottie simply forgot that some loads might have a metal case / chassis but no plug. See previous post (#164).