Marine AC - Why Tie Neutral and Earth?

[Wotname]

Originally Posted by 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.

Err... I don't think Hollywood is a great example of what to do / not do. Nor is aircraft static build up useful to this thread but we can discuss it if you want to start another thread . I do have decades of experience in that field....

[Wotname]

Originally Posted by Jammer

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.
.......

Question - in IEC terminology, is there any distinction between using a isolation transformer and using an inverter to create a floating output (L1,L2). In this instance I am assuming an inverter with isolated DC input, isolated AC output and an isolated chassis / case.

I have been assuming both sources to be considered an IT system, maybe I am wrong(?).

[Thumbs Up]

If we connect the outlet safety grounds as in options D, E, or F is it really no longer a floating output? I think it is in the sense that it is completely isolated from shore power. It is a floating output but we then are using that and creating a TN distribution by designating neutral and connecting the outlet safety grounds to it. The supply coming out of the transformer (up to that point) is floating. (this achieves what we want which is isolation from shore ground). Or in other words a TN distribution system derived from a floating (IT) source.
The correct way to wire it is option C with D:
C. interconnect and terminate at a chassis ground point. And
D. interconnect and terminate at L1 or L2 of the transformer outlet.

This is exactly how Promariner says to do it. https://www.hodgesmarine.com/amfile/...product/31082/ The drawings for isolation configuration (not polarization) show this connection to the chassis, the white output, and the outlet safety ground.

What they are not saying here is that if you have an RCD breaker on the secondary output (after the neutral/ground connection), the safety ground wire now acts as a sensing wire for the RCD.

With this setup I don't think you would need dedicated GFCI outlets. Comments Please.

[Thumbs Up]

Originally Posted by Wotname

Question - in IEC terminology, is there any distinction between using a isolation transformer and using an inverter to create a floating output (L1,L2). In this instance I am assuming an inverter with isolated DC input, isolated AC output and an isolated chassis / case.

I have been assuming both sources to be considered an IT system, maybe I am wrong(?).

I am curious too. Also a generator can also be a floating output. (Honda eu2000)

[Wotname]

Originally Posted by Wotname

Question - in IEC terminology, is there any distinction between using a isolation transformer and using an inverter to create a floating output (L1,L2). In this instance I am assuming an inverter with isolated DC input, isolated AC output and an isolated chassis / case.

I have been assuming both sources to be considered an IT system, maybe I am wrong(?).

To be clear, I am asking the question only in relation to IEC terminology.

Electrically speaking there is no useful distinction (within this thread) between an isolation transformer, an isolated DC/AC inverter and a say a Honda eu2000 generator.

[Jammer]

Originally Posted by Wotname

Question - in IEC terminology, is there any distinction between using a isolation transformer and using an inverter to create a floating output (L1,L2). In this instance I am assuming an inverter with isolated DC input, isolated AC output and an isolated chassis / case.

I have been assuming both sources to be considered an IT system, maybe I am wrong(?).

The IEC terminology presumes that an electric utility is the source of power. Hence the great focus on distinctions between US wiring practice (TN-C-S) and England (TN-S) which differ only on where the neutral and ground are bonded together.


The IEC and the NEC both mostly treat independent sources the same way, regardless of whether an inverter, generator, or isolation transformer. The NEC has some provisions specific to isolation transformer use in medical and laboratory settings that don't apply on boats. The ABYC treats generators and inverters the same way but has shore power provisions specific to isolation transformers.

[SailRN]

Originally Posted by Frankly

First off shipboard inverter/ inverter generator powered AC system and generator systems are different in significant ways. A 5 or 10 KW generator will....

Recently installed a 10 KW...

What is a kelvin watt?

[SailRN]

Originally Posted by mitiempo

I assume you are referring to a GFCI without a ground connection - eg only hot and neutral. In this case a "ground fault" is when there is 5 millivolts or more difference between hot and neutral.

It is a ground fault because the user either becomes the grounded conductor or there is leakage to another item - moisture for example - from either the hot or neutral.

GFCIs work just fine without a ground connection.

Hmmm, I'm measuring 120 V AC RMS with a Fluke 289 DMM between my hot wire and the neutral wire..., but the GFCI is not tripping with this > 5mV difference.

[AKA-None]

Originally Posted by SailRN

What is a kelvin watt?

Ummm try kilowatt

[Lake-Effect]

Originally Posted by SailRN

Hmmm, I'm measuring 120 V AC RMS with a Fluke 289 DMM between my hot wire and the neutral wire..., but the GFCI is not tripping with this > 5mV difference.

I suspect they meant a >5 mA difference in current running through L and N.

It seems to me now that fully floating AC is like soldering vs crimping: in the right hands, both offer advantages, but there aren't that many people who have the knowledge or skills to do them properly.

[markcouz]

We are staring to get into the grammar police part of any thread kW = kilowatt (not KW).

But if you understand that KW refers to kilowatt then all good, move on!

Originally Posted by AKA-None

Ummm try kilowatt

[s/v Jedi]

It’s hopeless right? Hunt for Red October, making a floating power distribution work like one with grounded neutral, absolute cluelessness about earth not being a return path behind an IT... it doesn’t stop. The most basic principles are not understood or even worse, ignored.

The problem is that there are people here who want to install the safest possible system but, not being EE’s, get confused by ignorant and even trolling remarks. As I’m just here trying to help people, I’ll only post for them from here on.

So take the diagram I posted and remove the 50A shore power part of it. This is as safe, reliable and as efficient as it gets. You can change it to just one inverter/charger, just one distribution group etc. If not US power, you can delete the auto transformers all together.

What is not in the diagram is the wiring from distribution breakers to outlets etc. Everything wired to the double pole breakers must also have a ground wire which connects to the boat ground busbar.

With all the naysayers attacking the diagram, remember that in all the years this has been posted about, nobody ever could describe a single thing that is not better than a non floating power distribution. Everything brought up was proven incorrect or just too silly to even discuss.

Ask me anything about that diagram

[SailRN]

Originally Posted by AKA-None

Ummm try kilowatt

No, K is kelvin, a measure of thermodynamic temperature, so KW is kelvin watt.



If they meant kilowatt, they would have used the correct term kW, as k is the prefix kilo.


It's more than just "grammar police" another poster mentioned. You have to know enough of the basics to not only understand yourself, but to be able to communicate correctly to others. I guess going to work for Rickover and then later becoming a critical care/trauma nurse one knows the importance of this. Which do you want, a 5 mg infusion of potassium chloride, or does it matter to you, your family, and your heirs if it's 5 Mg?


You just have to be wary of all of this advice when the poster doesn't know (as demonstrated) what they're writing.


Watch for my soon to be published book, The Care and Feeding of My Pet Peeves, due out in 2055. It has a chapter on incorrect usage of terms and symbols.

[Lake-Effect]

Originally Posted by s/v Jedi

The problem is that there are people here who want to install the safest possible system but, not being EE’s, get confused by ignorant and even trolling remarks.

The joys of a public forum...

One of the most important criteria for the "safest possible system" is how likely it is that it will be understood and duplicated successfully by the most people. Your technical rationales and explanations are top-shelf... but unless people are prepared to approach your level of knowledge, and/or follow your recipe to the letter, and fight with surveyors, insurers and techs who won't deviate from the existing standards... would they not be better off overall by following the current standards, which are more widely understood by more users and marine techs, and better-supported in the technical literature ("boat books" and websites), and by the available panels, breakers, and and supplies?

[SailRN]

Originally Posted by Lake-Effect

I suspect they meant a >5 mA difference in current running through L and N.

It seems to me now that fully floating AC is like soldering vs crimping: in the right hands, both offer advantages, but there aren't that many people who have the knowledge or skills to do them properly.

My point in an earlier post, if they don't know the difference between 5 mV and 5 mA, how can you believe what they write?


I've always like soldering personally, but for mission-critical applications crimping is required. NASA for instance does not permit soldered connections. A gas-tight mechanical crimp is required.