Marine AC - Why Tie Neutral and Earth?

[SailRN]

Originally Posted by Wotname

Err... you haven't confused even once so far. I get it, you asked about the 'kelvin watt' (and about voltage levels). You may have be been serious, you may have been a smart aleck or you may have had other reasons - it didn't matter. However after others answered your queries, you double down about the accuracy and trust etc.

The double down you mention is a hazard with reading postings serially and addressing them as they are read.



I'll have to send you an autographed copy of my new book, The Care and Feeding of My Pet Peeves due out in early 2055.

[Wotname]

Originally Posted by SailRN

The double down you mention is a hazard with reading postings serially and addressing them as they are read.



I'll have to send you an autographed copy of my new book, The Care and Feeding of My Pet Peeves due out in early 2055.

I've reserved a spot in my reading list for it!

[AKA-None]

Originally Posted by 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!

Just trying to move from the mistake of kelvin to the more correct kilo

That’s not grammar police

[Jammer]

Originally Posted by s/v Jedi

The isolation transformer in my diagram has no neutral so it’s impossible to ground it. It only has L1 and L2, i.e. it is 240V only. The whole diagram has no neutral beyond the primary winding of the isolation transformer.

I know it feels weird to not have a neutral, but there’s plenty of appliances that only take L1 and L2. My A/C, circulation pumps, the 1.5hp motor of the watermaker high pressure pump, the heating element of my water heater, they all connect to just L1 and L2 240V. There’s even outlets that don’t have the neutral... it’s the NEMA 6-x range.

If I wanted to make a neutral, then it would be all the way at the end where the auto transformers are. They have a center tap that I use to create 120V. If you ground that, then it would become a neutral. But if you do,you can only have one transformer.

Well, again, your boat, do what you want, but it is unclear to me what you are trying to accomplish with the unusual system you have.


If it were my boat, I would make some choices:


1) I would either choose to have a boat wired for 120v only (one hot and a neutral, following North American practices for smaller boats), 120/240v (two hots and a neutral following North American practices for somewhat larger boats) or a boat that is 240v only (one hot and a neutral following European practices). I would not have a boat that is half 240v without a neutral and half 120/240v.


2) I would either choose to have an isolation transformer for shore power or not and would not have the curious and confusing arrangement of having a 50a inlet without an isolation transformer and a 30 inlet with one.



Based on these choices I would simplify the wiring. Isolation transformers are available with center taps in the secondary in sizes that would allow you to take advantage of 50a/240v shore power, see for example this 7500 va transformer and this 10 kva transformer.


Those transformers would allow you to bond the neutral at the source per ABYC. 120v/240v generators and inverters are also available, using them along with a center tapped isolation transformer would allow you to ditch the autotransformers entirely. The point being that your lack of a neutral to bond is a design choice on your part, not an absolute. Again, your boat your choices -- but your posts make it sound like physics painted you into a corner and it didn't.

Let’s say we have only one auto transformer and we connect it’s center tap to boat ground. What do we win? .... nothing. Nothing is gained by that action. Does it get easier to understand? Nope. On the negative side,it does limit us to one auto transformer and in my case, I can’t switch to a regular grounded shore power like I do with the 50A shore power connection.

Well, as pointed out upthread, what you win is that the voltage relationship between ground and the current-carrying AC conductors is constrained. There will not be a DC voltage between ground and N, L1, or L2. There will not be an AC voltage between them in excess of the system voltage. And any RCD/GFCI type devices in the system will trip reliably in the event of a ground fault where they otherwise might not.


Now, you can say you don't care about these benefits, or that they are less important than whatever cost savings, weight savings, or redundancy improvements your system gives you, but they are still there and I am confident that you're smart enough to understand that.


And I guess you could make the case that the nominally isolated ground improves safety because assuming isolation is maintained you can stand barefoot on a metal deck and grab the hot wire without dying. You could acknowledge the benefits of a bonded neutral and make the case that the benefits of an isolated ground are greater. I don't believe you've done that in this thread.

[Thumbs Up]

Originally Posted by s/v Jedi

Well, you’re wrong and your proposed installation would be rejected by engineering.
This is the core of the matter and all isolation transformer documents explain this.

I have posted isolation transformer installation documents. They say to install as I have described. With an RCD installed immediately after the transformer, what fault condition would trip it? Then why do they say to put one there? Why do they say to connect outlet grounds to L2 or a center tap. Why does Goboatingnow say to do it that way? What is the advantage of full floating distribution on a small yacht and how do you know if there is a fault? You would still be isolated from shore power and ground. If you read my posts I won't have to keep repeating. Show me the isolation transformer documents that you are talking about. The manuals that I looked up says to connect neutral (L2) to ground in order for the output RCD to function. Are you not opening my links? Is this not your manual? https://www.victronenergy.com/upload...r1800-3600.pdf On page 8 does it not say:

Figure 5: Output neutral grounding
A Residual Current Device (RCD) or Ground Fault Circuit
Interrupter (GFCI) must be installed in the output cable of
the isolation transformer. For this RCD to operate
correctly, the output neutral must be connected to ground
(= all the metal parts in the boat). This is achieved by
placing a jumper on male push-on connectors J21, J33
(see fig 5), and by grounding the enclosure of the isolation
transformer.

You have said "ask me anything about my diagram" but now you want to defend your installation by saying "you are wrong and you are not an engineer therefore I am right". Show me anywhere that allows a full floating multi circuit installation that does not have any fault sensors that is approved by engineering. Show me any other electrical engineer that approves of your setup. If I am wrong, I will admit it. I will wait for another engineer to weigh in.

[boatpoker]

Originally Posted by Thumbs Up

I have posted isolation transformer installation documents. They say to install as I have described. With an RCD installed immediately after the transformer, what fault condition would trip it? Then why do they say to put one there? Why do they say to connect outlet grounds to L2 or a center tap. Why does Goboatingnow say to do it that way? What is the advantage of full floating distribution on a small yacht and how do you know if there is a fault? You would still be isolated from shore power and ground. If you read my posts I won't have to keep repeating. Show me the isolation transformer documents that you are talking about. The manuals that I looked up says to connect neutral (L2) to ground in order for the output RCD to function. Are you not opening my links? Is this not your manual? https://www.victronenergy.com/upload...r1800-3600.pdf On page 8 does it not say:

You have said "ask me anything about my diagram" but now you want to defend your installation by saying "you are wrong and you are not an engineer therefore I am right". Show me anywhere that allows a full floating multi circuit installation that does not have any fault sensors that is approved by engineering. Show me any other electrical engineer that approves of your setup. If I am wrong, I will admit it. I will wait for another engineer to weigh in.

My good friend and dock neighbour consults with the US Navy on their electric warships. He retired as President of IEEE 2 years ago ... He laughed when I showed him this thread

[SailRN]

Originally Posted by boatpoker

My good friend and dock neighbour consults with the US Navy on their electric warships. He retired as President of IEEE 2 years ago ... He laughed when I showed him this thread

Which of the 201 postings amused your friend so much?

[EngNate]

Where is this going now? I want to go back to post 1, after I say that I shake my head, roll my eyes in some cases, at much of what I read here, but at least for now I won't sort through it.

Whether on land or in water, the earth is connected to the ground circuit (the equipment grounding conductor) for the reason of including it, the earth or water in proximity, in the equipment grounding circuit. It is treated as other exposed conductive objects in the system. The purpose of the grounding conductor is to maintain all such objects at the same potential in the event of any fault. This purpose is accomplished whether or not the circuit is also connected to a neutral. Engineers have debated the subject for as long as electrical power has been commercially available and there is yet no conclusive determination that one system is safer than the other. Arguments focus more on potential for equipment damage in the event of faults.

Both floating and neutral bonded grounding systems are used and approved on inspected commercial vessels including passenger vessels in Canada and at least much of the rest of the world. The following is not unusual at all: Primary system is 480 Volts, floating ground, and runs all the major machinery. On the secondary side of a transformer the 120/240 or 120/208 domestic service is neutral bonded. The equipment grounding circuit is contiguous throughout the system. Indication of ground faults is required but it is often not sophisticated. Don't say it ain't so, I've built these systems, they were inspected, designs approved by Transport Canada in advance.

For domestic service, neutral bonded systems are the convention throughout the world. Note that for ELCI/GFI devices to work they must be connected to a neutral bonded system, so the practice is not outdated due to the advent of these devices.

There is another subject on the neutral to ground connection(s) on a vessel. The practice differs between pleasure and commercial applications. In short, for pleasure vessels neutral and ground are connected only at a source of power, and that connection is contiguous with the system only when that source is providing power.

[Lake-Effect]

Originally Posted by EngNate

Both floating and neutral bonded grounding systems are used and approved on inspected commercial vessels including passenger vessels in Canada and at least much of the rest of the world. The following is not unusual at all: Primary system is 480 Volts, floating ground, and runs all the major machinery. On the secondary side of a transformer the 120/240 or 120/208 domestic service is neutral bonded. The equipment grounding circuit is contiguous throughout the system. Indication of ground faults is required but it is often not sophisticated.

This thread is sometimes fast and loose with the definitions. So to clarify:

By floating ground, what do you mean exactly? That the 480v primary is floating; it has no connection to ground (eg no center tap or Y center connected to ground)? Because, as you state - equipment grounding circuit is contiguous throughout the system. In other words, there is a ground, it's not floating, it's just that no part of the 480v primary is connected or referenced to it. Yes?

[boatpoker]

Originally Posted by EngNate

There is another subject on the neutral to ground connection(s) on a vessel. The practice differs between pleasure and commercial applications. In short, for pleasure vessels neutral and ground are connected only at a source of power, and that connection is contiguous with the system only when that source is providing power.

+1. Also supported by Transport Canada referencing ABYC E-11 in TP1332

[Wotname]

Originally Posted by EngNate

Where is this going now? .............


For domestic service, neutral bonded systems are the convention throughout the world. Note that for ELCI/GFI devices to work they must be connected to a neutral bonded system, so the practice is not outdated due to the advent of these devices.

.........

I am sorry to have to inform you that ELCI/GFI devices do not have to be connected to a neutral bonded system to work. These devices will operate whenever there is a sufficient differential in the current flow of the two legs of the circuit. Plenty of discussion upthread explains why this is so.

[Thumbs Up]

Originally Posted by Wotname

I am sorry to have to inform you that ELCI/GFI devices do not have to be connected to a neutral bonded system to work. These devices will operate whenever there is a sufficient differential in the current flow of the two legs of the circuit. Plenty of discussion upthread explains why this is so.

Individual outlet GFCIs do not need to be connected to a neutral bonded system to protect against certain line to line faults (from separate outlets) , however the RCD breaker that is recommended just after the transformer needs this ground wire as a sensor. This wire does not only act as a return path to ground for triggering overcurrent devices. It will also cause an RCD circuit breaker to trip (usually 30ma).

[EngNate]

Floating ground means that there is no connection to any normally current-carrying conductor. That means that in a fault condition the grounding conductor can be driven to the full line potential. However, since all the parts are at the same potential the shock hazard is eliminated. In a floating ground system a fault can occur to one live line without interrupting operation. The idea is the fault will be detected and removed before a subsequent fault occurs to another phase. At that point the ground is no longer floating and behaves the same as if neutral bonded - a short circuit will occur and circuit protection will be tripped if the fault current is high enough.

A ground circuit is not floating if connected to any current-carrying conductor. In domestic systems the neutral is defined as the grounded current carrying conductor. Note that does not mean there is any path for said current through the grounding conductor, which is not to carry current except in a fault condition. In the example I noted, the grounding conductor is contiguous throughout the system and it is floating with respect to the 480V primary side and bonded with respect to the 240V secondary side. It may be easier to picture the other way around - think of the grounding conductor as a fixed potential, the 480V generator output is floating while the 240V transformer output is bonded, referenced, to the ground.

Usage of terms ground/grounding, earth & bonding, introduces confusion because they aren't applied consistently.

Ground/Grounding - the equipment grounding conductor, the circuit that connects parts that are to be protected from shock hazard. Earth is not ground, but an element connected to it, a grounded component.

Bond - connection to a conductor of the power source, specific usage with respect to grounding systems.

[EngNate]

Originally Posted by Wotname

I am sorry to have to inform you that ELCI/GFI devices do not have to be connected to a neutral bonded system to work. These devices will operate whenever there is a sufficient differential in the current flow of the two legs of the circuit. Plenty of discussion upthread explains why this is so.

Well, correct that if there is a leakage current it will trip. But unless there is already a ground fault from another phase there will be no leakage current to sense and the device will not trip. As TU said, it would trip on a fault to another phase line to line, or the same or another phase line to neutral, coming from another receptacle or circuit. This would be a most unusual circumstance. GFCI/ELCI/RCD is an ornament on a floating ground system.

[Wotname]

Originally Posted by Thumbs Up

Individual outlet GFCIs do not need to be connected to a neutral bonded system to protect against certain line to line faults (from separate outlets) , however the RCD breaker that is recommended just after the transformer needs this ground wire as a sensor. This wire does not only act as a return path to ground for triggering overcurrent devices. It will also cause an RCD circuit breaker to trip (usually 30ma).

Agreed and discussed at length upthread. It was remiss of me not to distinguish between an RCD at the source and an RCD outlet however I had considered this aspect to be already well explained.