Marine AC - Why Tie Neutral and Earth?

[Wotname]

Originally Posted by EngNate

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.

Agreed - almost anyway. I do see some merit in RCD single outlets but no merit in an RCD at the source.

[Lake-Effect]

Originally Posted by 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. ...

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...

Thanks for the definitions. To my perhaps too-literal mind, "floating ground" is an oxymoron, and as can be seen in the thread, some were even discussing or suggesting that the ground (aka earth) terminals of AC outlets and switches not be connected (bonded) anywhere, which is clearly wrong.

[s/v Jedi]

Originally Posted by Lake-Effect

Thanks for the definitions. To my perhaps too-literal mind, "floating ground" is an oxymoron, and as can be seen in the thread, some were even discussing or suggesting that the ground (aka earth) terminals of AC outlets and switches not be connected (bonded) anywhere, which is clearly wrong.

What is called “floating ground” here is often called “chassis ground” and it is the norm for mobile installations. Just that a boat can have an underwater ground plate doesn’t mean it is the norm. Most boats in the world don’t have that and thus have a floating ground... until they connect to shore power which is when the trouble begins.

Other examples: cars, airplanes. They don’t have grounding rods/plates.

Now about the reason for my setup for US power using EU inverters plus auto transformers, which is so confusing I’m told: when you have a split phase with L1, N and L2, then the rated power is split over L1 and L2. Let’s take a 50A 120/240V shore power as an example. These have a double pole breaker of 50A. Both L1 and L2 go through that breaker (not N). This feed supplies us with 50A x 240V = 12kW. Now we start connecting 120V loads to this, starting with an 1,800W induction cooktop. It is wired to an outlet that has L1 and N. This means that L1 gets a current of 1800/120=15A, while the current through L2 is 0A... because the N is the return path.

Now you want a second induction cooktop and you wire a second outlet and connect that to L2 and N. Now L2 gets 15A as well, but what happens to N? Well, the current there becomes ZERO Amps. The reason is that L1 and L2 are 180 degrees shifted in phase. You effectively connected two equal 120V appliances in series to a 240V source. Any imbalance translates in a current through N.

So what happens when L1 carries 45A, L2 carries 15A (7.2kW) and we switch on a 3000W battery charger, which is wired to an outlet that has L1 and N? Well, the shore side breaker trips. L1 becomes 45+25=70A which trips the breaker and because the double pole is mechanically linked, L2 trips as well and we are without power even though we didn’t use 12kW yet.

So: to utilize all available power requires perfect balancing over the split phase.... which is impossible.

Now, shore power is one, let’s look at another source, the diesel genset. It has two 120V output windings that are jumpered in series for 240V and the jumper becomes the Neutral for split phase service. But.... each winding has a breaker..... exactly like the shore power breaker. This means that loads must be perfectly balanced to use all available genset power... that’s a much bigger problem!

Last source is inverters. For split phase all serious manufacturers support two 120V inverters in a bridge configuration. Here again, you can only pull up to half the power from each of L1 or L2.

This is why the Neutral from the source is dropped like in my diagram. When you don’t use the N from the source, you automatically have perfect balancing over L1 and L2 and thus can utilize maximum power at all times. The catch is when you want to power 120V loads... and this is where the auto transformer pops up. It not only creates the split phase but it also balances the load. The imbalance that normally runs back to the source through N, now runs through the auto transformer. This is why a 32A auto transformer is big enough for a 50A source: it can handle 32A of imbalance.

In my setup you see two Multiplus 3000 inverter/chargers in parallel. These are EU 230V 50Hz units, that have been reprogrammed for 240V 60Hz. The parallel operation programming synchronizes the phase so you really can take all that power from them without any limitations on balanced loads, which is taken care of by cheap, simple auto transformers. Same for the genset, it will always deliver maximum power without balancing issues.

This is not a scary odd rare strange setup. It is recommended by manufacturers like Victron and common on boats... maybe not on small boats that only have 120V like suggested in this thread, but common on 120/240V split phase installations at 12kW or more.

[Lake-Effect]

Originally Posted by s/v Jedi

What is called “floating ground” here is often called “chassis ground” and it is the norm for mobile installations. Just that a boat can have an underwater ground plate doesn’t mean it is the norm. Most boats in the world don’t have that and thus have a floating ground... until they connect to shore power which is when the trouble begins.

Other examples: cars, airplanes. They don’t have grounding rods/plates.

Appreciated. As you point out, the ground connections of AC outlets, fixtures, appliances, etc are usually "grounded" to the metal chassis/frame/hull/motor where they exist.

[s/v Jedi]

I have started a new thread with a basic system recommendation instead of our advanced system. Some will rejoice with the grounding even though it doesn't connect to earth

Here's the link: https://www.cruisersforum.com/forums...ml#post3405770

[goboatingnow]

I never understand why there is so much confusion

In a direct shorepower feed you never bond neutral to the earth wire onboard and in Europe the view is you don’t provide any actual earth connection either ( ie your boat is treated like an extension lead ) bonding neutral to earth or providing earth paths , means your earth wiring could potentially carry other peoples fault currents. Not a good idea

It’s why earth rods are a thing of the past round here ( for more then this reason )

The situation changes if you introduce a local “ generator “ , ie an isolation transformer

In that case , the function of the earth wire on board is largely redundant , but to preserve existing conceptions of how fault detection works , the earth wire is connected to neutral and the “ generator “

This ensures hot failover to case triggers the protection devices even though such hot faults cannot transmit a shock. ( easily , as they can on shore ) It’s viewed as “ unwise “ to allow a hot case to persist as it can go undetected and offers the opportunity for someone inserting themselves into the circuit ( ie by touching a neutral and a hot case )

Hence it not wise to remove the protective earth.

Note if you have an isolation transformer you have a floating system. It’s not good practice in Europe to establish a ground connection to earth in the boat with or without an isolation transformer

[s/v Jedi]

Originally Posted by goboatingnow

I never understand why there is so much confusion

Note if you have an isolation transformer you have a floating system. It’s not good practice in Europe to establish a ground connection to earth in the boat with or without an isolation transformer

Exactly. It is disappointing to see that some have a drive to make that earth connection. I think it has to do with a false sense of security that earth is safe.

[Thumbs Up]

Originally Posted by goboatingnow

........................
Note if you have an isolation transformer you have a floating system. It’s not good practice in Europe to establish a ground connection to earth in the boat with or without an isolation transformer

In Europe it is a given that there will be RCD (GFCI) protection. AYBC promotes the use of the earthed ground connection on a boat which I too think is bad. Once the current passes through the transformer you are isolated from shore power in every way (floating). To re-establish the Neutral to earthing conductor downstream is standard practice and required by ISO and ABYC code. On Jedi's boat he has elected to leave neutral floating. It can be argued that this is very safe but there is an important distinction because it does deviate from code.

[Thumbs Up]

Originally Posted by goboatingnow

......................
It’s why earth rods are a thing of the past round here ( for more then this reason ) ...........................

North America and Australia (among other places) use a multigrounded neutral distribution system.

Here is some good reading on the topic:
The Hazardous Multigrounded Neutral Distribution System
And Dangerous Stray Currents https://www.ecs.csun.edu/~bruno/Mult...nal_4-17-7.pdf

[coopec43]

I think I'm more confused now than when I read the first comment.

Is this relevant? (I'd give you a link if it was possible but I don't think I can as it is an email)

(Apologies if someone has posted this article already)




Three Common Errors Made in Boat Electrical Systems

Boats with AC power systems continue to give their owners problems. Many of these problems are avoidable, and some are deadly. The major problem in using power from shore is that the risk of lethal shock is substantially greater in and around a boat than it is in a house. Moreover, the nature of the shock hazard changes when the boat is moved from freshwater into saltwater.

An AC electrical system must be designed and installed on the boat in such a way that the crew and any nearby swimmers are protected, no matter what kind of water is underneath your hull. Any stray current in the system will always work its way back to the source of power. And when you tie into shore power, it’s going to head for the AC ground that is established somewhere up on shore, possibly hundreds of feet away. The return point for shorepower is a conductive rod driven into the earth. All leaking current heads for the driven rod.

When it comes to shorepower systems, it’s a serious mistake to treat a boat like a floating house. The electrical standards established by the American Boat and Yacht Council (ABYC) clearly indicate that you should handle the entire boat as a grounded-type portable tool. Never ground both the hot wire and the shore-grounded neutral on the boat. The three most common errors boat owners make are: (1) connecting the grounded neutral (white wire) to the grounding wire (green wire); (2) omitting (or cutting) the green grounding wire connection to the engine; and (3) using equipment that requires both alternating and direct current, and which is not specifically designed for use in a marine environment. Making these mistakes can be very serious, if not disastrous.

For more details these common errors and other wiring advice, download Marine Electrical Systems, Volume 5, AC Systems from Practical Sailor.

Or, buy the entire Marine Electrical Systems five-part series!

[mitiempo]

Originally Posted by coopec43

I think I'm more confused now than when I read the first comment.

Is this relevant? (I'd give you a link if it was possible but I don't think I can as it is an email)

(Apologies if someone has posted this article already)




Three Common Errors Made in Boat Electrical Systems

Boats with AC power systems continue to give their owners problems. Many of these problems are avoidable, and some are deadly. The major problem in using power from shore is that the risk of lethal shock is substantially greater in and around a boat than it is in a house. Moreover, the nature of the shock hazard changes when the boat is moved from freshwater into saltwater.

An AC electrical system must be designed and installed on the boat in such a way that the crew and any nearby swimmers are protected, no matter what kind of water is underneath your hull. Any stray current in the system will always work its way back to the source of power. And when you tie into shore power, it’s going to head for the AC ground that is established somewhere up on shore, possibly hundreds of feet away. The return point for shorepower is a conductive rod driven into the earth. All leaking current heads for the driven rod.

When it comes to shorepower systems, it’s a serious mistake to treat a boat like a floating house. The electrical standards established by the American Boat and Yacht Council (ABYC) clearly indicate that you should handle the entire boat as a grounded-type portable tool. Never ground both the hot wire and the shore-grounded neutral on the boat. The three most common errors boat owners make are: (1) connecting the grounded neutral (white wire) to the grounding wire (green wire); (2) omitting (or cutting) the green grounding wire connection to the engine; and (3) using equipment that requires both alternating and direct current, and which is not specifically designed for use in a marine environment. Making these mistakes can be very serious, if not disastrous.

For more details these common errors and other wiring advice, download Marine Electrical Systems, Volume 5, AC Systems from Practical Sailor.

Or, buy the entire Marine Electrical Systems five-part series!

What confuses you? Makes perfect sense to me.

[coopec43]

Originally Posted by mitiempo

What confuses you? Makes perfect sense to me.

Well the article is quite understandable but when I read comments on the thread I get confused.

We had a 16 hour power failure just a couple of days ago so I started my Gentrax suitcase generator up for the first time and noticed an "earthing" terminal on the generator. Now I'm going to have to understand earthing on land and earthing on a boat when the time arrives.

That problem is a little down the track but I am book-marking all the relevant articles (and threads) so I can quickly come to grips with what has to be done.

[Lake-Effect]

Originally Posted by mitiempo

What confuses you? Makes perfect sense to me.

If something makes perfect sense to a marine professional, that's reassuring, but besides the point.

To coopec43, the OP's starting point was in regards to onboard AC sources - generators, inverters, transformers - whereas that Practical sailor article is referring to shorepower, without an isolating transformer onboard.

From the quoted email:

When it comes to shorepower systems, it’s a serious mistake to treat a boat like a floating house. The electrical standards established by the American Boat and Yacht Council (ABYC) clearly indicate that you should handle the entire boat as a grounded-type portable tool. Never ground both the hot wire and the shore-grounded neutral on the boat.

... I like the mental picture of the boat on shorepower as a grounded portable tool; it reflects all the key concepts.

...but the warning to "Never ground both the hot wire and the shore-grounded neutral"... show of hands please: who has deliberately grounded the AC hot wire ANYWHERE?

[mitiempo]

Originally Posted by coopec43

Well the article is quite understandable but when I read comments on the thread I get confused.

We had a 16 hour power failure just a couple of days ago so I started my Gentrax suitcase generator up for the first time and noticed an "earthing" terminal on the generator. Now I'm going to have to understand earthing on land and earthing on a boat when the time arrives.

That problem is a little down the track but I am book-marking all the relevant articles (and threads) so I can quickly come to grips with what has to be done.

Neutral and ground re connected at the source of power.

When on shore power they are connected at the substation providing the power.

When on shore power with an isolation transformer it becomes the power source. It is wired to connect neutral and ground.

When away from the dock the neutral and ground are connected at the inverter (only when inverting) or at the generator.

[coopec43]

Lake-Effect and mitiempo thank you both (and others) for your comments.

While building the yacht I've learned I must concentrate on the job at hand - one job at a time. Right now I am finishing off the aft cabin carpentry and then I'll finish of the electrical work with all the earthing, shunts, circuit breakers etc.

I'm downloading lots of articles (and this thread) so that I know what I'm doing when the time comes.