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Old 15-11-2017, 18:18   #31
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Re: Grounding an inverter

gfi's work fine with no ground. it only measures the diff between hot and neutral

the n-g bond is what allows the circuit breaker to blow if an appliance shorts out internally. otherwise it would not.
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Old 15-11-2017, 18:27   #32
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Re: Grounding an inverter

As a commercial electrician I have to giggle a little at the the extreme lack of a basic electrical theory among most laymen.

Without a reference the safety ground is doing nothing. it can't. The only path back to the source is the two ungrounded conductors provided. The source is not referenced to ground so either wire (but obviously not both) can harmlessly touch unguarded metal and absolutely nothing will happen.

A floating neutral is used at times as a safety feature and a "safety ground" does absolutely nothing in these instances. I've installed isolated transformer systems in operating theaters where there was an equipment ground but it was not connected to the secondary of the source in any way other than only as a very sophisticated sensor device to measure and ensure there was no leakage from either of the two floating ungrounded conductors being used with the medical tools on the table. If there was any leakage nothing bad would happen intially -no shocks, no current flowing to ground, other than an alarm going off at a set threshold and a meter on the panel moving to show the amount of leakage potential.

The first ground is free, and will only reference the leg that is grounded to earth. The second, and subsequently later ground in the other leg is the dangerous one if it should ever happen. The issue with this system is the monitoring for and of that first fault reference.

This same thing happens with a 3-phase ungrounded delta system used in some manufacturing systems. The system has no neutral and is not bonded on the secondary of the transformer in any way. If any leg were to short out to ground nothing at all would happen. It is not until there is a second ground fault on another leg that there WILL be a problem.

Again, the issue comes in being able to monitor for that first fault so it can be found and corrected before the second one ever happens. Cheap ungrounded inverters do not have this sensing capability. They are totally safe until there is a short in one of the wires. That short will go unnoticed until there is a second short, when fault current may flow and someone may be injured by energized parts.

Did you get all that? I didn't think so.
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Old 15-11-2017, 18:54   #33
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Re: Grounding an inverter

A GFI receptacle will work just fine on a system without a neutral ground connection ( technically if one high side of the system is not connected to the ground then it is not called a neutral). The reason it might not trip with a load between the high side and ground is that there is not enough current flow to trip the GFI protector and by definition not enough current to cause harm to a human in the same current path.

The primary reason for a connecting one high side to the green safety conductor at the source (and calling it a neutral) is to provide a low impedance path for a typical short to a metal case to cause a large current to flow, causing the feeding breaker to open almost immediately and remove the power from the device, thus rendering the device electrically dead and safe. The reason on dirt dwelling situations we drive a metal rod into the dirt and connect it to this neutral/ green wire connection has more to do with lightning and step touch potential situations.

There are lots of ungrounded (no neutral connection) electrical systems around and if done correctly are safer and more electrically reliable than the systems we are normally exposed to.
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Old 15-11-2017, 19:43   #34
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Re: Grounding an inverter

Transformerless inverters will always present some user risks because their outputs must be isolated from the boat's grounding/bonding system. A marine rated 3KW inverter sells for over $2200 whereas a transformerless EBay inverter can be had for about $300.

These things can be made safer by taking the inverter's output directly to an isolation transformer. The transformer's output will mimic AC dock power and one side of the transformer's secondary can be bonded to the boat's gound/bonding system. A Square D transformer can be purchased for under $300. Can also use the transformer's center taps to get 120/240 with the centertap bonded to ground.

Above does not mean I recommend unqualified people to do this type of installation.
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Old 15-11-2017, 20:36   #35
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Re: Grounding an inverter

From Victron Energy's manual for the European (230v) version of the Multiplus Inverter/Charger:

The MultiPlus is a safety class I product (supplied with a ground terminal for safety purposes). Its AC input and/or output terminals and/or grounding point on the outside of the product must be provided with an uninterruptible grounding point for safety purposes.

The MultiPlus is provided with a ground relay (relay H, see appendix B) that
automatically connects the Neutral output to the chassis if no external AC supply is available. If an external AC supply is provided, the ground relay H will open before the input safety relay closes. This ensures the correct operation of an earth leakage circuit breaker that is connected to the output.

─ In a fixed installation, an uninterruptable grounding can be secured by means of the grounding wire of the AC input. Otherwise the casing must be grounded.

─ In a mobile installation (for example, with a shore current plug), interrupting the shore connection will simultaneously disconnect the grounding connection. In that case, the casing must be connected to the chassis (of the vehicle) or to the hull or grounding plate (of the boat).

In case of a boat, direct connection to the shore ground is not recommended because of potential galvanic corrosion. The solution to this is using an isolation transformer.
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Old 15-11-2017, 22:03   #36
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Re: Grounding an inverter

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One reason ABYC requires neutral and safety ground connected at the source is so GFCI outlets will work.
A Point of order, A GFCI outlet measures the difference between hot and neutral (US) or hot to hot (EU). A ground conductor is NOT required for the GFCI to work. It trips when the current in and out are not balanced between the two legs.
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Old 15-11-2017, 22:05   #37
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Re: Grounding an inverter

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Originally Posted by BlackHeron View Post
As a commercial electrician I have to giggle a little at the the extreme lack of a basic electrical theory among most laymen.

Without a reference the safety ground is doing nothing. it can't. The only path back to the source is the two ungrounded conductors provided. The source is not referenced to ground so either wire (but obviously not both) can harmlessly touch unguarded metal and absolutely nothing will happen.

Did you get all that? I didn't think so.
Cough... I actually got all that. But then I hung around EE's way too much....
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Old 16-11-2017, 02:43   #38
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Re: Grounding an inverter

Ok, somewhere along the line the notifications stopped working for this thread, so I missed the responses until now.
Thank you everyone for the input!

To address some of the concerns:
I am a electrical engineer with a good understanding of 12 and 230V systems but when it comes to inverters in boats I fall short. Hence this thread.
Indeed I am in Europe, and the unit is a 230V one.
I have a very basic 230V system onboard, just a charger and a couple of outlets, with a RDC breaker panel. Boat is not bonded, as typical for EU boats.
I am fully aware this is not a marine unit I've bought. It was bought to serve a specific, short-time purpose and I'll not be installing it permanently in the boat.
The unit is double-isolated.

I've read that some inverters are 2-phase, with the neutral not connected at all elsewhere on this forum. If mine is such a device, it seems it would be impossible to create a working ground reference.
I'll most likely get a Victron Inverter/Charger for permanent installation.
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Old 16-11-2017, 05:00   #39
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Re: Grounding an inverter

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A Point of order, A GFCI outlet measures the difference between hot and neutral (US) or hot to hot (EU). A ground conductor is NOT required for the GFCI to work. It trips when the current in and out are not balanced between the two legs.
It is required. Without a safety ground the device test function will not work and some devices will not show the proper green/red indicators.
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Old 16-11-2017, 05:08   #40
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Re: Grounding an inverter

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It is required. Without a safety ground the device test function will not work and some devices will not show the proper green/red indicators.
The safety feature of the device is not sacrificed when the grounding conductor is not connected.

The point is that when used in an ungrounded power system, a GFCI protects against faults the same as it does in a grounded power system.
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Old 16-11-2017, 05:25   #41
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Re: Grounding an inverter

There are at least two reasons a GFCI must have a proper safety ground.

A GFCI that fails a self test must be replaced. So if the self test function does not work it cannot be relied upon and the unsuspecting boater will be forever replacing them.

How exactly can there be any "ground fault current" if there is no system safety ground circuit? A person would have to somehow connect themselves between hot and neutral at two different outlets to trip the GFCI. In which case both of them would trip. But how likely is that?

If a person were being electrocuted by a single GFI outlet in a system with no safety ground the GFI function would not shut off the power. This is one reason ABYC requires safety ground connected to neutral at the source. It is also why they have the, at first, seemingly nonsense requirement to switch open the ground-neutral connection at the inverter when powering from shore or generator. Because then the neutral and ground connection has to move back to the actual source. Thus ABYC compliant inverter/chargers with auto transfer relay cannot be two phase and they must automatically disconnect neutral-ground when in charge/pass through mode.
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Old 16-11-2017, 06:02   #42
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Re: Grounding an inverter

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There are at least two reasons a GFCI must have a proper safety ground.

A GFCI that fails a self test must be replaced. So if the self test function does not work it cannot be relied upon and the unsuspecting boater will be forever replacing them.

How exactly can there be any "ground fault current" if there is no system safety ground circuit? A person would have to somehow connect themselves between hot and neutral at two different outlets to trip the GFCI. In which case both of them would trip. But how likely is that?

If a person were being electrocuted by a single GFI outlet in a system with no safety ground the GFI function would not shut off the power. This is one reason ABYC requires safety ground connected to neutral at the source. It is also why they have the, at first, seemingly nonsense requirement to switch open the ground-neutral connection at the inverter when powering from shore or generator. Because then the neutral and ground connection has to move back to the actual source. Thus ABYC compliant inverter/chargers with auto transfer relay cannot be two phase and they must automatically disconnect neutral-ground when in charge/pass through mode.
You must understand the architecture of a GFCI/RCD in order to realize how it works.

An on-board test function on a GFCI receptacle that requires the grounding conductor in order for the test function to operate is the choice of the manufacturer, IOW, a test function can be implemented that works regardless of the presence of the grounding conductor. Take a look at the GFI/RCD circuit breakers, no grounding conductor required for the test function to operate properly.

No one is arguing the value of the grounding conductor in a grounded power system.

An ungrounded power system has better safety attributes over a grounded power system. No grounding conductor required!
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Old 16-11-2017, 07:20   #43
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Re: Grounding an inverter

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................ An ungrounded power system has better safety attributes over a grounded power system. No grounding conductor required!
If this is correct, why are systems grounded?
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Old 16-11-2017, 08:24   #44
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Re: Grounding an inverter

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If this is correct, why are systems grounded?
Due to transmission line issues.

According to the NEC, the purpose of connecting an electrical system to the physical ground (earth) is to limit the voltage imposed by lightning events and contact with higher voltage lines, and also for voltage stabilization.

https://en.wikipedia.org/wiki/Ground_(electricity)

These reasons don't exist on a recreational boat AC power system.

There are many applications where an isolation transformer or inverter is used and the secondary power system is ungrounded to provide a safe(r) environment. A lot of medical applications deploy this scheme.

In small controlled environments, an ungrounded power system is safer.
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Old 16-11-2017, 08:50   #45
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Re: Grounding an inverter

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Originally Posted by transmitterdan View Post
There are at least two reasons a GFCI must have a proper safety ground.

A GFCI that fails a self test must be replaced. So if the self test function does not work it cannot be relied upon and the unsuspecting boater will be forever replacing them.

I have just given this a practical test and my GFCI still trips when the test switch is activated with the ground wire removed on both the inlet and outlet.
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