About grounding... again

[marksman]

After looking through threads on this forum and reading my country's national electric code I'm more confused than when I started.

I have a small 24 V system w/ 920 Wp of panels and a 3000 VA AiO EAsun/Powland inverter. Its main purpose is to power my shed (mainly my heater's water pump - about 120 W) and my internet access gear - about 55 W. Its secondary purpose is to power the house (essential hardware only) during a severe power outage.
Currently the pump and internet gear are plugged into a 500 VA UPS that charges the battery from a house outlet.

Now, while testing the AiO (NOT connected to the grid power) I found out that it has what I believe is called a "floating neutral". Even though I was supplying ground from the house ground, testing with a plug tester was showing no ground. Testing for live showed live only on the live wire, as expected. I decided to bond the neutral to the ground at the inverter output, right before the 16 A GFCI that supplies the (test) power to the gear. Now the plug tester shows the plug is wired correctly, but this is not the real working condition because:
1. I need to connect the AiO to the grid to charge the batteries in winter, when the solar would not be enough to top them off.
2. I need to wire the same output into the house (there is a house manual transfer switch that switches the live and neutral only) and with the N-G bond at the inverter output my understanding is that I will either be tripping the house GFCI or route the whole neighborhood through my property ground.

As the pump and the internet gear are on a separate connection from the house (they share the ground through the UPS when the power is out - I hear a relay clicking and I believe it is the neutral-ground bonding connection that is being made when the power goes out), how can I get the setup to work correctly with grid powered AiO? The AiO gets the grid from the same panel as the house, but through a different, 16 A breaker. House has a 25 A breaker.

 

[meetyg]

There is a long discussion above solving this issue with AIOs that don't bond N-G internally.
Are you running single phase or split phase AC ?
If single phase, basically you have two options, each with its own caveats:
1. Create a "common Neutral": Connect AC IN Neutral (only the Neutral!) to AC OUT Neutral.
This way, when inverter is running from grid, you get the bond from your grid supply. When running in battery/off-grid mode, it will use the same bond.
This is the simplest way, but you need to trust your grid supply N-G bond. Depending on your electrical grounding method (TN-C/TN-C-S etc...) this may or may not be a good idea. Also, if you have a GCFI/RCD before the inverter input, it might trip. That means that you might need to power the AIO before the main GFCI/RCD.

You also need to check with local regulations, to see if it's legal to do this or not. Some regulations require a full isolation between grid and another power source.

2. Use a Normally Closed relay/contactor, connected to grid power, that will do the N-G bond when grid is out. This allows isolation of the Neutral from the grid. Most of these AIOs will keep the grid bond, if grid is connected to AC IN (and grid is alive), even if they supply power from the solar/battery. You need to check this though, to make sure your specific model also does this.
The downside to this method is that when grid comes back, the relay will open immediately. The inverter can take a while to return to grid power, as it has to sync with the grid's sine wave. So for that short while, you will have a floating output. For added safety, you should be using a GFCI or RCD on the inverter output.

Personally, I prefer method #2, as I think it conforms to my local regulations better (isolated sources of power) and is more robust in case of loosing the grid's N-G bond (which could happen due to various reasons, again depending on you local grounding method). On the other hand, you rely on this relay/contactor, that could also fail.

 

[marksman]

I went with the second option. However there is something odd: when checking with a socket tester (picture below, situation in 1st row on the explanations) I do get that the ground connection is present but the rightmost LED is dimmer than the other one. When I turn off the grid connection breaker the contactor closes and the LED is lit at the same brightness level as the other.


I thought that the contactor could be at fault but no, I get the same situation if I manually make the connection between neutral output and ground. The only thing I did not do at the AIO was to connect the ground to its case because I tested with a DMM and found that case and ground input are connected.
Any thoughts?

 

[marksman]

I have tested with a DMM and with input breaker open I found I have 218V between hot output and ground and 12 V between neutral output and ground. These voltages add up to 230 V, exactly the value between hot output and neutral output. I can't figure out why the LED is dim with the input breaker on.

 

[marksman]

There is a long discussion above solving this issue with AIOs that don't bond N-G internally.
Are you running single phase or split phase AC ?
If single phase, basically you have two options, each with its own caveats:
1. Create a "common Neutral": Connect AC IN Neutral (only the Neutral!) to AC OUT Neutral.
This way, when inverter is running from grid, you get the bond from your grid supply. When running in battery/off-grid mode, it will use the same bond.
This is the simplest way, but you need to trust your grid supply N-G bond. Depending on your electrical grounding method (TN-C/TN-C-S etc...) this may or may not be a good idea. Also, if you have a GCFI/RCD before the inverter input, it might trip. That means that you might need to power the AIO before the main GFCI/RCD.

You also need to check with local regulations, to see if it's legal to do this or not. Some regulations require a full isolation between grid and another power source.

2. Use a Normally Closed relay/contactor, connected to grid power, that will do the N-G bond when grid is out. This allows isolation of the Neutral from the grid. Most of these AIOs will keep the grid bond, if grid is connected to AC IN (and grid is alive), even if they supply power from the solar/battery. You need to check this though, to make sure your specific model also does this.
The downside to this method is that when grid comes back, the relay will open immediately. The inverter can take a while to return to grid power, as it has to sync with the grid's sine wave. So for that short while, you will have a floating output. For added safety, you should be using a GFCI or RCD on the inverter output.

Personally, I prefer method #2, as I think it conforms to my local regulations better (isolated sources of power) and is more robust in case of loosing the grid's N-G bond (which could happen due to various reasons, again depending on you local grounding method). On the other hand, you rely on this relay/contactor, that could also fail.

As the output is still floating in battery mode even if it is connected to the grid, I've scrubbed the contactor and went with the common neutral option. This way I have the N-G bond provided by the grid and got rid of the voltage between N and G.