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diy solar

Need to bond neutral + ground for off grid shed using PIP 3048LV-MK 48V 3kW 120V Inverter

OK.... Sorry. Is this the diagram?

View attachment 86057

That one is confusing because the 'highlighted' current does not go through a load.
It is correct to say that there would be no objectionable current with a single 240 V load (or any perfectly balanced load).

However, with any imbalanced load there will be an objectionable current. (Unbalanced loads is the typical case if you have 120V loads)

There will be an imbalance with a 120V load, current will return to the source (inverter providing the phase).

In the diagram above, it's all messed up. It appears the poster is has connected 2 neutrals to each inverter. Have no idea where they get this idea from. I believe it is the poster that wanted to tie input and output neutrals together which is not approved by MPP.
The primary differences between the drawing I showed and the one above is that there is no Grid on the AC input and no xfer switch.

Not what is being discussed.
 
In the diagram above, it's all messed up. It appears the poster is has connected 2 neutrals to each inverter.
I am not sure what diagram you are referring to, but the one I posted has one neutral per inverter.

Law 1: The total current entering a junction or a node is equal to the charge leaving the node as no charge is lost.
That is absolutely true. It is also true that current will split and take all paths back to the source. The amount of current on each path is inversly proportional to the resistance of that path.

So... I modified my drawing to show smaller lines on the return current after it splits to the unwanted path:

1646428018056.png

Not what is being discussed.
OK. I am not sure what you are referring to. Please let me know. Meanwhile, I am pretty sure @mccljs have been referring to this diagram.
 
There will be an imbalance with a 120V load, current will return to the source (inverter providing the phase).

In the diagram above, it's all messed up. It appears the poster is has connected 2 neutrals to each inverter. Have no idea where they get this idea from. I believe it is the poster that wanted to tie input and output neutrals together which is not approved by MPP.


Not what is being discussed.
There are not two neutrals connected to each inverter although the highlighting does obscure the wire colors. FilterGuy's version is much clearer so we should use it for discussion. Compare the highlighted and un-highlighted diagrams and you will see that there is only one neutral to each inverter.

Note that the objectionable current will occur ONLY IF BOTH inverters bond the neutral to ground in battery mode, which has not yet been determined. If the inverters are smart enough to bond the neutral to ground only in the top inverter in these examples, the objectionable current problem will not exist. In his Advanced Level Grounding Discussion video at about 3:30, Will says that there are neutral-ground bonds in both inverters so maybe he would be willing to do some testing. Is there a way to tag him so that he will receive a notification of this post?


Resolving this question is very important because objectionable current (current flowing on the green EGC) can kill you.

For example, the neutral to the top inverter is not connected. Maybe it was a loose connection and slipped out of the terminal. Normally, you would notice this because some of your loads would no longer work because they were not receiving power. In the below diagrams, those loads would continue to receive power because now all of the neutral current to the top inverter is flowing through the EGC (green wire).

Then you decide to redo the EGC connection. You leave the inverters on because you don't expect the EGC to be carrying current. You remove the EGC from the top inverter and touch the case of the inverter with one hand and the EGC with the other hand. Now 120V is flowing through your heart. Not a good thing. It would also happen if you removed the EGC from the bottom inverter, the panel, etc. Anything that would put you in series with the objectionable current flowing on the EGC.


This is also very interesting if you have time to watch the entire video.


1646426415847.png
1646426439371.png
FilterGuy's version.
1646426483713.png
 
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Note that the objectionable current will occur ONLY IF BOTH inverters bond the neutral to ground in battery mode, which has not yet been determined.
Based on everything @Will Prowse has said in his video and comments in other threads, I am 99% sure that the bonding relays on both inverters engage when the inverters go to battery mode. This is reinforced by the fact that to disable the relay, you must physically remove the screw. (If the SW could control it, there would almost certainly be a setting for it)
 
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@mccljs : I really like the Mike Holt Videos. He really knows his stuff!!

In one of his videos (I am not sure which one), he mentions the RFI issues with the objectionable current that is very pertinent to what we do with Inverters.

The inverters job is to rapidly switch high currents on and off in order to convert DC to AC. *Any* time a large current is switched on or off, it generates RFI noise. The parallel path created by having two N-G bonds creates a big loop antenna that will resonate at certain frequencies and broadcast noise. A small noise from the inverter can 'pump' this resonance a create some really bad RFI. This RFI can interfere with any radio signal including but not limited to Bluetooth, Wifi, AM, FM, and Cell phones. What is worse, all but the AM and FM have error correction built in so they still work.... but at a reduced performance. Consequently, it may be impacting you and you don't even know it.

I have actually observed the RFI issue from an inverter. I had a hunting cabin in the mountain that had terrible cell phone service. I noticed that sometimes the service was better than others (It was never good). One day a realized that it was always better when the inverter was off.

I did not have ground loops or dual NG-Bonds, but the inverter was noisy enough to impact the weak cell signal. If you add ground loops or neutral loops, it would almost certainly be even worse. (I played with the install for quite a while and never did solve the problem)

My joke is that you may not be able to tune in to your favorite 70s disco station.... but maybe that is a good thing:)
 
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