Connecting neutral of single phase hybrid inverter to the neutral of a 3 phase home grid supply

[MOHDHAQ]

Hello,
I have a 3 phase grid supply and a neutral (4 wires coming from the transformer). Phase to Neutral is 220VAC/50 Hz.
I have s single phase Hybrid inverter type Noor Power Plus Model 2SSP3119C 5KVA with two wires for AC input and two wires for AC output. Of course it also has an input for solar panels and also for a battery bank.
I am using a knife switch to select manually between the grid (three phases and neutral) on one side and the only one phase and neutral coming from the inverter on the other side (looped to supply the three phases of the house when grid id not available). Each side has its own neutral wire. I am using a 2x3 phase RCD to supply the 2 floors in my house. The switching is done manually by operating the knife switch to either inverter input or grid.
I want to automate this operation. I have dedicated two phases for the heavy loads (4 A/Cs on each phase). These should work only when there is grid available. The third phase is the same phase supplying the inverter with its AC input. This phase I want to use for the other loads i.e. lights, fridges, washing machines, receptacles, water pumps etc...
I intend to connect the phase of the inverter AC output at the knife switch in place of one of the grid three phases (the same used to supply AC input to the inverter). I have to connect the neutral of the inverter AC output to the grid neutral at the knife switch. I am scared to death to do that. Can anyone with better experience help? the instruction manual is very poor and does not tell you anything in this regard.

appreciate your help.

 

[sunshine_eggo]

Sounds like a job for...

@timselectric

 

[timselectric]

In your situation I don't think you want to connect both (input and output) neutrals to the grid neutral. I believe it would probably conflict with the RCD.
If you can separate the 3rd phase neutral for the loads you want to feed. And connect it to the output neutral of the inverter (AIO? ). And then connect the input neutral to the grid neutral.
Everything should work properly.

 

[MOHDHAQ]

In your situation I don't think you want to connect both (input and output) neutrals to the grid neutral. I believe it would probably conflict with the RCD.
If you can separate the 3rd phase neutral for the loads you want to feed. And connect it to the output neutral of the inverter (AIO? ). And then connect the input neutral to the grid neutral.
Everything should work properly.

thanks timselectric for your response.
actually yesterday few hours after posting my question, I connected the inverter output neutral to the grid neutral and of course the AC input of the inverter has also its neutral connected to the same grid neutral. I was scared but it worked without causing any problem. In fact I also thought the RCDs will not like it but they did nothing. I even thought of replacing the 2x3phase RCDs with 6x1phase RCDs so that every single phase has two separate RCDs (one for each floor). How does that sound?. Having a separate neutral for the inverter circuit output is of course the best but it requires running wires, drilling holes and making changes in the distribution panels which I am reluctant to do. I will have to check to be sure that the RCDs are really functioning properly in case of a ground fault anywhere. I was more scared to have the inverter to act as a load instead of a generator when the grid comes on with all consequences one can imagine. Luckily Nothing happened.
of course the 3 phases are now way out of balance and neutral carries a current of more than10A with the 6 AC units working on the 2 dedicated phases and all other appliances on the third phase.

 

[MOHDHAQ]

thanks timselectric for your response.
actually yesterday few hours after posting my question, I connected the inverter output neutral to the grid neutral and of course the AC input of the inverter has also its neutral connected to the same grid neutral. I was scared but it worked without causing any problem. In fact I also thought the RCDs will not like it but they did nothing. I even thought of replacing the 2x3phase RCDs with 6x1phase RCDs so that every single phase has two separate RCDs (one for each floor). How does that sound?. Having a separate neutral for the inverter circuit output is of course the best but it requires running wires, drilling holes and making changes in the distribution panels which I am reluctant to do. I will have to check to be sure that the RCDs are really functioning properly in case of a ground fault anywhere. I was more scared to have the inverter to act as a load instead of a generator when the grid comes on with all consequences one can imagine. Luckily Nothing happened.
of course the 3 phases are now way out of balance and neutral carries a current of more than10A with the 6 AC units working on the 2 dedicated phases and all other appliances on the third phase.

in addition when the grid is available it seems that the inverter, beside charging the batteries from the grid, it also switches the output directly to the input which implies connecting the grid neutral to the output neutral anyway.

 

[timselectric]

in addition when the grid is available it seems that the inverter, beside charging the batteries from the grid, it also switches the output directly to the input which implies connecting the grid neutral to the output neutral anyway.

Correct.
In bypass, they are connected together.
In inverter mode, they're not.
I expected that switching between the two modes would trip the RCD. Luckily your RCD isn't as sensitive as most.

 

[MOHDHAQ]

Correct.
In bypass, they are connected together.
In inverter mode, they're not.
I expected that switching between the two modes would trip the RCD. Luckily your RCD isn't as sensitive as most.

Or maybe the RCDs are not working at all since they are old from the original home installation ?. I have to check them. In order to check the RCD I intend to connect a 5w bulb between phase and ground (third pin of the receptacle). That will allow some current to flow from phase through ground conductor to earth (assuming this path is still in tact). That current will not return through the neutral conductor which should trip the RCD. Is there any better way to test the RCDs?. I used to utilize this method to check the old Earth Leakage device I was having in another house somewhere else. I have never used it to check RCDs but I assume it is the same principle (just looking for small current difference between phase and neutral). thanks.

 

[timselectric]

Or maybe the RCDs are not working at all since they are old from the original home installation ?. I have to check them. In order to check the RCD I intend to connect a 5w bulb between phase and ground (third pin of the receptacle). That will allow some current to flow from phase through ground conductor to earth (assuming this path is still in tact). That current will not return through the neutral conductor which should trip the RCD. Is there any better way to test the RCDs?. I used to utilize this method to check the old Earth Leakage device I was having in another house somewhere else. I have never used it to check RCDs but I assume it is the same principle (just looking for small current difference between phase and neutral). thanks.

That will work fine for testing it.