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Causes of high voltage neutral-ground? (Axpert)


Nuno

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Just finished setting up my first ground rod, and decided to test the voltage difference between the ground and the output of my inverter. I'm getting super odd values and have no idea what could be causing this (note: I tested directly with the ground rod, so its not a case of bad wiring):

Live-Neutral: 230V
Live-Ground: ~110V
Ground-Neutral: ~100V

Is this the inverter acting up or could be some other issues? Or is this just standard Axpert behaviour? 😕

Not sure if its related or not, but it also doesnt seem to make my GFCI breaker work properly! It breaks when I press the test button, but if I short the Live wire with a ground, nothing happens, which means all my ground wires (and potentially any metal casings) will be live at 110V 😮

It's an Axpert 5kva 48v

Edited by Nuno
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6 hours ago, Nuno said:

It's an Axpert 5kva 48v

As @Javi Martínez suggests, that covers a lot of models: older 4 kW models, newer 5 kW models, 450 V MPPT models or 145 V MPPT models, Value models and standard models, and so on.

The only models I'm fairly certain that have the AC out neutral to ground relay contact are the ones that come with main firmware 73.00.

If you're off-grid, you can probably get away with connecting neutral AC out to earth. The GFCI breaker relies on a neutral to earth connection somewhere.

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9 hours ago, Nuno said:

super odd values

Not really. It means your neutral is not bonded to earth. The reason it floats in the middle is because there is some capacitive leakage leakage between conductors, plus any appliance with a surge protector or EMI filter will likely leak a little bit to earth too. Combined, this causes an unbonded earth to float up to the middle of the supply (110V), or rather (if you want to be pedantic), it causes live and neutral to float towards 110V on either side of earth.

If there is a grid connection, then check if there is a voltage between neutral and earth while connected to the grid (as in, inverter is in bypass). If the voltage is zero while on grid, but floats at 110V when running from the inverter, then you have one of the older Axperts that didn't have a bonding relay. You then have to figure out if your axpert has the firmware option for controlling an external bonding relay through it's dry-contact relay, and if you also don't have that... well then you're pretty screwed.

If you are completely off-grid, simply tie neutral to earth.

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thank you both.

I'm not sure which model it is... where can I check that? I am offgrid, but the inverter does support utility connection.

right now there is NO ground bonding, and thats why I guess I have a voltage of 110V between neutral and ground...

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2 minutes ago, Nuno said:

right after the inverter neutral output?

^^^^ This one.

If you do it after the GFCI (aka RCD, aka earth leakage) it will cause it to trip as current bypasses the RCD on the earth return.

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34 minutes ago, Nuno said:

Like this, right?

Pretty much. Although the proper way to do it is to have another short neutral bus before the GFCI, and then have a physical wire bonding this bus to the ground, and then from this bus you go to the neutral side of the GFCI... and that then feeds the long neutral bus where all the loads connect to. A bit like this (this was at @The Terrible Triplett's house where they had to install an earth spike and make it TN-C-S). Note the bond indicated by the green arrow.

On an off-grid setup I believe you can do this right in the main DB.

tncs.png.92abe10164fa9c2ca47c36464711bf8d.png

Edited by plonkster
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8 minutes ago, plonkster said:

... where they had to install an earth spike and make it TN-C-S ...

FWIW. If it was not for me going grid tied, with the tests required as per the form, we would not have found the problem with the bonding.

The faulty original bonding was one of the reasons my Multigrid kept on tripping the Ac-Out1 DB.

My advice, as said before, have ones DB tested before connecting anything UPS / solar to it. Saves a lot of frustrations IF there is a latent defect.

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thanks guys, I'm going to try this today and see what happens!

(fingers crossed because in another forum someone told me not to bond neutral-ground in this kind of inverters as they could go up in smoke :x)

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48 minutes ago, Nuno said:

Like this, right?

Heh. I see you have labelled your AC out with + and —. I friend of mine told me that in countries that use black for neutral and red for active (most house wiring in Australia is like that), there are plenty of electricians that believe this to be literally true, i.e. active is always positive with respect to neutral.

In Australia, we use the international colours of brown / light blue / green-yellow for active / neutral / earth, in our flexible cables. But for reasons that escape me completely, our house wiring is red / black / green-yellow.

@Nuno, I hope that you realise that AC (Alternating Current) also implies Alternating Voltage, even though we never call it that. So half the time, the red wires are negative with respect to the red ones. And a hundred times per second, they cross zero.

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1 minute ago, Coulomb said:

Heh. I see you have labelled your AC out with + and —. I friend of mine told me that in countries that use black for neutral and red for active (most house wiring in Australia is like that), there are plenty of electricians that believe this to be literally true, i.e. active is always positive with respect to neutral.

In Australia, we use the international colours of brown / light blue / green-yellow for active / neutral / earth, in our flexible cables. But for reasons that escape me completely, our house wiring is red / black / green-yellow.

@Nuno, I hope that you realise that AC (Alternating Current) also implies Alternating Voltage, even though we never call it that. So half the time, the red wires are negative with respect to the red ones. And a hundred times per second, they cross zero.

Yes, I know :) It was just a quick way to represent it in the drawing :)

For what I was told, this kind of (cheap) inverters generate AC on the live and the neutral sides, half in each, and thats why I got 110v live-ground and 110v neutral-ground...

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15 minutes ago, Nuno said:

someone told me not to bond neutral-ground in this kind of inverters as they could go up in smoke :x)

That only happens in very unusual inverters with split phase outputs (for those legacy countries still using 120 V), and when they also internally bond one of the phases to the metal case and to earth. As @plonkster told me recently (thanks!).

Edited by Coulomb
Linked to the mentioned post.
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15 minutes ago, Nuno said:

For what I was told, this kind of (cheap) inverters generate AC on the live and the neutral sides, half in each, and thats why I got 110v live-ground and 110v neutral-ground... 

This one is actually not THAT cheap... I mean it is cheap... but it is actually kind of decent for how cheap it is.

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There are actually a few 120 V Axpert models, but you have to buy two of them to make a split phase system. This is possibly one of the reasons that Axperts are not all that common in the USA.

So yes, I can confidently say that bonding AC-out neutral to earth on a single Axpert inverter won't blow it up.

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24 minutes ago, Nuno said:

Yes, I know :) It was just a quick way to represent it in the drawing :)

For what I was told, this kind of (cheap) inverters generate AC on the live and the neutral sides, half in each, and thats why I got 110v live-ground and 110v neutral-ground...

To know what your inverter does, measure voltage Nout-Ground in line mode, and the same in battery mode.

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I think that there is still a problem with though, if a generator is used. A generator presumably bonds its neutral output to earth, so that some of the generator neutral current would travel through the earth conductors. I think the GFCI won't trip, though these things do my head in when I can't see the whole schematic.

I don't know the answer to that one. Especially when the generator can be disconnected. [ Edit: apart from an external earth to AC-out neutral relay. ]

Edited by Coulomb
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44 minutes ago, Javi Martínez said:

Join AC In Neutral - AC Out Neutral.

That works great (I do it myself) if you are on-grid and have the one and only permanent connection from neutral to earth where the grid comes in. But with a generator, the "grid" can be unplugged. If you bond AC-in neutral to earth, then when you run a generator, again the generator earth wire takes some of the neutral current (or so it seems to me). Maybe that's not such a disaster.

Edit: also, in places where they like to isolate the neutral as well as the active when isolating the grid input to the inverter, you can't rely on the grid's neutral to earth bonding. It seems that this is common in South Africa.

Edited by Coulomb
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2 hours ago, Coulomb said:

I think that there is still a problem with though, if a generator is used. A generator presumably bonds its neutral output to earth, so that some of the generator neutral current would travel through the earth conductors. I think the GFCI won't trip, though these things do my head in when I can't see the whole schematic.

Oh yes! I have a story about this, dating back to high school. So as the locals know, around 1989 the South African military withdrew out of Namibia as part of UN resolution 435, and as a result of that heaps and heaps of vehicles and equipment got sold on auctions. One of the items the military had were 3-phase Diesel generators on a trailer, but these things had a 36V DC charger built in for charging some batteries, and because 32V lighting was commonly available, both the military and most farms had lighting systems running on 36V.

A farmer in our area bought such a generator and had an electrician connect it to his existing setup (which had a single-phase generator, also with 36V charger).

These generators had a built-in earth/neutral bond, as they should. They also had a bond between battery negative and ground.

So upon connecting these two generators to the same battery bank, the RCD started to trip. We eventually fixed the problem by trial and error culminating in the removal of one of the battery/earth bonds -- a fix which turned out to be the correct one -- but it took me years before I realised what was going on: These two generators had completely separate AC systems, each with its own DB and RCD, and a changeover so the main farm house could be powered from L1, but because the battery system was in common, and the negative was bonded both ends, we had inadvertently bypassed the neutral of both DBs.

So... similar story here. If you already have a generator in there, tied to the AC-in of the inverter, and you add another bond on the inverter out... well, it may or may not cause problems. Would have to consider the whole picture.

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On the side of my house I have this old meter - no longer used, and the wiring you see.

My power comes through here since throwing that breaker turns off everything.

The red sign says "Earthed to neutral" or "aan Neutraal geaard"

i do have an earth rod down below which I assume is connected somewhere here too,

The breaker is 80A. Not sure what’s in the box bottom right that is labelled 25A.

The R10 fine is a scary thought!

Can anyone tell me how this works and what goes where?

Is this TNC-S?

 

71DF8691-6807-4F01-BC68-5491D2B3F8B4.thumb.jpeg.4c6a2ba9434dcfff9d6d021aa9133492.jpeg

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1 hour ago, Elbow said:

i do have an earth rod down below which I assume is connected somewhere here too,

 

1 hour ago, Elbow said:

Is this TNC-S?

Since you have an earth rod on the premises, yes, it is probably TN-C-S.

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