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240v inverters and ground-neutral bonding?

Rumbaar

New Member
Joined
Jul 23, 2022
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30
Location
Melbourne, Australia
I understand basic DC and AC (240v) principals, but within the off-grid setup I have I'm having some questions on how I setup a 'safe' system. I've installed an array, solar charge controller, battery and inverter (240v) AU. I've also installed a grounding rod and have the solar charge controller and inverter all back to a common busbar and that is connected to the earthed rod.

But when I use a plug tester I get an error on the earth connection, as in it doesn't show as an LED to indicate all is working. Now I've watched some YouTube videos and found that it's due to the way the inverter generates 240v power. With a normal AC (Australia household) setup being L+N = 240v, N+L =240v and N+E = 0v, but with an inverter it's L+N = 240v, N+L = 120v and N+E = 120v and thus it'll never have a correct test result.

On the inverter it has 3 output, L, N & E and I've attacted my grounding to that E output connector (only grounding point for the inverter and per contacting supplier).

So my question is, am I protected from an Earthing issue? With the two 120v phases making a single 240v out, is it even possible for me to achieve what I'm trying to do? I've read, and I need to look into more, about ground-neutral bonding, but I'm not sure if I should attempt this or how to go about it.

What is the best way to go about this? Any help appreciated. Here is a photo of the current setup, if that might help.

shed_20221013_172841.jpg
 
I believe everything is correct and safe. The inverter seems to create a floating neutral similar to most generators in our 120v world of the US systems. Still the ground is connected same and all works fine.
 
You should contact the Inverter manufacturer to see you can have N-G bonding without damaging the unit or not first.
 
I think you are getting horribly confused by our crazy 120/240 USA split phase power system.
There are some great "down under" forums that will answer your questions more clearly.
 
I understand basic DC and AC (240v) principals, but within the off-grid setup I have I'm having some questions on how I setup a 'safe' system. I've installed an array, solar charge controller, battery and inverter (240v) AU. I've also installed a grounding rod and have the solar charge controller and inverter all back to a common busbar and that is connected to the earthed rod.

But when I use a plug tester I get an error on the earth connection, as in it doesn't show as an LED to indicate all is working. Now I've watched some YouTube videos and found that it's due to the way the inverter generates 240v power. With a normal AC (Australia household) setup being L+N = 240v, N+L =240v and N+E = 0v, but with an inverter it's L+N = 240v, N+L = 120v and N+E = 120v and thus it'll never have a correct test result.

On the inverter it has 3 output, L, N & E and I've attacted my grounding to that E output connector (only grounding point for the inverter and per contacting supplier).

So my question is, am I protected from an Earthing issue? With the two 120v phases making a single 240v out, is it even possible for me to achieve what I'm trying to do? I've read, and I need to look into more, about ground-neutral bonding, but I'm not sure if I should attempt this or how to go about it.

What is the best way to go about this? Any help appreciated. Here is a photo of the current setup, if that might help.

View attachment 116254
I don’t understand… if L+N = 240V then N+L MUST ALSO = 240V… electrically I mean… the N+G equaling 120V should not be though…
 
Just about all high frequency inverters use an H-bridge output stage. This allows their high voltage DC to be just slightly greater than the AC sinewave peak voltage by the H-bridge effectively switching the polarity of the high voltage DC to get the positive half and negative half AC cycles.

The battery to HV DC to DC converter has a closed loop feedback to regulate HV DC level. Problem comes into play when the DC to DC converter does not have an electrical galvanic isolation between its HVDC output side and battery input side common connection within inverter. Most all newer inverters use opto-isolators in regulation feedback control path to do this input to output isolation.

An older (years) HF inverter may not have the isolation so the battery lines will have output AC riding on them if AC output neutral is grounded. If battery negative is also grounded when AC output neutral is grounded it can blow out inverter. If you stand on ground and touch floating (ungrounded) battery lines connection you can get a nasty shock. It is more common to see this issue on older modified sinewave inverters. True sinewave inverters are modern enough to have the galvanic isolation.

A little confusing detail is most HF inverters have small capacitors from their chassis/case ground and each AC output prong to reduce digital noise EMI output. When nothing is connected to inverter AC output and you measure AC voltage with a high input impedance AC voltmeter from each AC prong and its ground prong you will see about half the AC voltage on each AC prong because of the small EMI capacitors. You can still ground the neutral AC side even when you see this, as long as the inverter has the battery side to AC side isolation.

All this also applies to inverter-generators. Some inverter-generators do not allow their AC neutral to be connected to their ground.
 
Last edited:
I understand basic DC and AC (240v) principals, but within the off-grid setup I have I'm having some questions on how I setup a 'safe' system. I've installed an array, solar charge controller, battery and inverter (240v) AU. I've also installed a grounding rod and have the solar charge controller and inverter all back to a common busbar and that is connected to the earthed rod.

But when I use a plug tester I get an error on the earth connection, as in it doesn't show as an LED to indicate all is working. Now I've watched some YouTube videos and found that it's due to the way the inverter generates 240v power. With a normal AC (Australia household) setup being L+N = 240v, N+L =240v and N+E = 0v, but with an inverter it's L+N = 240v, N+L = 120v and N+E = 120v and thus it'll never have a correct test result.

On the inverter it has 3 output, L, N & E and I've attacted my grounding to that E output connector (only grounding point for the inverter and per contacting supplier).

So my question is, am I protected from an Earthing issue? With the two 120v phases making a single 240v out, is it even possible for me to achieve what I'm trying to do? I've read, and I need to look into more, about ground-neutral bonding, but I'm not sure if I should attempt this or how to go about it.

What is the best way to go about this? Any help appreciated. Here is a photo of the current setup, if that might help.

View attachment 116254
 
Last edited:
You should contact the Inverter manufacturer to see you can have N-G bonding without damaging the unit or not first.
I've gone ahead and setup a N-G inline with a RCD (safety switch) breaker I setup for the output line of the inverter and it's appears to have 'fixed' the AC plug error and I get 'all good' LED indicators. For all outputs.

N-G_bond_20221016_132723.jpg
I think you are getting horribly confused by our crazy 120/240 USA split phase power system.
There are some great "down under" forums that will answer your questions more clearly.
This is an AU model of a 240v inverter, to which most US based inverters are 120v systems. But yes, it seems default 240v systems are a different beast compared to the US based 120v and require a different way of thinking or needs.
I don’t understand… if L+N = 240V then N+L MUST ALSO = 240V… electrically I mean… the N+G equaling 120V should not be though…
Yeah, I don't understand it either, but that's how the inverter seems to produce a 240v output or how some do. My house 240v (N-G bonded configuration) has the 240/240/0, but the inverter output has the 240/120/120 when testing via a multimeter. I stumbled upon this, as it was new to me, via a YouTube video (UK) based that explained why I was getting the plug probe error for non ground.
Just about all high frequency inverters use an H-bridge output stage. This allows their high voltage DC to be just slightly greater than the AC sinewave peak voltage by the H-bridge effectively switching the polarity of the high voltage DC to get the positive half and negative half AC cycles.

The battery to HV DC to DC converter has a closed loop feedback to regulate HV DC level. Problem comes into play when the DC to DC converter does not have an electrical galvanic isolation between its HVDC output side and battery input side common connection within inverter. Most all newer inverters use opto-isolators in regulation feedback control path to do this input to output isolation.

An older (years) HF inverter may not have the isolation so the battery lines will have output AC riding on them if AC output neutral is grounded. If battery negative is also grounded when AC output neutral is grounded it can blow out inverter. If you stand on ground and touch floating (ungrounded) battery lines connection you can get a nasty shock. It is more common to see this issue on older modified sinewave inverters. True sinewave inverters are modern enough to have the galvanic isolation.

A little confusing detail is most HF inverters have small capacitors from their chassis/case ground and each AC output prong to reduce digital noise EMI output. When nothing is connected to inverter AC output and you measure AC voltage with a high input impedance AC voltmeter from each AC prong and its ground prong you will see about half the AC voltage on each AC prong because of the small EMI capacitors. You can still ground the neutral AC side even when you see this, as long as the inverter has the battery side to AC side isolation.

All this also applies to inverter-generators. Some inverter-generators do not allow their AC neutral to be connected to their ground.
I'll have to assume my 'newer' inverter has this isolation in place and I don't have to worry about DC to AC bridging within the inverter and DC never touches the AC side of the output.

Yes, I've gone ahead and ground the neutral AC side and it's 'solved' my current plug probe error I was getting. I've installed an RCD Safety Switch breaker on the output connections and it's 'fixed' the test on all outputs (2 x 240v plugs). I guess if it wasn't isolated, it would have blown up or errored the inverter immediately?

Thanks all for your replies.
 
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