24V off-grid system - theoretical and practical questions around NG bonding, grounding and earthing.

[GeeGee]

Hi,

I'm building a 100% off-grid system (i.e. no AC inputs, AC outputs not connected to house wiring or Grid) and have some questions around Neutral-to-Ground (NG) bonding, grounding equipment and earthing.

I've read the grounding PDF manuals from this site and many posts. The purpose of this post is to get additional forum input, thoughts and advice as an overall sanity check. I intend to call out an electrician once I've set up my system. I've included diagrams and images for reference.

These are my basic components:

• 24V 3000VA Victron Phoenix Smart Inverter
• 2 x 12V 200Ah Renogy batteries in series
• 2 x Victron MPPT charge controllers
• ~830W solar panels
• AC Consumer Panel with 1 x MCB (25A) and 2 x RCBO (16A and 10A) (which feed wall outlets)

Please see Images below for more detail.

Things you should know and some of my assumptions:

• The solar panel frames are grounded to earth.
• Victron's manual (see image below) says the inverter is internally NG bonded. I tested the inverter AC voltage from Hot to Neutral, Hot to Ground and Neutral to Ground and the test results came up 240V, 240V and 0V, respectively, which I'm told is one way to confirm an internal bond. I haven't done any other tests.
• I've grounded my Inverter chassis as recommended by Victron's manual (see image below), which says you should ground the inverter chassis using "at least half the cross-section of the conductors used for the battery connection". I grounded using a 4 AWG wire running from the Inverter earth lug to DC neutral. I did it like this because from what I've read, you should ground DC components to DC neutral so that excess current generated by a ground fault can trip a fuse/breaker.
• I'll admit, there is one electrical concept I don't quite fully understand and that is how do you treat the AC side and DC side of an off-grid setup like mine - that is, as one entire system or as separate systems with their own electrical rules to follow. I think my lack of understanding here may be the reason why I'm asking some of the questions below and the way I'm asking them. So apologies in advance for my ignorance.

So here are my questions please:

1. Should I leave the inverter internally bonded OR should I unbond the inverter and move the NG bond to my consumer panel? Are there pros/cons to where the NG bond should exist? I've read differing opinions on this forum.
2. Does the rule "There can only be one (1) neutral to ground bond across the entire electrical system" refer to both the AC and DC side of the system? That is, if you create a NG bond on the AC side, does that mean you shouldn't NG bond anywhere else, including anywhere on the DC side?
3. As stated above, on the DC side I have grounded my Inverter case to DC Neutral. Does this mean I've unintentionally created a 2nd NG bond? If yes, then how do I reconcile this with general consensus that you should ground DC-side components to DC neutral?
4. Where in my system should I earth? I'm thinking on the AC side (e.g. from the Consumer Panel ground busbar) and the DC side (e.g. from DC neutral busbar). Is that ok?

If you've read this far I thank you.

Looking forward to your responses.

Regards



*** Image - High-Level Schema





*** Image - Overall system setup (pre-solar connection)





*** Image - Inside view






*** Image - Victron Manual > Internal Bond (page 21)





*** Image - Victron Manual > Grounding the Inverter (page 5)

 

[sunshine_eggo]

So here are my questions please:

1. Should I leave the inverter internally bonded OR should I unbond the inverter and move the NG bond to my consumer panel? Are there pros/cons to where the NG bond should exist? I've read differing opinions on this forum.
2. Does the rule "There can only be one (1) neutral to ground bond across the entire electrical system" refer to both the AC and DC side of the system? That is, if you create a NG bond on the AC side, does that mean you shouldn't NG bond anywhere else, including anywhere on the DC side?
3. As stated above, on the DC side I have grounded my Inverter case to DC Neutral. Does this mean I've unintentionally created a 2nd NG bond? If yes, then how do I reconcile this with general consensus that you should ground DC-side components to DC neutral?
4. Where in my system should I earth? I'm thinking on the AC side (e.g. from the Consumer Panel ground busbar) and the DC side (e.g. from DC neutral busbar). Is that ok?

0: Why not a Multiplus? What if you need to use a generator?

1: It's whatever makes sense to you. If you plan to run a generator, it may make sense to move it to the AC panel because the MP would decouple the N-G bond and generators have a floating neutral with no ground. If you stick with the Phx, it's your preference.
2: No. That's on the AC side. You bond your battery (-) to the AC ground system. With 12V, you don't have to. In the states, it's required for 60-300VDC systems, but most do it anyway. It's certainly done that way in RVs.
3: You're good.
4: Agree.

 

[timselectric]

Hi,

I'm building a 100% off-grid system (i.e. no AC inputs, AC outputs not connected to house wiring or Grid) and have some questions around Neutral-to-Ground (NG) bonding, grounding equipment and earthing.

I've read the grounding PDF manuals from this site and many posts. The purpose of this post is to get additional forum input, thoughts and advice as an overall sanity check. I intend to call out an electrician once I've set up my system. I've included diagrams and images for reference.

These are my basic components:

• 24V 3000VA Victron Phoenix Smart Inverter
• 2 x 12V 200Ah Renogy batteries in series
• 2 x Victron MPPT charge controllers
• ~830W solar panels
• AC Consumer Panel with 1 x MCB (25A) and 2 x RCBO (16A and 10A) (which feed wall outlets)

Please see Images below for more detail.

Things you should know and some of my assumptions:

• The solar panel frames are grounded to earth.
• Victron's manual (see image below) says the inverter is internally NG bonded. I tested the inverter AC voltage from Hot to Neutral, Hot to Ground and Neutral to Ground and the test results came up 240V, 240V and 0V, respectively, which I'm told is one way to confirm an internal bond. I haven't done any other tests.
• I've grounded my Inverter chassis as recommended by Victron's manual (see image below), which says you should ground the inverter chassis using "at least half the cross-section of the conductors used for the battery connection". I grounded using a 4 AWG wire running from the Inverter earth lug to DC neutral. I did it like this because from what I've read, you should ground DC components to DC neutral so that excess current generated by a ground fault can trip a fuse/breaker.
• I'll admit, there is one electrical concept I don't quite fully understand and that is how do you treat the AC side and DC side of an off-grid setup like mine - that is, as one entire system or as separate systems with their own electrical rules to follow. I think my lack of understanding here may be the reason why I'm asking some of the questions below and the way I'm asking them. So apologies in advance for my ignorance.

So here are my questions please:

1. Should I leave the inverter internally bonded OR should I unbond the inverter and move the NG bond to my consumer panel? Are there pros/cons to where the NG bond should exist? I've read differing opinions on this forum.
2. Does the rule "There can only be one (1) neutral to ground bond across the entire electrical system" refer to both the AC and DC side of the system? That is, if you create a NG bond on the AC side, does that mean you shouldn't NG bond anywhere else, including anywhere on the DC side?
3. As stated above, on the DC side I have grounded my Inverter case to DC Neutral. Does this mean I've unintentionally created a 2nd NG bond? If yes, then how do I reconcile this with general consensus that you should ground DC-side components to DC neutral?
4. Where in my system should I earth? I'm thinking on the AC side (e.g. from the Consumer Panel ground busbar) and the DC side (e.g. from DC neutral busbar). Is that ok?

If you've read this far I thank you.

Looking forward to your responses.

Regards



*** Image - High-Level Schema

View attachment 187195



*** Image - Overall system setup (pre-solar connection)

View attachment 187198



*** Image - Inside view

View attachment 187199




*** Image - Victron Manual > Internal Bond (page 21)

View attachment 187200



*** Image - Victron Manual > Grounding the Inverter (page 5)

View attachment 187201

1. Optional, it really depends on how you design the rest of your system.

2. Only the AC has a neutral. DC is positive and negative.
I would only bond the negative on a mobile application. (RV or camper)
But you can do what you prefer.

3. Again, DC is positive and negative. (Not neutral)

4. Consumer panel ground bus bar. There should only be one connection to earth.

Bonus:

The solar panel frames are grounded to earth.

The solar panel frames, should be connected to the grounding system.
Again, there should only be one connection to earth.

 

[GeeGee]

Thanks for the super quick reply.

I choose the Phoenix for no big reason except that it wasn't an all-in-one system from Victron. I tend to prefer the resilience of having separate components in a system as opposed to an Inverter + charger unit. But that's just me. Tbh, I'll probably change my mind in the future (for instance, those all-in-one EG4 systems look sweet).

0: Why not a Multiplus? What if you need to use a generator?

2: No. That's on the AC side. You bond your battery (-) to the AC ground system. With 12V, you don't have to. In the states, it's required for 60-300VDC systems, but most do it anyway. It's certainly done that way in RVs.

Re: # 2, and just so I'm 100% clear, what you're saying is:

• The '1 NG bond' rule only apples to the AC side?
• Even though you say it's not required to earth a 24V off-grid system, but if I was to, then I should ground DC negative to earth AND that I should ground to my property's existing earth rod (which just happens to be used by my home's AC system)?

Just quickly, I just saw Tim's reply (which I'll respond to seperately)........yes, I've been referring to DC negative as DC neutral......my bad

 

[sunshine_eggo]

Thanks for the super quick reply.

I choose the Phoenix for no big reason except that it wasn't an all-in-one system from Victron. I tend to prefer the resilience of having separate components in a system as opposed to an Inverter + charger unit. But that's just me. Tbh, I'll probably change my mind in the future (for instance, those all-in-one EG4 systems look sweet).




Re: # 2, and just so I'm 100% clear, what you're saying is:

• The '1 NG bond' rule only apples to the AC side?
• Even though you say it's not required to earth a 24V off-grid system, but if I was to, then I should ground DC negative to earth AND that I should ground to my property's existing earth rod (which just happens to be used by my home's AC system)?

Just quickly, I just saw Tim's reply (which I'll respond to seperately)........yes, I've been referring to DC negative as DC neutral......my bad

Tim is always righter than I am. You can disregard my comments except for the multiplus bit... and you're wrong. If you're willing to trust the Phoenix inverter, the MP is probably even more reliable.

It's like you're saying you trust cars, but not if they have tires on them because you want to keep them separate...

 

[GeeGee]

Hi Tim,

Thank you for calling out my misuse of DC Negative vs AC Neutral terminology. I now see why it's important to get the nomenclature correct. I should have been more careful with usage of terms as it just ends up adding confusion and diluting the focus of my post.

2. Only the AC has a neutral. DC is positive and negative
I would only bond the negative on a mobile application. (RV or camper)
But you can do what you prefer.

Just so I'm clear:

• When you say you don't really need to bond in my type of application, are you saying there's no need to run a ground wire from the Inverter chassis to DC Negative OR are you saying there's no need to ground DC negative to earth? Just trying to understand what you're referring to as 'bonding' in this case. I'm guessing the latter, as in 'bonding' to earth?

3. Again, DC is positive and negative. (Not neutral)

Uhuh!!!!!!!!!

For sake of accuracy of this thread, I'm updating Q3 to the following because this is what I actually wanted to ask:

3. As stated above, on the DC side I have grounded my Inverter case to DC Negative. Does this mean I've unintentionally created a 2nd NG bond? If yes, then how do I reconcile this with general consensus that you should ground DC-side components to DC negative?

However, I now see that the rule "There can only be one (1) neutral to ground bond across the entire electrical system" actually answers itself. Neutral refers to AC, not DC. Thus the rule is applicable to AC only.....as @sunshine_eggo pointed out.

 

[timselectric]

Just so I'm clear:

• When you say you don't really need to bond in my type of application, are you saying there's no need to run a ground wire from the Inverter chassis to DC Negative OR are you saying there's no need to ground DC negative to earth? Just trying to understand what you're referring to as 'bonding' in this case. I'm guessing the latter, as in 'bonding' to earth?

I wouldn't connect DC negative to any grounding or earth.
It's done in mobile applications. Because there are usually interconnections between the house battery and starting battery. And the starting battery is already negative bonded to the mobile chassis.
Reason:
If the house system happens to fault to the chassis. It needs a good path for fault current. Without traveling through the starting system.

 

[GeeGee]

I wouldn't connect DC negative to any grounding or earth.
It's done in mobile applications. Because there are usually interconnections between the house battery and starting battery. And the starting battery is already negative bonded to the mobile chassis.
Reason:
If the house system happens to fault to the chassis. It needs a good path for fault current. Without traveling through the starting system.

So you're saying don't connect the Inverter Chassis to DC Negative.

Ok, so what happens in the situation if the Inverter has a ground fault and the chassis becomes active / hot? How does the ground current get diverted back to the proper path (i.e. DC negative) in the hope tripping a breaker or fuse, so no one gets shocked?

 

[timselectric]

So you're saying don't connect the Inverter Chassis to DC Negative.

Ok, so what happens in the situation if the Inverter has a ground fault and the chassis becomes active / hot? How does the ground current get diverted back to the proper path (i.e. DC negative) in the hope tripping a breaker or fuse, so no one gets shocked?

The inverter chassis needs to be grounded.
Not the DC system.
The grounding system is there to protect you from the AC system.

 

[GeeGee]

Cheers

The inverter chassis needs to be grounded.
Not the DC system.
The grounding system is there to protect you from the AC system.

Cheers. I'll update my schema with your replies and others.

FWIW, I tried to be productive and went searching for clarity in the background and found your consistent reply in another thread.

Eath rod grounding inverter chassis AC-out with grid connection

I would like to ask a few questions about earthing grounding and bonding on my system which will be UK based. To make things clear (hopefully ?) with terminology I will try to specify words and what I mean by them first. Earth or earthing connection will be a coper rod in the ground (pink in...

diysolarforum.com

 

[mikefitz]

wouldn't connect DC negative to any grounding or earth.

Why?
UK regs require this negative grounding and perhaps other countries.

 

[timselectric]

Why?
UK regs require this negative grounding and perhaps other countries.


View attachment 187240

I prefer to keep the DC system isolated. The only connection I want between the DC and AC system, is the inverter. A lot of the arc fault and ground fault detection devices, in AIO's. Need the system to be floating in order to monitor them correctly.
It used to be common place to ground the negative, here in the states. Now, it's only done on mobile applications.
And since Victron equipment was designed for the mobile market. I assume that is why it shows it in the manual.

 

[GeeGee]

Why?
UK regs require this negative grounding and perhaps other countries.


View attachment 187240

Interesting schematic....I see multiple ground rods but bonded nonetheless.

 

[timselectric]

I just see a really bad idea.
Or several.

 

[Peteandjian]

Seems you have a grip on the concept, I think the question you need answered is "is it safe".
I also installed an off grid system and I found great comfort after watching this video.

hope this helps

 

[GeeGee]

For those interested, I have summarised the general feedback of this thread so far as per below.

Grounding takeaways:
AC

• There should be only 1 Neutral-to-Ground (NG) bond across the entire AC system
• Ground on the AC side (i.e. ground to earth or other grounding system)

DC

• 'Bond' the Inverter chassis and MPPT Charge Controller (and any other component metal frames) to DC negative
• Differing opinions exist (please read posts above) on whether to ground the DC side
  • e.g. UK regs require grounding, US regs require grounding only above 60V
  • If you do ground, then be sure to ground to the same grounding system as the AC side
• There should be only 1 connection to your grounding system (e.g. earth) across your entire system

General Schema

FWIW, I'll be getting a sparky / electrician to check over my system. I'll post back with any comments he may have.

 

[GeeGee]

Seems you have a grip on the concept, I think the question you need answered is "is it safe".
I also installed an off grid system and I found great comfort after watching this video.

hope this helps

Oh yes, that's the video that helped me so much, esp. regarding the concept of Neutral to Ground (NG) bonding and how to test if your Inverter has an internal NG bond (I used his 1st test - I think he lists 3 in total?). Thank you for adding that video to this thread.