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EDITED TITLE: Why not common Neutral? What is dangerous about it?

FilterGuy said:
You may be on to something there. If the system is set up with a common neutral, the bonding screw must also be removed in order to ensure there is not a double bond. Since they don't document the bonding screw (and some distributors say removing the screw voids the warranty), maybe their assertion it is dangerous has a built-in assumtion the screw is in place
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That's the reason I believe common neutral is not supported. Hybrid inverters (can run with grid sharing) like the Deye/Sun Synch do not bond N-G under inverter power and rely on the N-G bonding at the service panel, that is why a contactor is needed to be installed if off grid or grid down operation is desired. The video was quite interesting, it clearly showed the inverter did not bond N-G.

Remember the thread way back with the member using a Reliance transfer switch where the neutrals remained in the main panel? Only hots were moved to the transfer switch, as soon as it was turned on it tripped the breaker on the inverter. Of course the bonding screw was in place.

You know my stance on this subject, I prefer installing a 3 pole double throw in order to disconnect neutral from main service panel under inverter power and leaving the bonding screws in the inverters.
 
Pappion said:
Main neutral could have 100A moving thru it. The NG bond point could be farther back (wire resistance). There is voltage on the neutral and the inverter just connected it to the gnd. I don't think the Inverter NG bond relay and wiring is rated for that kind of current.
You might be connected to a panel with the NG bond, but code has to consider more situations, including failures.
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Inverters with internal N-G bonding have a built in transfer switch that switches N when moving from grid power to inverter mode. There is a delay for a reason, it prevents multiple N-G bonds at the same time, plus there isn't any current flowing.
 
Zwy said:
That's the reason I believe common neutral is not supported.
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It is the only explanation I have found that makes any technical sense.
Zwy said:
You know my stance on this subject, I prefer installing a 3 pole double throw in order to disconnect neutral from main service panel under inverter power and leaving the bonding screws in the inverters.
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That is the only solution that does not violate the manufacturer's instructions.... but if there are multiple inverters it does create multiple NG bonds which is not a good thing and does not follow the NEC.

Like I said, these value-priced inverters force a Sophie's Choice.
 
Zwy said:
That's the reason I believe common neutral is not supported.
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It is the only explanation I have found that makes any technical sense.
Zwy said:
You know my stance on this subject, I prefer installing a 3 pole double throw in order to disconnect neutral from main service panel under inverter power and leaving the bonding screws in the inverters.
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That is the only solution that does not violate the manufacturer's instructions.... but if there are multiple inverters it does create multiple NG bonds which is not a good thing and does not follow the NEC.

Like I said, these value-priced inverters force a Sophie's choice.
 
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Your all assuming that it is all wired correctly. Don't assume that about an RV Power Pedestal. The wires might be mixed up. That's why we have RV Circuit Analyzers. My Surge protector has it built in.
RV Circuit Analyzer
If Neutral is actually Hot you will see smoke with Common Neutral NG Bond.
 
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The reason for a common neutral is to carry the existing N/G bond through the system. After removing bonding screws from multiple paralleled AIO's. To avoid the dangers of multiple N/G bonds.
If you don't remove the extra N/G bonds. A common neutral causes more problems than it removes.
The only safe solution for multiple AIO's with grid input is a common neutral.
The Tier 1 manufacturers have known this for years.
 
Pappion said:
Your all assuming that it is all wired correctly. Don't assume that about an RV Power Pedestal. The wires might be mixed up. If Neutral is actually Hot you will see smoke with Common Neutral NG Bond. Thats why we have RV Circuit Analyzers. My Surge protector has it built in.
RV Circuit Analyzer
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If the incoming neutral is actually hot and the incoming hot is actually neutral, the system is already very dangerous. There is a very good chance the vehicle chassis is hot even if there are no inverters installed.

Furthermore, if the inverters are installed with a common neutral and no bonding screws, nothing additional would happen. The inverter can't tell a hot-neutral swap unless they measure voltage between neutral and ground.... and we know they don't based on how they behave. The inverter will happily take in the swapped input and pass it right through.

If the inverters are installed with the bonding screws there would be a short if the inverter engages the bonding relay before it disengages the hot and neutral relays. If this were to happen the breaker at the monument should pop.... If the inverter on the monument doesn't pop, all hell will break loose.....and that is without a common neutral arrangement.
 
timselectric said:
The reason for a common neutral is to carry the existing N/G bond through the system. After removing bonding screws from multiple paralleled AIO's. To avoid the dangers of multiple N/G bonds.
If you don't remove the extra N/G bonds. A common neutral causes more problems than it removes.
The only safe solution for multiple AIO's with grid input is a common neutral.
The Tier 1 manufacturers have known this for years.
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I agree.... mostly. Victron is a Teir 1 inverter company that has dynamic bonding and has worked through all of these issues. They provide ways of dealing with all the various scenarios (including multiple inverters)

With that small caveat, I agree that for a stationary install, a common neutral system is the best. It completely removes bonding relays from the equation which makes it safer and more flexible.

However, people are buying and using these value-priced inverters so I would like to get down to the specifics of the risks so they can make informed decisions.
 
FilterGuy said:
Victron is a Teir 1 inverter company that has dynamic bonding and has worked through all of these issues. They provide ways of dealing with all the various scenarios (including multiple inverters)
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Victron is primarily aimed at the marine (and mobile) market where dynamic bonding is a critical need. However, they also sell to the stationary market and have made accommodations for the differences in the markets.

MPP, EG4, Growatt, and the other value-priced inverters of the same ilk seem to be shipping inverters with mobile/marine characteristics but are not making proper accommodations for the stationary market.

As best as I can tell, the design of these inverters was intended to accommodate the stationery market but someplace between the product design and the presentation to the end customer the accommodations got dropped. However, I can not say that for sure without understanding the reason they won't support common neutral.
 
In my opinion, these value priced units should never be connected to the grid. But then, you lose some of the features that make them alluring.
My system is no longer connected to the grid. But I am adding the features back, externally.
 
FilterGuy said:
Victron is primarily aimed at the marine (and mobile) market where dynamic bonding is a critical need. However, they also sell to the stationary market and have made accommodations for the differences in the markets.

MPP, EG4, Growatt, and the other value-priced inverters of the same ilk seem to be shipping inverters with mobile/marine characteristics but are not making proper accommodations for the stationary market.

As best as I can tell, the design of these inverters was intended to accommodate the stationery market but someplace between the product design and the presentation to the end customer the accommodations got dropped. However, I can not say that for sure without understanding the reason they won't support common neutral.
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My guess is that their lawyers won't let them publicly support anything that isn't in the manual. And since they are selling like hotcakes, there's no reason to change anything.
 
timselectric said:
In my opinion, these value priced units should never be connected to the grid. But then, you lose some of the features that make them alluring.
My system is no longer connected to the grid. But I am adding the features back, externally.
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I agree with your assessment. They can be reasonably safe and follow the manufacturer's instructions if it is a single inverter without a transfer switch between it and the powered load. The moment multiple inverters are involved or a transfer switch is added, things start to get sketchy.
In my documentation, I even say that I can not recommend stacking them because of what the manufacturer says.

However, people ask me about these inverters all the time....often *after* they have purchased them. I would like to be able to give them all of the technical pros and cons of the situation. I would not be satisfied with an answer of "Because that is what the manufacture said' so I would like to provide the technical reasoning why something is safe or not. Furthermore, I don't view it as my role to tell someone what they can or can't do, I try to provide the information so *they* can make a good decision.
 
FilterGuy said:
I agree with your assessment. They can be reasonably safe and follow the manufacturer's instructions if it is a single inverter without a transfer switch between it and the powered load. The moment multiple inverters are involved or a transfer switch is added, things start to get sketchy.
In my documentation, I even say that I can not recommend stacking them because of what the manufacturer says.

However, people ask me about these inverters all the time....often *after* they have purchased them. I would like to be able to give them all of the technical pros and cons of the situation. I would not be satisfied with an answer of "Because that is what the manufacture said' so I would like to provide the technical reasoning why something is safe or not. Furthermore, I don't view it as my role to tell someone what they can or can't do, I try to provide the information so *they* can make a good decision.
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Agreed
I'm dealing with this issue a couple of times a week.
And it's always, trying to help them use what they already purchased. All that we can do is make recommendations. And try to help them understand why.
 
Here is the question that goes through my mind.

I think we can all agree that it is possible for the neutral or negative wire to short to ground, even multiple times. How would you detect the condition and prevent the short from causing a problem?

I honestly do not know the correct answer, but since it can happen and that it is probably possible to detect the short. Why wouldn't the solution to it be better than grounding and why shouldn't codes be amended?
 
LeoThomson said:
Here is the question that goes through my mind.

I think we can all agree that it is possible for the neutral or negative wire to short to ground, even multiple times. How would you detect the condition and prevent the short from causing a problem?

I honestly do not know the correct answer, but since it can happen and that it is probably possible to detect the short. Why wouldn't the solution to it be better than grounding and why shouldn't codes be amended?
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In NEC parlance, the Grounded Conductor is what we call neutral and it is NOT a Grounded Conductor (neutral) if it is not connected to the ground. So, yes, a neutral not only can be shorted to ground but by definition, it IS shorted to ground. If the short is not at the system grounding jumper, it becomes a 2nd Neutral-Ground bond. As @timselectric says, a GFCI upstream from the short will trip, but other than that, there is very little to indicate the problem unless you are measuring current on the ground and neutrals. (Too much current on the ground or too little current on the neutral between the N-G bonds will indicate a problem)
 
timselectric said:
The reason for a common neutral is to carry the existing N/G bond through the system. After removing bonding screws from multiple paralleled AIO's. To avoid the dangers of multiple N/G bonds.
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Post #20 https://diysolarforum.com/threads/e...-what-is-dangerous-about-it.53252/post-679478

BTW: Your comment about the rating of the bonding relay brings up a related point that I recently ran into while helping someone that has decided to go with a common neutral layout using six inverters. When doing more than 2 inverters in split-phase or more than one inverter in single-phase, you need to do one of two things:

  1. Leave multiple bonding screws in place so multiple bonding relays can split the large amount of ground fault current that multiple inverters could source. This means the system would have multiple N-G bonds....which is a bad thing.
  2. Use a common neutral with none of the bonding screws in place. This allows a very large neutral wire to go from the main breaker box to the critical load box and bypass the small bonding relays. In a ground fault scenario, the large neutral and the main bonding jumper will handle the fault current, not the bonding relays. Unfortunately, this violates the blanket statement from the manufacturers that common neutral layouts must not be done.
The issue is whether or not the bonding relay in only one inverter (all other bonding screws removed) could handle the fault current under #1. Under #2, ground fault current should return to source (inverter mode) on the N. What size of N would be required with 6 inverters?

EG4 6500EX is 54A per inverter x 6 = 324A. That's 4/0 territory. Sec. 310-15(b)(4) applies as it is a current carrying conductor, I don't believe it can be derated.

timselectric said:

If you don't remove the extra N/G bonds. A common neutral causes more problems than it removes.
The only safe solution for multiple AIO's with grid input is a common neutral.
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3 pole neutral switching where the system is SDS works too.

These are sold as off grid inverters,basically standalone, thus the classification requires N-G bonding to be at the inverter.

timselectric said:
The Tier 1 manufacturers have known this for years.
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It's not an issue when used as an SDS.
 
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