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MondayCreek
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# Posted: 16 Mar 2016 09:23pm
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I'm planning on a building a 24v off grid solar power system with a generator to supply backup power/battery charging. I already have a 24v 1500w inverter with the n-g permanently bonded and an 1800w generator (floating neutral) so the remainder of the system will need built around these items. The system will also be located in a shed and the plan is to supply power to the cabin via an RV 30A transfter switch. Because this is a small 15A or less system, I'm also debating whether to have a formal service panel/breakers in the cabin or to just use a power strip/surge protector that the different circuits in the cabin plug into. And although safety is of concern, the system will not be subject to an inspection, nor will it likely ever be wired to the grid due to needing an easement from an unsupportive neighbor.
I think I have a good handle on how to add in the other components of the system but the the question I can't find an answer to is how to connect the ground wires going from the inverter and generator to the transfer switch. I realize the bond must remain in the inverter but I am a little lost beyond that, especially when I starting mixing in proper grounding of the remainder of the sytem - panels, batteries, charge controller, etc.
I'll turn it over to you solar gurus on the forum and will probably be asking a lot of follow-up questions!
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MtnDon
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# Posted: 16 Mar 2016 10:23pm
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The main question is, what does the transfer switch do when it switches from one source to the other? Does it switch the hot and neutral only or does it also switch the ground?
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MondayCreek
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# Posted: 16 Mar 2016 10:45pm
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Good question MtnDon, and it's part of my confusion - I don't know if I need to switch the ground but assume that I do based on the system I've outlined. That being said, the typical RV-style PowerMax ATS has a ground buss and no switching capability. And I cannot find good reference info on ground switching on this forum or NAWS.
And to answer your question another way, I want to have a system that is grounded safely, can support GFCI, and can be powered by an inverter and a generator through a switching mechanism. Does this help?
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MtnDon
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# Posted: 17 Mar 2016 12:07am
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The RV ATS switches I am familiar with switch the hot and the neutral, not the ground as you state. That is probably the norm, but that is also a guess.
The way I see it, if the inverter has a permanently bonded neutral and ground you need to emulate that at the generator. If the generator has dual 120 VAC outlets like many do you can do that with a plug. Wire the neutral to the ground with a jumper. Plug it into one socket and plug the ATS input into the other outlet.
Do not have any further neutral - ground bonding anywhere else in the entire system. Then when the ATS switches from one source to the other the bonding remains consistent.
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bldginsp
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# Posted: 17 Mar 2016 12:24am - Edited by: bldginsp
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The ground should never be switched. All grounds always connected together, connected to earth at the ground/neutral bond, and bonded to neutral in one of two ways when using a transfer switch.
The NEC allows two means of connection with a transfer switch. Either the transfer switch does switch the neutral, or it does not. If it does switch the neutral, this can compromise the ground/neutral bond, if it is upstream of the transfer switch (which it should be unless you use a transformer rule). Therefore, if the neutral is switched, a ground/neutral bond must exist at each power source. If the neutral is not switched, the ground/neutral bond at the primary power source is not compromised and will function to clear ground fault. In this case, do not put a ground/neutral bond in the second power source. This would create a second ground/neutral bond in ordinary use which creates the potential for a parallel path for return current, through the grounding system. That's why they make "floating neutral" generators, to accommodate transfer switch setups without a switched neutral.
Hope this makes sense. Transfer switches are confusing and therefore present the danger of being incorrectly connected. If in doubt, hire a licensed electrician.
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MtnDon
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# Posted: 17 Mar 2016 01:10am
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I checked the two user manuals I have for two different RV ATS. They both have relays that switch both hot and neutral simultaneously. First breaking the circuit and then a split second later making the new connection.
So with the inverter you have I am certain the generator must have a similar bond. And a bond nowhere else. Right bldginsp?
Of course the whole thing falls apart if the inverter has an internal switching bond... a bond that switches in and out, on or off as the unit is powered up. Some do that...
....part of the reason that an inverter that is equipped with built in outlets is not meant to be used as a hard wired device, as a permanent part of a bldg..
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MondayCreek
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# Posted: 17 Mar 2016 08:21am
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Thanks for your responses, here are a few more points on my set-up related to the feedback:
1. I already have a plug with a jumper for neutral-ground bonding - this allows the GFCI on my RV to function when connected to this generator.
2. Per the inverter instruction manual I believe there is a permanent bond- The neutral (common) conductor of the power inverter AC output circuit is connected to the chassis ground. Therefore, when the chassis is connected to ground, the neutral conductor will also be grounded.
3. The ATS I was considering does in fact switch the hot and neutral.
Given the comment that the NEC requires a separate ground/neutral bond for each power source when the neutral is switched, will connecting all ground wires in the transfer switch cause any issues?
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bldginsp
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# Posted: 17 Mar 2016 01:15pm - Edited by: bldginsp
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Quoting: MtnDon So with the inverter you have I am certain the generator must have a similar bond. And a bond nowhere else. Right bldginsp?
Right, if the neutral is switched. If it is not switched, no bond at the generator or anywhere else besides the inverter.Quoting: MtnDon Of course the whole thing falls apart if the inverter has an internal switching bond... a bond that switches in and out, on or off as the unit is powered up. Some do that... ....part of the reason that an inverter that is equipped with built in outlets is not meant to be used as a hard wired device, as a permanent part of a bldg..
Never heard of an inverter with internal switching bond. Bizarre. But I agree that inverters meant to be used only from their receptacles should not be used to power up a building, partly because if the thing switches the bond somehow you don't know what's going on. Also, some inverters and generators achieve 120 at the receptacle with 60 in each wire, in which case there is no neutral as I understand it, so you can't make the bond for ground/neutral for basic safety.
Quoting: MondayCreek I already have a plug with a jumper for neutral-ground bonding - this allows the GFCI on my RV to function when connected to this generator.
Hmm... Not sure about this. A GFCI works by detecting difference in amperage between two wires. I didn't think it depended on a ground/neutral bond.
Quoting: MondayCreek 2. Per the inverter instruction manual I believe there is a permanent bond- The neutral (common) conductor of the power inverter AC output circuit is connected to the chassis ground. Therefore, when the chassis is connected to ground, the neutral conductor will also be grounded.
Good. Always nice when they clarify the issue.Quoting: MondayCreek The ATS I was considering does in fact switch the hot and neutral.
So your 'floating neutral' generator needs a bond. The plug method works but can be easily removed. I'd install a permanent one inside. Trace the neutral back as close to the windings as possible, bond from that point to frame, at the same point bond grounds and a ground rod.Quoting: MondayCreek Given the comment that the NEC requires a separate ground/neutral bond for each power source when the neutral is switched, will connecting all ground wires in the transfer switch cause any issues?
No it won't. Be sure the ground connection from the transfer switch is permanently connected to both power sources and all other equipment.
This does, however, raise an interesting issue. If the transfer switch does not switch the neutral, and therefore the only bond is at the inverter (or service meter when a genset is used as backup), then look what happens to a fault current path when the generator is operating and a fault occurs. The whole idea with grounds and the ground/neutral bond is that fault current (say from worn through insulation that allows the hot wire to energize metal, like your lamp or washing machine frame) will go from the energized metal to the ground wire, through the ground system, to the neutral and back to the power source. This low impedance path for return current causes the amperage in the circuit to spike, which throws the breaker and opens the circuit, removing the danger. That's the main thing they are trying to do with all this grounding stuff.
So- why can't we have more bonds between the grounds and neutrals? Why do they require it happen only once in a given system? Because if the neutral is bonded to ground, that puts neutral current in the grounding system, potentially delivering neutral amperage to whoever touches it. But if this bond occurs only once, and as close to the power source as possible, the neutral current won't go through the grounds, even though directly connected, because the neutral current is going to take the easiest path back to the power source. If you bond ground and neutral further away from the source, say in the subpanel or in a j-box, there is a greater likelihood that return neutral current will take both the neutral AND the ground system back to the power source. This is called a parallel path for return current, and it's a hazard because it energizes the ground system which ordinarily should have no current in it at all. You can get shocked by this or it can cause arcing or fire.
Back to transfer switches- if the switch does not switch the neutral and the bond happens at the inverter (or service meter), then when the generator is operating and the switch is thrown, if there is a ground fault, where does that fault current go? It goes from the washing machine frame to the ground wires, to the ground/neutral bond at the inverter (or service meter) AND THEN FROM THERE to the source, the generator, which is X distance from the inverter, that fault return current can then return to the generator via the neutral which is continuous and unswitched to the generator, thus tripping the breaker. But it can also return to the generator via the grounds that exist between the inverter and generator. More importantly, in normal operation, some return current will travel back to the generator via the ground wires. So, we deliberately created a parallel path for return current, which we are trying to avoid. This doesn't keep me awake at night, but it just points to the fact that there is no way to make a 100% safe electrical system, and the code writers have designed what they feel is the safest configuration.
So, why not just bond ground and neutral at both power sources regardless of whether the neutral is switched? If the neutral is switched you HAVE to bond at the second power source or else you have no ground/neutral bond. But why not bond there also if the neutral isn't switched? Be honest with you I don't know, I don't see that it presents any immediate danger other than the parallel path for return current from the second power source neutral to the ground system, which is likely to be short. But still it is a second bond downstream of the main power source which is what they are trying to avoid.
Hope I haven't bored you with this lengthy dissertation- we building inspectors are supposed to know this stuff so I bothered to research it. Also, as I said, transfer switches are a bit complicated and you need to understand the how and why of it to implement it correctly, given the particular features of the equipment you are using.
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MondayCreek
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# Posted: 18 Mar 2016 07:56am
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bldginsp - many thanks for the thorough response! I will be referring back to this thread many times before I complete my system.
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redside
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# Posted: 11 Apr 2019 04:27pm - Edited by: redside
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In response to bldginsp I think that there may not be a parallel path back to a floating neutral generator since electricity will only travel back to the generator on the neutral line (back to the internal winding). I am assuming the ground wire carries no current since it is unbonded at the generator and will only travel down the neutral wire to get back to its source. Would be interesting to get clarification as this is a very confusing topic. I do see what your saying, that if the generator is grounded then there could be some current that flows through the ground wire, but I think the breaker would trip since the neutral would carry the majority of the current since it has less resistance than earth, thus making the system safe.
I'm in the process of designing my system now and will need to bond the floating neutral Yamaha 2000 since my inverter switches the neutral when an external AC source is detected.
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Jebediah
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# Posted: 12 Apr 2019 07:11am
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I'm not an electrician and I see this topic all over the internet...if everything is fused shouldn't we be ok....or is it a safety redundancy, just in case....
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neckless
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# Posted: 13 Apr 2019 01:35pm
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ok iam going to get fried here lol.....in my system i am the transfer switch ... i un plug my cord from my panel from gen and plug into inverter , panel is grounded and when i need geny i just plug the cord back into the geny too simple may be wrong but did not have 500$ for transfer switch
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creeky
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# Posted: 13 Apr 2019 08:00pm
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the truly manual transfer switch 
good quality 30amp RV transfer switches can be had for under 100 bucks.
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