Ground showing voltage?

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375matt
375matt Solar Expert Posts: 39 ✭✭
I have a modified sine wave inverter. I have my entire inverter system grounded to my house earth ground rod while I assemble and test before taking to off vrid cabin.The in inverter system is floating neutral (not sure if inverter is inside?). When I turn on inverter and touch battery negative to ground it shows 1.7 volts. When I shut off inverter it goes to 0. I thought ground should show no voltage?
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  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: Ground showing voltage?

    Where are you meaasuring this voltage? Are you saying that you have 1.7 volts dropping across the ground conductor? tell us where you are placing the meter leads.
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    I place one on earth ground and one on battery negative. When I shut off inverter voltage goes to 0. I wanted to ground my battery negative to earth ground is why I checked.
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    It's 1.5 volts now.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
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    Re: Ground showing voltage?

    if you did NOT disconnect the inverter there is still power going to it ... somewhere. see what you get after disconnecting.
     
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  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    If i turn the inverter off it goes to 0 from 1.5 volts. Turn it on and 1.5 volts between battery negative and ground.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Ground showing voltage?

    Put a filament lamp between the two grounds... If the light does not light (and you can connect an amp meter to measure current)--Then you should be able to safely hard connect the two grounds.

    A digital volt meter has very high input resistance and any leakage current is usually enough to take the meter from zero volts across two arbitrary circuits.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    I took my filament test light n connected it from bat neg to earth ground, no light. I left it on n tried my multimeter from bat neg to both sides of earth ground on each side of filament light n 0. ?
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    My inverter also got quiter?
  • boB
    boB Solar Expert Posts: 1,030 ✭✭✭✭
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    Re: Ground showing voltage?
    375matt wrote: »
    I took my filament test light n connected it from bat neg to earth ground, no light. I left it on n tried my multimeter from bat neg to both sides of earth ground on each side of filament light n 0. ?

    With inverter on and off, they both read 0 volts with the light connected ??? If so, then it is most likely leakage and nothing to worry about.

    You are measuring AC voltage and not DC voltage, right ??

    boB
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    Both read zero with light connected.

    How do I know if im readin ac or dc?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Ground showing voltage?

    It depends on the meter... Some meters when on AC will read zero volts (or current) when measuring DC... Other meters will read a voltage (or current) when set to AC...

    True RMS meters will read AC or DC voltage when set to AC (root mean square). "simple/cheaper" meters will not read any DC voltage on AC scale or something other than the correct DC voltage (i.e., they do not do the RMS measurements/calculations).

    Take your meter, set to 20 VAC full scale and read your 12 VDC battery bank (assuming the battery reads 12.7 on VDC scale) and see what it reads (~0, 12.7 volts, or something different).

    -Bill

    And, if the light "shorts" out the voltage, then it is "safe" to connect the two grounds together (and usually a good idea to connect battery return to safety/frame ground).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    Meter will read the battery correctly set on 20 vac, 13.6 or so on float charge n between 1.5 & 1.7 volts from batt neg to earth ground. So should I ground the battery negative and move on ? Part of me was wondering is my modified sine wave inverter may be neutral bonded and causing this? The inverter manual doesn't say so I may try a call to the manufacturer since email isnt working.... On their end.
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    Is the filament light just to test or do you leave one wired to kill the voltage?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Ground showing voltage?
    375matt wrote: »
    Meter will read the battery correctly set on 20 vac, 13.6 or so on float charge

    Are you asking me or telling me? If the meter is an RMS type meter then on VAC scale it will read DC voltage too (better to set to VDC scale though to read DC voltages). You will not damage the meter (VAC reading on VDC for example).
    n between 1.5 & 1.7 volts from batt neg to earth ground. So should I ground the battery negative and move on ?

    If it is a small cabin power system--driving a ground rod and connecting your battery return to the ground rod does not really do too much for safety.

    If, however, this is a larger system and/or you have lightning in the area--Then a ground rod for PV frame grounding and connecting the DC Ground back to the Ground rod is a good idea.

    The whole grounding from a power systems point of view (ignoring lightning for the moment) is based on creating a "grounded neutral" so you don't have to put double pole breakers everywhere (like we do on 240 VAC split phase circuits for North American 120/240 VAC power).

    With MSW inverters, in general, you cannot create a grounded neutral on the AC output and have the DC battery bank grounded. If you tried doing this, it creates a dead short through the inverter (from battery ground through inverter to AC output to "neutral ground bond").

    So--from a general safety point of view, ground the DC "return" (usually negative) to a ground rod. And let the MSW AC output "float".

    With most TSW inverters, you can ground one of the 120 VAC outputs and make a "grounded neutral"... Again, check the manual before doing this to be safe (there may be some TSW inverters that cannot be neutral bonded to ground--I don't know).
    Part of me was wondering is my modified sine wave inverter may be neutral bonded and causing this? The inverter manual doesn't say so I may try a call to the manufacturer since email isnt working.... On their end.

    The minor voltage you were reading with your meter is "leakage" current--and probably much less than a few milliamps--Which is perfectly normal (inductive/capacitive coupling, a bit of leakage current through an insulator, etc.). A typical Digital Volt Meter has very high input resistance (millions of ohms or more)--And will read a few volts almost anywhere. The filament lamp "shorts" out the very small leakage currents and makes the readings more stable.

    I would not plan on grounding the "neutral" of the MSW inverter--It just is not possible (your battery bank would then become "hot" if you ground referenced the AC "Neutral".

    If you are into "safety"--Then you should use double pole breakers (both hot leads) for fire/shock safety.

    If you use GFI outlets (near sinks, outside, etc.). They may work--But you may have to try different brands/models to find some that are reliable (no false trips).

    Do not use a GFI breaker on the output of your inverter--For example, if you have some lighting and an outlet near a sink--and a blender falls in the sink tripping the GFI--It would also turn off the lights too... That is why you want GFI only near point of use. And not one GFI (or breaker) controlling everything in the cabin/home.

    Sorry--Grounding is a very complex subject.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    If you think Im on track, I have a final ground issue before I think my system is complete. I have the xantrex xw mppt 60 with 2 prefer solar 250 watt panels on metal frames. How do I rig the ground for this? I see it was talked anout here:
    http://forum.solar-electric.com/showthread.php?5881-xw-mppt-60-installation-problems/page2

    Do I need a ground for panels or is this done through the negative terminal in the cc?

    Do I need to do anything like disable the ground fault protection in the xw or make the xw my first stop from ground rod then continue grounding to the other components?
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    Here is a pic. 2600 watt vanner inverter on top, auto transfer, 12 circuit panel, outlet box tied to gen to plug in the iota iq4 55 amp charger, xw 60 cc, c&d kct 450 ah in series n paralleled for 12v and a midnite solar switched led battery monitor stuck off to the side.
    Attachment not found.
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    It is for a small cabin off grid and I'm far from a safety freak but don't want to die early either. I'm not scared of backwoods redneck engineering, just scared of magic smoke!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Ground showing voltage?
    375matt wrote: »
    If you think Im on track, I have a final ground issue before I think my system is complete. I have the xantrex xw mppt 60 with 2 prefer solar 250 watt panels on metal frames.

    You have "two grounds" to talk about here... One is the "safety ground" for the frames/mounting racks/etc... That is usually a 6 AWG wire that connects to each solar panel frame and then runs down to a ground rod right at the base of the array or wall of the home (if roof mounted).

    If the array ground rod is not the same as the main panel/home ground rod, then you should run a 6 awg wire from the array ground rod to the main ground rod--This allows a short circuit from the array to be electrically connected to main home ground rod.

    The other ground is the DC "ground" or negative wire--That is carried back from the array back to the solar charge controller--treat it just like a "hot" wire that needs to be insulated from everything else.

    Newer controllers (and there are stand alone options too) have a ~1 amp fuse/breaker between DC ground and Safety Ground. If there is a "ground fault" in the system somewhere, the current flows the 1 amp "sense" fuse/breaker which then stops current flowing elsewhere in the system (there are variations of solutions between vendors).

    You can follow the vendor's installation instructions--Or make you own choice and (essentially) use a heavy jumper wire between the DC battery ground and the Safety ground (ground rod) connection. I have an opinion that the NEC method of DC GFI is not safe at all--And offers little additional safety when implemented as specified.
    How do I rig the ground for this? I see it was talked about here:
    http://forum.solar-electric.com/showthread.php?5881-xw-mppt-60-installation-problems/page2

    You have to follow the manual (or specifically do something different, in my humble opinion, and "defeat" the DC GFI).
    Do I need a ground for panels or is this done through the negative terminal in the cc?

    The Safety ground wire to local ground rod. And the DC return back to charge controller--no grounding.
    Do I need to do anything like disable the ground fault protection in the xw or make the xw my first stop from ground rod then continue grounding to the other components?

    Which XW? I assume you mean the XW MPPT charge controller (there is also a XW Hybrid GT/Off Grid inverter too).

    The code is--Install the NEC/UL approved DC GFI connections as documented.

    If you want to talk more about the DC vs Safety grounding issue (fuse vs hard bonding between DC and Earth/Safety Grounding)--We can talk more. It is easy to defeat the DC GFI system--But you need to make a decision to defeat (or not). I am not there and I am not your installer/inspector.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Ground showing voltage?

    A 2,600 watt 120 VAC inverter will output ~22 amps--So you need breakers/fuses on 14 AWG and 12 AWG circuits to ensure that the wiring does not overheat if something gets shorted.

    IF you DC ground your battery bank to a ground rod (and any major metal in the cabin like metal water pipes/etc.). Then you should use two pole ganged breakers (double pole breakers) and wire your 120 VAC circuits like they were two wire 240 VAC circuits.

    This is because with a MSW inverter both 120 VAC leads are "hot" with respect to ground. You want both leads "breakered" and you want both to turn off (ganged) if one or the other "hot lead" shorts to ground.

    If you have no "grounded metal" inside the cabin (i.e., plastic sink/PVC water/waste piping/etc.)--Then, in theory, you only need one breaker per branch circuit--because there is no way to "short to earth" one of the hot wires. You only have to protect against a short circuit (hot to hot short--i.e., a shorted AC appliance).

    I hope I am being clear here--This is tough to describe in words and not be confusing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    I don't anticipate seeing any inspectors unless they have claws, fangs n weigh over 500 lbs.

    Its the xantrex xw mppt 60 charge controller.

    So...
    Ground the panel frames to my main ground with #6.

    You say you would defeat the gfi in charge controller which is done doing this:

    http://forum.solar-electric.com/attachment.php?attachmentid=1940&d=1309566063

    So if i defeated the ground fault there will be one ground wire coming from the xw 60 mppt, just like I have done inthe picture?
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    A 2,600 watt 120 VAC inverter will output ~22 amps--So you need breakers/fuses on 14 AWG and 12 AWG circuits to ensure that the wiring does not overheat if something gets shorted.

    Not sure I follow where these breakers would go? Distribution panel?

    IF you DC ground your battery bank to a ground rod (and any major metal in the cabin like metal water pipes/etc.). Then you should use two pole ganged breakers (double pole breakers) and wire your 120 VAC circuits like they were two wire 240 VAC circuits.

    Seems like it would be easier to not ground the battery negative at all. Any concerns doing this?
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: Ground showing voltage?
    375matt wrote: »
    Seems like it would be easier to not ground the battery negative at all. Any concerns doing this?

    Battery and inverter ground should be connected to a common point (frame on mobile applications or earth ground for off-grid cabins) when using an inverter with a floating neutral. The purpose of the floating neutral is that you can come in contact with a hot conductor and will not get a shock from the conductor to ground because there is no neutral ground bonding. The downside is that you will have no ground fault protection since it is impossible for ground fault interrupters to measure any difference in current between the hot and neutral wires, since power from neither one can flow to ground. So if you come in contact with both the hot and neutral, and the current can flow thru you, you will be laid out on the floor quivering like a bowl full of jelly.

    A floating neutral is not legal in the US for residences. It may be legal for an off-grid cabin under some circumstances (probably where it won't get inspected). But it must be treated just like 240 split phase power because it is really 120 split phase. That means two pole switches and breakers on all the circuits to shut off BOTH the neutral and hot.

    The other bad thing about inverters that have a floating neutral is that their fault protection consists of letting the Factory Smoke out if the neutral comes into contact with inverter ground. That Factory Smoke is what makes those inverters work, and once it gets out the inverter is no good anymore.
    --
    Chris
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Ground showing voltage?

    Not trying to pick and argument with Chris (I probably would lose)... But grounding can get pretty complex and depending on lots of factors (brand/model/type of inverter) where grounds are made (ground rod, AC Neutral to Green wire vs floating), single vs double pole breakers (or two fuses--less good), what you are trying to protect against (a "floating" electrical system cannot cause a shock to ground--with the first connection--problem is the "second connection". The first connection creates a ground reference which is not usually detected, and then the second connection causes the current flow.).

    Also, the amount of current--Under 8 amps of available current is usually fairly safe if you are using 14 AWG or larger wiring (i.e., you cannot overheat the wire unless you are >15-20 amps, for example). However, for electric shock, we are worried about leakage currents on the order of 0.005 amps (5 milli-amps) which almost any slight short (wet wiring connection, a bit of salt fog, etc.) can cause to flow.

    So--While a floating power system is technically "safe" from ground faults--Because nobody goes around once a shift testing their floating/isolated power system--A "simple" current leak/fault will be hidden until the next time somebody touches a wet drill while standing in wet grass.

    And--Ground Neutrals (DC Return bus, etc.) -- This is what allows us to use only one pole breakers/fuses... Because, there is never enough voltage on a grounded neutral (you may see a few volts) to cause enough current to flow and burn out a faulted neutral connection. And the Fused/Breaker'ed Black/Red Hot wire limits the outgoing current from the power source, so the neutral will never carry more current than it is designed for--Assuming standardize wiring practices are followed and there are not multi-point connections (paralleling of Neutral/Green ground wire, etc.).

    MSW Inverters, since they are essentially split phase 120 VAC output--Have two hots and should use two pole/ganged breakers for protecting the "two hot" wires in the circuit... If one wire faults (hot to hot OR hot to ground), then the one breaker will trip and also trip its mate in the circuit--Helping safety because you "know" that both wires are killed in a short circuit (as always, never assume, measure/confirm/kill power before doing any work). (and add that the MSW wave form has its own issues with GFI breakers/outlets and AC loads designed for sine wave power).

    Add grid power, different size generators, rocky/dry soil, lightning, etc... And you have a semester or two college electrical engineering course.

    Because of the complexity--That is why I like to know about your installation, needs, and expectations--Throwing everything out there at once is confusing and usually counterproductive to a "simple" ground showing voltage? question.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
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    Re: Ground showing voltage?
    BB. wrote: »
    MSW Inverters, since the are essentially split phase 120 VAC output--Have two hots and should use two pole/ganged breakers for protecting the "two hot" wires in the circuit...

    -Bill

    One way of characterizing this would be 60/120VAC with the "neutral" not actually being exposed at the output terminals, but being connected within the inverter to the battery ground (Battery + or - input to the inverter.)
    It causes problems that are often mentioned in the MSW inverter documents but overlooked by the installer or user.
    SMA SB 3000, old BP panels.
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    So should i have my panel full of 2 pole ganged breakers and one leg/side for the black wire amd one for the white wire? Then I wouldnt use the neutral bus bar in the panel at all?
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
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    Re: Ground showing voltage?
    375matt wrote: »
    So should i have my panel full of 2 pole ganged breakers and one leg/side for the black wire amd one for the white wire? Then I wouldnt use the neutral bus bar in the panel at all?

    That is certainly one way to go. Perhaps overkill, but it does call attention to the exact nature of the problem and it is probably as close to code compliant as you can get.

    But if you go that route and want to stay totally technically code compliant, you should use black and red to the outlets instead of black and white, and use 240 volt outlets into which you will plug 120 volt loads wired L1 to L2 instead. :-)

    More realistically, the critical thing is that you do not want to make a bond between the white wire and ground anywhere in the circuit. (While for POCO power input, a bond between their neutral wire and your ground in one place is mandatory. )

    The safety issue comes in because a short between the white wire and ground (or more realistically a high resistance arc or other fault) will not trip a single breaker located in the black wire. Hence the need for a breaker in each.

    Also, under no circumstances try to use an integrated GFCI receptacle!
    SMA SB 3000, old BP panels.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Ground showing voltage?

    Yes--That would be the "optimum" solution with a MSW (or floating TSW) inverter.

    Regarding "colors"--You probably would want to use non-white wire for the circuit... (Or use colored tape at the junction boxes). White and Green (green/yellow are reserved colors and could confuse those following you later).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Ground showing voltage?

    Hmmm... Going with non-standard AC outlets because of the "two hots" is probably over kill...

    Modern appliances are either three wire (the two power leads are insulated, exposed metal is connected to green safety wire in case a single layer of insulation fails) or two wire with double insulation (again, the two power leads are insulated from the user).

    Old appliances (very old mixer, old power drill with metal case) and light bulb sockets (the socket is supposed to be connected to neutral to avoid shocks when changing the bulb)--Yes, newer household power devices will carry polarity/grounding/isolation through the entire home.

    Older appliances (and homes) may not.... Chances of killing yourself is not high (i.e., changing a screw in outdoor security light bulb standing on a metal ladder in wet grass), but there is the slight increase in chance of something going wrong every time one of the overlaying "safety features" is bypassed/ignored.

    I would not worry about it--If you have five year olds changing bulbs on metal ladders--The risk is higher (not trying to be flip--just trying to frame the risks).

    Rewiring receptacles and plugs is probably a bigger risk (miss-wiring, non-molded/strain relieved cord sets, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ChrisOlson
    ChrisOlson Banned Posts: 1,807 ✭✭
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    Re: Ground showing voltage?
    BB. wrote: »
    Hmmm... Going with non-standard AC outlets because of the "two hots" is probably over kill...

    Bill is right - your appliances don't know which wire is neutral and which is hot - they could care less if you swap them around with a floating neutral system. The main thing is that with a floating neutral if you only switch or fuse one wire, you're only shutting off half the power. Think of it as being identical to 240 split phase, except with no neutral reference or winding center tap at the power source - only a ground.

    The neutral bus in your panel can be used for grounding - just like it is in RV's or boats with inverter power onboard. The only difference is that you install a green bonding screw in the neutral bus to bond the neutral bus to the panel itself instead of installing a separate grounding bar. Then you attach all your grounds to the neutral bus.

    If you have any doubts just go to an RV dealer someplace that has an RV on the lot with a factory installed Xantrex or Magnum inverter/charger and ask them if you can look inside the AC service panel. You'll see immediately how they wire floating neutral AC systems. However, one difference with RV's or boats is that they have a transfer switch with a switched neutral. So they can connect to shore power and switch from floating neutral to ground bonded neutral when on shore power.

    Edit:
    Just make sure your grounds are all common - inverter case, battery bank, ground rod, panels, etc. - all grounded to the common grounding point. You don't want multiple grounds with potential (voltage difference) between them. Ground should be ground everywhere in the system. From that point you have two hot wires. Neither of those can come in contact with ground. On good inverters like Xantrex or Magnum it'll just trip or fault the inverter and it'll require a reset. On the cheaper Chinese ones it typically starts leaking Factory Smoke.
    --
    Chris
  • 375matt
    375matt Solar Expert Posts: 39 ✭✭
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    Re: Ground showing voltage?

    Ok, u got my attention on the transfer switch. I have my inverter going into a powermax 30 amp transfer switch. Is this switch ground bonded neutral?