Inverter grounding

xp190
xp190 Registered Users Posts: 6
Hi everyone

I just happened across this form and I thought I'd stop by for some advice.

I have a cabin out in the country where I setup an off-grid system.

It is made up of 3x80watt solar panels, and a battery bank.

Initially I had a simple 1000watt inverter which worked great providing there was enough sun, but without sun, charging the batteries from a generator became a problem.

I found a solution to my problem in the form of the Xantrex Freedom HF 1000watt inverter/charger. I plan on installing it in the next few days when I get to go up to the cabin.

My concern is grounding the unit.

I'll describe the installation.

The cabin is wired with standard 14/2 wiring just as it it were meant for a typical hook up. I have a breaker panel, but the main service breaker is removed and in it's place I have wiring running to the inverter. The panel has it's own grounding rod.

The inverter I am using currently does not require a ground, I think it's too small and simple, however this new one has to be grounded.
When I looked through the manual, one option of grounding the inverter is to connect it to the negative buss of the battery bank. I don't have a buss, I just have 1/0awg cables going from battery to battery in a parallel configuration (each is a 12V battery) would connecting the ground to the negative side of the batteries be sufficient for the grounding needs?

I'm a bit skeptical of this setup as I have not come across it before, and I'm looking for some type of verification that this is a safe way to go about it.

I also came across diagrams where the DC and AC grounds are bound together, but I'm not sure I need to consider this at all.

If you take a look at the Freedom HF manual:

http://statpower.com/documents/Inverter-Chargers/Freedom-HF/Freedom%20HF%201000-1800%20Install%20Guide%20%28975-0395-01-01_Rev-A%29.pdf

On Page 1--6, I will have something very close to what you see, except for the car installation ie: battery isolator, alternator and starting battery, and on the other side I will not have a AC source panel, the genny will connect right into the inverter. With that diagram in mind, is it okay to tie the inverter ground to the negative side of the battery bank?

Your expertise will be greatly appreciated.

Thank you

xp

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Inverter grounding

    Unfortunately, the middle 2/3'rds of the installation manual is blank on my PDF reader...

    From the owner's manual, it is a MSW type inverter and unless it says otherwise, I would not ground the "neutral" anywhere in its AC output (typically, the "main panel") has the neutral and earth ground connected together... You will need to replace that/mount the neutral bus so that it is fully isolated.

    Grounding the DC Battery Negative to earth ground, and running a ground wire to the inverter chassis ground stud is all normal.

    Basically, there are two major reasons for "earth ground". One, if any AC (hot/neutral) wire touches a piece of metal, grounding the metal prevents it from becoming "hot" and creating a shock hazard (mixer wire breaks, makes chassis "hot" and person touches mixer/kitchen faucet and gets electrocuted).

    If any hot to ground connection is made--then the breaker/fuse will open.

    The other reason for grounding is to prevent lightning/static charges from energizing part of the system.

    Grounding the negative battery post and metal chassis parts to "safety ground" is normal and a good way of protecting equipment and people.

    Because the typical MSW inverter (Modified Square Wave) does not have an "isolated" output (no isolation transformer)... If you attempt to ground both the battery input and one of the output (make a "grounded neutral")--it can create a short circuit from the battery, through the MSW inverter, to the "neutral ground"... Which will blow the power transistors inside the inverter (and/or pop fuses/beakers).

    So--Earth ground the battery, ground the inverter case, float all AC "Neutral" bus bars and you should be fine.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • xp190
    xp190 Registered Users Posts: 6
    Re: Inverter grounding

    Thanks Bill

    I don't have the Battery Negative terminals grounded, I will do that and ground the inverter to that.

    The AC panel has the neutrals floating, just the panel itself is earth grounded, sorry I should have specified that earlier.

    Thanks for all your help.

    xp
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Inverter grounding

    xp,

    Not a problem--I try to explain what the theory is behind the issues (such as grounding) so you can make the best decision for your installation as it is difficult for me to do that from X,XXX miles away behind a keyboard. :roll:

    Regarding DC (and AC too) Grounding. You want to make sure you have only one ground connection per "circuit" so that you don't get parallel current paths (example, grounding the DC "-" at the solar panel and again grounding the Battery Bus "-" terminal). You can get parallel current flow (or lightning induced current surges) when there are multiple paths (another common multi-path ground is when you have the battery grounded and running a car or HAM radio from 12 VDC--The case is DC ground and Safety Ground, and so is the Antenna braid ground too--Running a DC ground to the radio chassis plus a safety green wire to the antenna braid ground can create such a loop).

    Regarding the solar panel ground--I am assuming that you are Earth Grounding the solar panel aluminum frame... and NOT Grounding the "-" wire from the solar array anywhere. Correct?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • xp190
    xp190 Registered Users Posts: 6
    Re: Inverter grounding

    Hi Bill

    Thanks again for all your help.

    You are correct, the solar panel frames are grounded, not the negative terminal. Currently to water pipes, but once the new ground is in, I will attach them to the battery ground instead to eliminate the parallel circuit.

    With regards to per "circuit"grounding, from what you describe I am under the impression that the AC side (a circuit) should have its own ground, which is the one from the AC panel to a grounding rod.

    Now the DC would be a separate circuit and requires another ground, it did not have one before, this is what I will be adding in. Once this new ground is added, I can use it to ground all other devices on that same DC circuit (ie: switching panels from water pipes to battery ground).

    I understand the parallel grounding situation you explained. I don't know how the inverter handles the DC and AC grounds at this point, and I don't recall anything from the manual along these lines, I will check again.

    It kind of sounds like I'm getting myself into that very situation through the inverter but I don't think it was built in such a manner.

    Once again, thank you for your help, 4 years ago when I put this installation up it was very hard to find help on this topic, I'm glad I found this forum.

    Regards

    xp
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Inverter grounding

    You are very welcome XP--I too have learned a lot from other folks on this forum.

    My person philosphy on grounding (and, like other areas of interest--some of this stuff becomes religious)--Is that you pick a really good spot for grounding (say that one 8+ foot ground rod. If you need more ground (dry soil, rocky, etc.) run #6 or larger wire out in a "star pattern" from that one ground rod to other ground rods/water pipes/etc.

    From that center ground rod--that is now your "zero volt" location (in telecom, also called the Ground Window). Run single cables to each of the other points that will need grounding. One to your solar panel frames. Another to your battery negative bus, and yet another to your AC Neutral point and common AC Green Wire ground (if you have one), etc.

    You do not aim to "isolate" your AC and DC grounds--you just aim to only have one path for current to flow...

    The problem with, for example an AC ground rod and a DC ground rod--If you get a lightning strike nearby--you will have a voltage gradent through the ground. And the two ground rods separated by XX feet will have hundreds to thousands of volts differences between them. And you would end up with your DC ground being at a different voltage vs your AC ground (and water pipes, etc.).

    Having everything referenced to a single ground at the edge of your home (per code) is a much safer setup.

    Also, I have seen grid power problems with different grounds. I had two grounds 100' apart at the side of a salt water tank (dolphins). There was 60+ VAC difference between them (I got shocked) because of AC leakage current between different pumping / power points in the facility.

    Sometimes, folks get in trouble because they have heavy DC Battery Return currents (100+ amps) and they have an incidental AC ground/neutral path that is common with the DC return. You can now start sharing the 100's of amps through the 14 awg neutral/safety ground of the AC circuit and overheat the AC wiring.

    Another issue to watch for with AC vs DC wiring (ground and Hot/Neutral). Generally, you always want to run AC wires right next to each other (Hots, Neutrals, etc.) because the AC current, if for example you run one hot lead through a box opening and run the neutral/return lead through another hole--the AC current can cause circulating currents to flow in the metal box and overheat the box itself.

    With DC, you do not have the problem...

    And then there is the lightning issue... You normally want to run the DC +/- lines in tight pairs too... If there is a nearby lightning strike, if the pairs of wires are separated by several feet--it becomes a "loop antenna" and can inject energy from the strike into your wiring system.

    Another issue--If you have metal on the roof/lightning rods--Those cables should be ran on the outside of the house--do not bring them into the center of the home. High Frequency Current (like lightning) wants to spread out as far as it can... So, two wires, one on the north corner and the other going to ground on the south corner of a home is much better than running a lightning ground down through the middle of your home to a ground rod (the lightning will tend to want to leave the central ground cable and spread out through your home).

    Obviously, a direct strike is going to take out equipment--but some basic safety practices can reduce the overall risk.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • arkieoscar
    arkieoscar Solar Expert Posts: 101 ✭✭
    Re: Inverter grounding

    Very good, clear answer to a contentious question. Someone should "sticky" it. Thanks.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Inverter grounding

    i'll add that separate grounding points can setup what they call ground loops and all kinds of crazy stuff can result.
  • xp190
    xp190 Registered Users Posts: 6
    Re: Inverter grounding

    Hi Bill

    Thanks for your assistance, over the weekend I went up to my cabin and fixed up the installation.

    I ran into a few issues, but I fixed up all the ground connections. For some reason my hardware stores don't carry grounding rods anymore, but rather plates, I believe there should be not much difference. I had rods before but they were not as long as required, so I ditched those and put the plates in, 8 feet apart connected with the heavy duty copper grounding cable.

    That cable runs to the DC circuit as well as the AC breaker panel.
    On the DC side, I have a fuse box which is grounded and to the fuse box I have the batteries and inverter grounded. On the AC side I have the household wiring.

    The inverter has it's own ground wire on the AC side that runs from its output to the AC panel. I ran into an issue where the GFCI outlet on the inverter kept tripping whenever I connected the household wiring. I tracked it down to a punctured 14/2 wire which had the ground and neutral bonded. Funny enough the other two inverters which I have used before never saw this as a problem, but I'm glad this one did and that I found it before it caused any issues.

    One question, I currently have the batteries disconnected from the ground as I was doing some troubleshooting. While troubleshooting I used a multimeter to check the Voltage between the grounding plates and the negative terminal of the battery bank, it showed 12V, I was a bit surprised, but this is a connection I'm not familiar with at all. My best guess is that it has to do with relative ground vs absolute ground, could you shed some light on this phenomenon?

    I will be investigating the lightning rods in the near future as I will be building a house with a metal roof, I am guessing lightning rods need their own grounding plates/rods and should not be connected to the same ones that the household wiring is connected to, would that be about right?

    Many many thanks for all your help Bill. The manual for the inverter does not even begin to scratch the surface of what you unveiled, and I'm glad I have it done right.

    Cheers

    xp
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Inverter grounding

    Plate? What sort of plate?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Inverter grounding
    dwh wrote: »
    Plate? What sort of plate?

    Some places sell grounding plates instead of rods. Usually in areas where you can't get the rod deep into the ground (minimum 5' - and that's in good soil).

    http://www.homedepot.ca/webapp/wcs/stores/servlet/CatalogSearchResultView?D=910158&Ntt=910158&catalogId=10051&langId=-15&storeId=10051&Dx=mode+matchallpartial&Ntx=mode+matchall&N=0&Ntk=P_PartNumber

    You need more than one to get the equivalent surface contact area.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Inverter grounding

    XP,

    You are very welcome.
    xp190 wrote: »
    One question, I currently have the batteries disconnected from the ground as I was doing some troubleshooting. While troubleshooting I used a multimeter to check the Voltage between the grounding plates and the negative terminal of the battery bank, it showed 12V, I was a bit surprised, but this is a connection I'm not familiar with at all. My best guess is that it has to do with relative ground vs absolute ground, could you shed some light on this phenomenon?
    A multi-meters typically has 10,000 to 1,000,000 ohms of resistance.

    Any slight, high resistance leakage current, from +12 volts to ground, will give you a "soft ground" referenced to +12 volts.

    For larger equipment, it is common practice to take a 12 volt light bulb (brake/tail light) and connect it between Battery Negative and Safety Ground.

    If it is a high resistance connection to +12 volts--the bulb will "short out" the leakage current and take the negative terminal to earth/safety ground.

    If there is a real short between +12 and Earth--The light bulb will limit the current to ~1 amp or so and prevent more smoke and fireworks.
    I will be investigating the lightning rods in the near future as I will be building a house with a metal roof, I am guessing lightning rods need their own grounding plates/rods and should not be connected to the same ones that the household wiring is connected to, would that be about right?

    Since I am no lightning expert--I will give you a personal opinion... Wherever your electrical safety ground is--You could attach one of your lightning arrestor ground wires there too. Basically, your job is to keep the water pipes, electrical circuits, safety ground, down spouts, roof, etc. all at the same voltage level (most of this will be hardwired to the same ground point at the edge of your building--in a grid tied home--that would be typically near the point where the down-feed/utility meter is located.

    For an off grid home--if you have a metal water pipe coming in from a tank/lake/etc.--I would probably drive my ground rod "next/near" the copper pipe and tie it all together.

    Remember that high frequency electrical current (AC and lightning strikes) likes to travel on the surface of conductors--does not like to travel down ferrious materials (example iron rebar)--And wants to spread out the perimeter of the building (no arrestor cable down through the middle of the home). And lightning does not like to take sharp corners (you end up with fairly thick braided cable with 18" radius minimum curves--as I recall).

    In the end, you are sitting there in the home/cabin. You don't want to see a nice arc jump from the wall outlet to your kitchen sink.

    A couple threads about Lightning:

    Off Grid Grounding Technique?
    Another Question, this time about Lightning

    Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

    And our host's consolidate FAQ page:

    www.windsun.com
    Lightning Protection for PV Systems

    From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • xp190
    xp190 Registered Users Posts: 6
    Re: Inverter grounding

    Thanks again Bill

    I'm definitely glad I stopped by here :)

    Your explanation about lightning arrestors makes perfect sense, I guess it was out of my league at the time of question, but now that I think about it, it's crystal clear.

    And just for the record I have two plates, I checked the local codes and if using rods it is the same deal, you need two of them about 6-8ft apart in my area.

    I'll ground the batteries as required, electricity is quite the fascinating topic, seems like a bottomless well of new concepts and ideas.

    Thanks again for the help. I'll open up another thread for a new question I have :)

    Cheers!

    xp
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Inverter grounding

    You are very welcome XP!

    By the way, are the plates you used something like these (lower part of page)... Around 2 to 9 square feet of copper with one or two mounting lugs?

    Never heard of them before--but they make perfect sense. Also, how deep did you bury them? Any special instructions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solorone
    solorone Solar Expert Posts: 257 ✭✭✭
    Re: Inverter grounding

    I don't mean to high jack the thread, but on a related issue, is it best to ground the land line phone to the panel/battery/array ground? Right now I have them separate. I get all kinds of fire coming in on the phone line, and we have suffered various losses.

    We are at the end of the line, and no manner of grounding seems to be able to stem surge problems.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Inverter grounding

    The old phone demark's had a pair of carbon blocks that would be a ~600 volt spark gap to earth ground (lightning protection).

    You should not tie one leg of the the telephone line to ground.

    Here is a nice little web page about how commercial surge suppressors typically are wired, and some information on building your own.

    They use a lot of gas discharge tubes for the suppressors--But I am not sure that they are the best solution--If I recall correctly, gas discharge tubes are relatively slow (when I was designing some phone/computer interface circuits).

    There is a really interesting telephone isolator (PDF) for ~$330 or so. But I am not sure that it solves any interference problems on the line side.

    The end recommendation is that all safety/earth grounds should be ran to one central point (typically the ground rod at the edge of your home) so that all equipment is at nearly the same ground throughout your home.

    Grounding by itself usually does not solve generic surge/noise issues on incoming phone lines. Different brands/models of phones/modems/DSL/etc. may be better or worse in terms of design and reliability...

    Has the phone company said anything about your issues? It is time to get something like DSL/cable (if available) with a digital internet phone service like Vonage and ditch the POTS line (plain old telephone set--yea, really what the abbreviation is for).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Inverter grounding
    BB. wrote: »
    It is time to get something like DSL/cable (if available) with a digital internet phone service like Vonage and ditch the POTS line (plain old telephone set--yea, really what the abbreviation is for).

    -Bill

    DSL has loop length limits - the longer the loop the lower the speed. Most phone companies won't install DSL if the local loop exceeds 13k feet even though technically you could go farther if you're willing to accept lower bandwidth (IF the phone line is clean enough). 20k feet or so is pretty much the practical limit though since the higher freqs of DSL attenuate pretty quickly.

    DSL won't work at all if there are load coils on the loop since they filter out the higher freqs.
  • cheapforlife
    cheapforlife Registered Users Posts: 6 ✭✭
    edited January 2021 #18
    Hello, I see this post is old. I guess everyone has figured this out except me. 
    I have the panels on a metal roof and grounded to a ground rod. My home is connected to the grid and the neutral/ground are bonded in the main panel. I am installing a critical loads panel that I’ll use a 4kw reliable inverter controlled by a manual transfer switch. The 120v reliable inverter has two hot wires that have 60v from them to ground. I am keeping the reliable inverter completely isolated from the grids wires. To do that the manual transfer switch has 3 poles where I will connect the ground and the inverters two hots. So the ground of the metal subpanel and subpanels grounding bar will be isolated from the grids ground when the manual switch is turned to the inverter. 

    Now my question is how do I get a ground for the critical loads in the subpanel when the inverter is powering them? Should I install another ground rod and connect it to the chassis screw on the inverter?
    Then the ac ground wire from the inverter would be the ground for the critical loads and the metal subpanel when the grid is fully disconnected from the subpanel? 
    This critical loads panel will hardly ever be powered by the inverter. The power rarely goes out where I live. 
    Also where do I ground my chargecontrollers case? And should I ground my batteries, where too? I’m thinking I should ground the chargecontroller to the solar panels ground rod, but wouldn’t that create a loop that isn’t good? I figured grounding the solar panels was good enough. Please correct me if I’m wrong.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    >  The 120v reliable inverter has two hot wires that have 60v from them to ground.

    Uh Oh.   That sounds like a Modified Sinewave inverter.   Motors, like in a fridge, don't like mod sine power.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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  • cheapforlife
    cheapforlife Registered Users Posts: 6 ✭✭
    It’s a pure sine wave inverter
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Assuming this is a PSW inverter--Then it is probably just a floating 120 VAC output (transformer isolated)--Possibly with a couple of filter capacitors on the output to chassis ground. Normally, you would just choose one output (L1 or L2) and ground that to the inverter chassis ground/your ground rod (and tie the local inverter ground rod with 6 AWG back to the house ground rod/cold water pipe/green wire safety ground).

    HOWEVER, read the manual closely about grounding. And before you tie Lx to chassis ground--You can use a small 120 VAC filament lamp connected between Lx and chassis ground (you can also try Lx to DC negative input too). If the lamp lights, then do not connect to chassis ground until you figure out what is going on. If the lamp does not light, and the Lx output drops to chassis ground (near zero volts), then the L1/L2 outputs are probably floating (and the capacitors are just keeping L1 and L2 at 60 VAC to chassis ground).

    Grounding with AC inverters is tricky... Most all PSW/TSW inverters have floating outputs (possibly with the white/neutral wire already connected to chassis/safety/green wire ground). And all is OK.

    If this is an MSW (modified square/sine wave) AC inverter--Then tying either L1 or L2 to chassis/safety ground (and with a the DC negative or positive grounded), this will generally smoke the MSW AC inverter.

    As always read the manual closely--But many AC inverter manuals do a pretty good job of not talking about AC neutral bonding (generally because they are MSW and cannot ground bond a neutral).

    Your other questions... Metal buildings/siding are not good for grounding or lighting grounding (poor connections, easy for lightning to blow holes in sheet metal). Always follow code for grounding solar panel frames/racking with (generally) a 6 AWG cable from frames/racking to the local ground rod.

    The "local" ground rod is for lighting (want a straight/short path from racking/etc.) to earth (ground rod/cold water pipe/etc.).

    HOWEVER, you also need to tie the local ground rod back to the home's main ground rod/grounding point with 6 AWG cable. This is to provide "safety" grounding between your two systems... If there is a short from the inverter's Lx (hot) to metal (sink, home's green wire safety ground, electrical boxes, etc.), you want the short circuit to find its way back to the AC inverter to pop the fuse (you don't want to energize the shorted metal to 120 VAC). That is the reason you need to tie the AC inverter's local ground rod back to the home's grounding system too (your need 6 AWG copper wire to pop the breaker... Two ground rods driven into the dirt will not carry enough current to pop a 15 amp @ 120 VAC breaker).

    You will generally tie all local solar equipment (and DC negative ground bus) to the local ground rod (and 6 AWG from local ground rod to home's ground rod). Note that with larger AC inverters--You can have >200 Amp fuse/breaker from the DC Bus/Battery Bank--So the "safety ground/chassis" of the AC inverter to local ground rod (and DC bus ground) can be as large as the main DC +/- wiring for the AC inverter.

    For a "critical" load sub panel--You need to check with the Inverter's manual. There are just so many variations on how to connect a sub panel back to the AC inverter's output, and your home's 120/240 VAC main panel. Does your inverter have an AC input and its own transfer switch? Or do you have to do your own transfer switch? Can the AC inverter have a ground bonded neutral or not? Do you want a manual or automatic transfer switch? etc...

    Regarding "ground loops"... That is why we talk about "single point" Neutral (or battery negative bus) bonding to safety ground (in main panel for bond between Neutral and Earth/safety ground) or at the ground rod (such as Battery Negative bus and DC side safety ground wiring/chassis ground/6 AWG back to main home ground rod). The idea is that all load current flows through L1/L2/negative/positive wiring... And only fault currents (short circuits to ground) flow through green wire grounding. We don't want (for example) a neutral/ground bond in the main panel, and a second neutral+ground bond in the sub panel at the same time--That puts the Neutral and Green wire cables in parallel, and the current flows in both Neutral/White and Ground/Green cabling (do not want).

    And the question about lightning--Is this an issue for you? If so, then get some good surge suppressors and put them on where the solar panel cables enter the building and the AC output of your inverter (and main house panel too). Not cheap, but will help protect your equipment.

    https://www.solar-electric.com/search/?q=midnite+surge

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • colinas
    colinas Registered Users Posts: 1
    Similar question. I just installed a 10KW 192V system. It is located in a welded metal building, making the whole building a big ground. After connecting the ground wire of the inverter l get 192V between battery positive and the metal building. Do l need to connect battery negative to my ground as well?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Welcome to the forum Colinas,

    If you want to have a longer discussion, go ahead and start a new thread about your system and needs.

    In general, metal skinned buildings are not really "lightning proof"... The thin sheet metal does not carry lightning current and the edge to edge connections are not good.

    https://www.metalconstructionnews.com/articles/installing-lightning-protection-systems

    Anyway... Back to why ground.

    There is grounding for safety--You don't want a short from AC or DC to a metal door frame energizing the frame--And somebody walking in from the rain, gets electrocuted from frame to puddle. So--Grounding the power source (battery bus, AC neutral, etc.) to cold water pipe, plumbing, gas stoves, metal electrical boxes/panels, etc. will cause a short from a device/power to something grounded to trip the local circuit breaker.

    And there is grounding to keep lightning energy from entering the home/work space. Phone Lines, Cable TV have a surge supressor installed where the signal wiring enters the building--With a wire to ground (ground rod, water pipe, etc.) to shunt energy away from inside the building.

    And there are other reasons for grounding (or proper grounding)... There is cathodic protection--Helps prevent rust/corrosion of metal/wiring that contacts the earth:

    https://cathwell.com/priniples-of-cpppppp/

    So--The question is what is the power system (is this a 192 Volts DC battery bus, or 208 Volts AC inverter output?) and what you are trying to prevent or protect against...

    In general--There are two types of power systems... One is "ground referenced" and is used for cars (your battery is negative grounded and the metal body/frame is used as the negative wire for returning power back to the battery). In homes/industry, our 120/208/240/etc. AC power is usually ground referenced. That way, if there is a short between AC power and metal (building frame, electrical box/conduit, kitchen sink, etc.)--A short will trip a circuit breaker or fuse and kill the power to the short. Because the neutral side is grounded, it never goes above zero volts above ground--So no breaker/fuse is used on the "neutral or return" side of AC power systems (i.e., in 120/240 VAC split phase power systems, the neutral is tied to ground in the main panel, and we only have one breaker in the panel for protecting 120 VAC circuits).

    And the second power system is "floating"... The power lines (+/-, Hot and Return, etc.) are floated from ground. If one leg is shorted (or a person touches one power leg), there is no current flow and is "safer" (in some ways). However, you need a (for example) two pole breaker to protect both +/- (hot and return) from short circuits (long explanation why) in case there are two or more faults to ground. Floating outputs are usually used for special applications.

    Anyway--Need more details to make any recommendations... Grounding can be a very complex subject and details matter here... In one system, tying return to earth ground/ground rod is the correct thing to do. With a different AC inverter, it can short the inverter out and smoke the device.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    colinas said:
    Similar question. I just installed a 10KW 192V system. It is located in a welded metal building, making the whole building a big ground. After connecting the ground wire of the inverter l get 192V between battery positive and the metal building. Do l need to connect battery negative to my ground as well?
    Is your inverter pure sinewave, or modified sine wave ?  Carefully check your + & - battery terminals to ground.   You have some dangerous voltages running around.
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