Need AC inverter for RV. Negative bonding to ground?

I am finding it difficult to get my questions answered with some vendors and hoping you guys can shed some light.  I need to get an inverter installed in an RV that does not currently have one.  I need to have AC available for small loads when not connected to a generator or grid power.  The concern is that in examining the wiring in the RV, it is apparent the negative of the batteries is certainly bonded to chassis as well as the AC side ground.  It also appears there is some bonding between the AC neutral and ground.  

Now, my original plan was simply plug my RV cord into the inverter which would then supply AC to all receptacles.  Obviously I would have to limit my loads while using the inverter but this would simply several things.  I have seen other RVs with an inverter but they did not run inverter AC to the entire coach, just a couple outlets, probably so people would not try to run their hair dryer on them!  

I currently have an inverter but it is not designed as an "RV inverter" and I am reluctant to attempt this wiring config without better information and my current inverter has about ZERO support.  

The wiring schematic for my RV is pretty pathetic and it seems I may have to resort to drafting one just so I can confirm a few things.  I think the merger of AC and DC in the same wagon is really confusing me.  

Comments

  • mcgivormcgivor Solar Expert Posts: 3,609 ✭✭✭✭✭✭
    Your questions regarding RV bonding  are very common and the opinions vary on what is the best practice. Some inverters will fault if neutral is established, the manual for one I have specifically states not to do so as it relies on earth leakage detection of both legs for shock protection, there is a ground lug provided. Futher the manual states that the intended purpose is to use only the receptacles provided for load connection, the appliances in other words.

    Sorry for not answering the question but do consult the manual or manufacturer, if support exists, on what is the best practice, or simply use the receptacles with floating neutral as the final load connection.




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  • BB.BB. Super Moderators, Administrators Posts: 31,171 admin
    As always, refer to the manual for your AC inverter (it is hit or miss if the manual will discuss AC grounding in any detail).

    If it is an MSW AC inverter (modified sine/square wave), then almost all of them will "short out and let out the magic smoke". MSW inverter typically do not have electrical isolation between DC input and AC output. If you ground the DC bus (positive or negative) and ground one of the output leads, it shorts through the inverter.

    If it is a PSW/TSW inverter, most (virtually all?) of them have isolation between DC input and AC output... So "ground bonding" on the 
    DC or AC side does not matter (with a major gotcha on AC ground bonding).

    For the most part, the AC shore power (North America) has the white wire "ground bonded" back somewhere in the RV park.

    That means that most RVs do not ground bond the White/Neutral wire if they have shore power (only). If the RV has a ground bonded neutral, it should trip the GFI breaker/outlet for shore power.

    If the RV is purely off grid and using a generator or PSW/TSW AC inverter, the Neutral/White wire is ground bonded to the RV frame (along with grounding the DC battery bank negative bus.

    The idea is that if there are any shorts from Hot (12 volt or 120 volt) to anything metal (sink/plumbing/stove/propane), the ground bond will short the power line and trip the protective fuse/breaker. Just normal North American 120/240 VAC split phase wiring.

    Where it gets more complicated... it seems to me that smaller gensets and AC inverters (even TSW/PSW types) simply "float" the AC output (think like an isolation transformer). In some ways, this is pretty safe--If you touch a "hot wire", there is not return path (sink, water, etc.) back to the other AC lead. There are some other issues (like you get one "short" that does nothing, then the second short can be bad news).

    For larger inverters and generators, they seem to always connect AC White/Neutral to chassis ground. Normally, you only want this done in one location. At the genset, or the AC inverter, but not both. Also, the Neutral+Ground bonding needs to be done before the GFI breaker/outlet inlet connection. Ground bonding after the GFI will usually trip the breaker (looks like a unsafe connection).

    Larger RVs, they can have a transfer switch that "floats Neutral" when connected to shore power, and ground bond the neutral when off grid. So, that is a possible solution, use an AC transfer switch that also transfers Neutral+Ground bonding.

    Now, there is an alternative... You can "float" the AC Neutral/white wire (no ground bonding), and use a GFI breaker or GFI outlet(s) to protect yourself. A small TSW/PSW AC inverter will "float" its AC output, and the GFI outlet will protect whoever uses the AC power will be protected against electrocution--The GFI will trip if there is any hazardous current leakage (remember, that no GFI will protect a person from touching Hot to Neutral--That looks like a "normal" AC load and will toast you very nicely).

    You can use a transfer switch to change connections from RV main AC power (whatever that is) to power your 120 VAC circuits from your small AC inverter (or even just unplug from one inverter and plug into the second inverter).

    So much of this depends on how you plan on using the power (shore power, genset, AC inverter 1, AC inverter 2) and such.

    If you float your AC power in the RV, it will be compatible with shore power. And if you use GFI branch circuit breakers and/or GFI outlets, you will be safe. If you use a transfer switch (or two) to control neutral+ground bonding, that works too.

    PSW/TSW inverters are almost always compatible with Neutral+Ground bonding. MSW inverters are almost always killed when ground bonding (technically ground bonded DC input and ground bonded AC output--But almost everyone negative ground bonds there battery bank for many reasons).

    Sorry for the word salad. RV (and boat), genset, inverter, grounding is a very complicated subject. Confused yet?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fastlinefastline Registered Users Posts: 31 ✭✭
    edited April 7 #4
    Not entirely confused, but will admit that dipping into RV stuff gets largely into "how will it be used?"  I will offer some updated info to see if we can pin this down, and honestly, I sincerely appreciate your response!  I have found this subject to be a white cloud for most.  

    The inverter in question is a smaller, PSW device.  I did quickly check for obvious internal connections with a meter and can confirm the inputs and outputs are not directly linked.  I can also confirm the AC output is floating and the ground pin in the receptacle is not connected to the inverter chassis.  

    Further, I have verified at least in the main electric panel (only one here), the AC grounds and neutrals are NOT bonded.  The neutral bar is specifically isolated with plastic.  

    Further, the battery charger (converter) built into the RV has its chassis grounded to AC ground, and the negative goes to another terminal bar that is screwed to the wood floor behind the actual main panel.  All AC neutrals connect to that terminal bar and it 'would be' insulated due to where it is mounted.  HOWEVER, the negative lead from the charger goes from that AC neutral terminal strip, then back to the negative of the batteries.  Then those batteries are very obviously grounded to the metal chassis.  

    What is really confusing here is though the AC neutrals and grounds are NOT bonded in the panel, they technically are because because that neutral strip gets to chassis ground through the connection at the batteries.  

    I think what I might be excusing is though we all know a neutral and ground 'should' always be at the same potential, I might be skipping is how stray current might travel and as it can throw current on a ground conductor, a GFI would pick that up.  ??

    Now, I can tell you the RV is currently plugged into grid power and has zero issue with that.  I am not certain if I have put my generator on it yet but I know the Honda EU3000 has a floating neutral but is designed to be bonded when needed and will accept that connection.  So if the RV is floating, and the generator is floating, it is likely the system would not be officially 'bonded' on a generator.  

    My use for this setup will be temporary and I am just fine unplugging something manually rather than a transfer switch, but I know  exactly what you are saying with that and I think that is pretty much a standard for OEM installations where an inverter is added.  What I had planned to do was just unplug the generator when not needed, and let the inverter run some loads.  

    I have realized though that running the entire RV AC side with the inverter will not work correctly without some rework because it will then try to power the battery charger.  


  • Ralph DayRalph Day Solar Expert Posts: 927 ✭✭✭✭
    Or just shut the battery charger breaker "off" on your rv breaker panel when you're using the inverter.  Mind uyou, where is the inverter getting power from?  The rv house battery?

    Once you shut off the charger circuit breaker you won't have any charging/load loop occur.  Just remember to turn it back on when y ou need to charge the house battery (back on shore or generator power).
  • BB.BB. Super Moderators, Administrators Posts: 31,171 admin
    One thing to be careful of... Transfer switches are (generally) designed to only allow one power source to be connected to your AC house mains.

    If you put several sources on to one main (shore, genset, AC inverter, etc.)--And just use one breaker (or dual breakers if 120/240 split phase power)--The danger is always that at some point that a breaker will be left on, and one power source backfeeds another--Not usually a good idea. You can get some breaker lockouts (aka keyed locks) or interlocks (sheet metal pivots/sliders/etc.) that can prevent two breaker (power sources) from being on at the same time...

    We can all have a "brain freeze" on occasion, and doing this with electrical stuff can be dangerous and $$$$.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tcrescontcrescon Registered Users, Users Awaiting Email Confirmation Posts: 1
    BB Thank you. Best explanation on this. of a month long research.
  • BB.BB. Super Moderators, Administrators Posts: 31,171 admin
    I glad I was able to help.

    Hope all works out for your project(s).

    If you have any questions or wish to show us what your solar power system design is and how it works for you... Please feel free.

    Take care,
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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