To ground or not to ground to chassis?

cralls
cralls Registered Users Posts: 26 ✭✭
Hi All,

Question, would it do any harm to test my inverter before grounding it to the chassis of my RV? Neither the battery bank nor the inverter are grounded to the chassis at the moment but everything else is hooked up. I'm just curious what difference it would make to have the bank/inverter both grounded to the chassis of the RV before running some tests.
:: 3 - Suniva OPT330-72-4-100 Silver Mono Solar Panels | Magnum Energy MMS1012
:: Midnite Solar Classic 200 | 4 - Interstate 6V GC2-XHD-UTL

Comments

  • oil pan 4
    oil pan 4 Solar Expert Posts: 767 ✭✭✭✭
    What does the manufacturer technical data say?

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

  • cralls
    cralls Registered Users Posts: 26 ✭✭
    edited March 2016 #3
    I don't really understand it to be honest, I'm not sure if I'm supposed to wire the battery bank and inverter to the chassis or just one or the other. The way I have this wired in the RV is I'm just plugging the RV with a 15amp adapter directly into the inverter to power the whole RV so the battery bank and inverter are a separate system.

    The manual says this...

    The inverter/charger should always be connected to a permanent, grounded wiring system. The idea is to connect the metallic chassis of the various enclosures together to have them at the same voltage potential, which reduces the possibility for electric shock. For the majority of installations, the inverter chassis and the negative battery conductor are connected to the system’s ground bond via a safetygrounding conductor (bare wire or green insulated wire) at only one point in the system. Per the NEC, the size for the grounding conductor is usually based on the size of the overcurrent device used in the DC system. Refer to Table 1 to select the appropriate DC ground wire based on the overcurrent device used for your inverter model.

    If the inverter is in a vehicle, DO NOT connect the battery negative (-) cable to the vehicle’s safety ground. Only connect to the inverter’s negative battery terminal. If there are any non-factory installed appliances onboard the vehicle, DO NOT ground them at safety ground. Only ground them at the negative bus of the DC load center (as applicable).
    :: 3 - Suniva OPT330-72-4-100 Silver Mono Solar Panels | Magnum Energy MMS1012
    :: Midnite Solar Classic 200 | 4 - Interstate 6V GC2-XHD-UTL
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    edited April 2016 #4
    It is safe (enough) to use a marine/mobile inverter (ie UL458) without any hard wiring, using a single appliance plugged directly into the inverter. I recomend double insulated appliances only and a plug in RCD/GFI (if the inverter doesnt have one). What you dont want to be doing is plugging a whole bunch of appliances in because thats when you increase the risk of shock from appliance grounding issues.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • cralls
    cralls Registered Users Posts: 26 ✭✭
    Thanks zoneblue, so if I were to set it up correctly would I want to ground the battery bank as well as the inverted to the chassis?
    :: 3 - Suniva OPT330-72-4-100 Silver Mono Solar Panels | Magnum Energy MMS1012
    :: Midnite Solar Classic 200 | 4 - Interstate 6V GC2-XHD-UTL
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Remember that vehicle electrical systems (many times) use the vehicle chassis as the "negative lead". Most off grid installations (with large inverters and battery banks) use an insulated negative lead back to the battery.

    What you want to do is have major metal components (chassis, sink, plumbing, metal siding) grounded to the chassis (small jumper wires) and/or directly to battery bank.

    That way, if there is ever a short circuit from the "battery hot" (fused/breakered) to any major metal in the vehicle--The metal will not become "hot" but will instead short the hot lead and trip the breaker.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cralls
    cralls Registered Users Posts: 26 ✭✭
    Hi BB, That makes sense and that's basically what I have now. I have the battery bank hooked up to the inverter which of course is hooked up to the positive and negative sides of the battery bank and then I'm plugging the entire RV into the AC Out side of the inverter. 

    I'm hearing two difference answers here though, 1) It's ok not to ground the battery bank / Inverter if I'm only using one appliance and 2) Everything should be grounded to the chassis or back to the battery bank... but in the case of #2 if the battery bank isn't grounded to the chassis then it doesn't help everything else that's grounded to the chassis so I should probably ground both the battery bank as well as the inverter to the chassis. Let me know if I'm understanding correctly? Thank you again!
    :: 3 - Suniva OPT330-72-4-100 Silver Mono Solar Panels | Magnum Energy MMS1012
    :: Midnite Solar Classic 200 | 4 - Interstate 6V GC2-XHD-UTL
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Yes, the battery bank negative is also grounded to the chassis. The idea is to have a complete current path from every major metal component back to the battery negative terminal (and the AC green wire ground too).

    But all devices with a negative terminal should be directly wired back to the battery negative bus. (many 12 volt car accessories use the metal chassis, such as a car radio, as the negative ground terminal too).

    Things get a bit more complex with AC power connections. Green wire ground wire and the AC neutral terminal are tied together back at the main AC panel in the RV park.

    When running from AC genset, sometimes the AC neutral is bonded to trailer ground (with many/most true sine wave inverters) and not tied to chassis ground (with many/most Modified Sine Wave ac inverters). To do things "right" needs a relay that can switch Neutral/green wire bonding options inside the RV.

    For smaller AC off grid power systems (MSW or TSW), many times AC neutral is left floating from the genset/AC inverter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cralls
    cralls Registered Users Posts: 26 ✭✭
    Thanks again Bill.. you are a wealth of knowledge, I really appreciate you taking the time to answer. What your saying makes sense so what I'm going to do is ground the bank and inverter to the chassis so everything else can find it's way back to ground.
    :: 3 - Suniva OPT330-72-4-100 Silver Mono Solar Panels | Magnum Energy MMS1012
    :: Midnite Solar Classic 200 | 4 - Interstate 6V GC2-XHD-UTL
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    You are very welcome Cralls.

    Note--You are grounding the "chassis" of the AC inverter to chassis ground. Not the Negative Terminal of the AC inverter.

    We do not want to "share current" between chassis and the negative return wire... It can cause problems (like overheating chassis bolts, bonding wires, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cralls
    cralls Registered Users Posts: 26 ✭✭
    Hi Bill,

    That's exactly right, I already have the positive and negative terminals on the AC inverter hooked up to the positive/negative ends of the battery bank. The inverter has a separate place to ground it to the chassis. I could see someone making that mistake though so I'm glad you mention it here... hopefully it will help someone in the future :)
    :: 3 - Suniva OPT330-72-4-100 Silver Mono Solar Panels | Magnum Energy MMS1012
    :: Midnite Solar Classic 200 | 4 - Interstate 6V GC2-XHD-UTL
  • cralls
    cralls Registered Users Posts: 26 ✭✭
    Ok so I have a problem... I went to connect the ground wire that's connected to the chassis to the negative side of the battery bank and it makes a buzzing noise haha... what could that be?
    :: 3 - Suniva OPT330-72-4-100 Silver Mono Solar Panels | Magnum Energy MMS1012
    :: Midnite Solar Classic 200 | 4 - Interstate 6V GC2-XHD-UTL
  • cralls
    cralls Registered Users Posts: 26 ✭✭
    cralls said:
    Ok so I have a problem... I went to connect the ground wire that's connected to the chassis to the negative side of the battery bank and it makes a buzzing noise haha... what could that be?
    Sorry, I seem to have resolved the problem. I didn't have the inverter grounded first before trying to ground the battery bank. Once I grounded the inverter... then I could ground the battery bank so now both are grounded to the RV chassis and no more buzzing :)
    :: 3 - Suniva OPT330-72-4-100 Silver Mono Solar Panels | Magnum Energy MMS1012
    :: Midnite Solar Classic 200 | 4 - Interstate 6V GC2-XHD-UTL
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Depending on the type of AC inverter--For example, MSW (modified square/sine wave) type inverters--They are "electrically" more noise and can cause loose sheet-metal to rattle (MSW wave forms are usually closer to square waves and the sharp edges can cause stuff to "rattle" more than Sine wave).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cralls
    cralls Registered Users Posts: 26 ✭✭
    The inverter is a Magnum Energy MMS1012 so it's a pure sine wave. Still not sure what was making the buzzing but once I grounded the inverter first and then the battery bank it went away...
    :: 3 - Suniva OPT330-72-4-100 Silver Mono Solar Panels | Magnum Energy MMS1012
    :: Midnite Solar Classic 200 | 4 - Interstate 6V GC2-XHD-UTL
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    edited April 2016 #16
    Cralls, for AC hardwired motorhomes, similar regulations apply as to that of houses. Ie you need an electrician or "competent person" to do the work, and an electrical certificate.

    As bill explained the process is similar to a house is that the neutral and earth are bonded at one place in the motorhome, and in turn bonded to the chassis. However where it gets interesting is around handling the shore power, verses no shore power. The correct gear must be fitted that lifts the neutral bond when shore power is applied, because other wise you have two neutral bonds. It really isnt that straightforward\ and an electrician is the best route.

    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • djgriff
    djgriff Registered Users Posts: 1
    Hey Bill I have an on topic question. I'm converting a cargo trailer and was wondering if the ac panel has its own separate ground that go's to the chassis or if the ground from inverter/converter is sufficient? Hope that makes sense. -dj
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    In my humble opinion, there are several major issues to look at when grounding.

    First, make sure there are no parallel paths between Neutral and green wire grounding. Neutral carries return current, and if you have a bond between neutral and ground in two or more places--You can have part of the current flow in the white wire and part of the current flow in the green wire. There should never be current flow in the green wire--Except for fault current.

    Note that the green wire/safety ground will usually have many multi-point connections. For example, you may have both a green wire and conduit. The green wire is grounded in the main panel, and so is the conduit. And you can have the green wire and conduit tied together at the end of the branch circuit (outlet, switch, device). That is "legal" and standard procedure.

    I have concern with mixing Green Wire and Chassis ground with DC and AC devices... For example, 12 VDC and 120 VAC... A 20 amp (2kWatt) current in 120 VAC line turns into a 200 Amp current in the 12 VDC side of the system (power=voltage*current--1/10th voltage means 10x current for "same power"). And because you have a mix of green wire ground wire sizes (you may have 14 AWG green wire in branch circuit, 6 AWG from main panel to ground rod/water pipe, and upwards of 4/O ground wire from inverter chassis to battery negative bus to ground rod/water pipe common ground). So--It is possible to have a DC fault that can "fuse" a branch circuit green wire safety ground if you have multi point ground between chassises (AC and DC devices), and various wiring boxes.

    A possible solution/example would be a Multi-function panel like an e-panel where you have inverter+AC+DC wiring all in the same box. Looking at the fault current means that the ground wiring from that box should be heavy enough to carry all fault currents (typically a minimum of 6 AWG if the largest breaker (AC or DC) is 200 Amps maximum). That way if there is a DC fault in the inverter, the 6 AWG carries all fault current to (for example) the common ground rod, and the ground cable from the ground rod runs back to the Battery Negative Bus... If you had too small of green wire in that path, it would be possible for fuse the green wire safety ground--Which would energize the e-panel--And now you have "e-panel sheet metal" and "hot ground" that will find some other path (through 14 awg ground wires, for example) and eventually make it back to some common DC ground/negative bus connection--And possible fuse something in that path too.

    The last major reason for green wire/safety grounds is lightning. And my humble suggestion is that all grounds that could handle lightning (from solar array to local ground rod, ground rod to ground rod connection, main panel sheet metal to ground rod, possibly even sub panels (such as where solar array enters building with surge protection devices to local ground rod) should all be a minimum of 6 AWG (to reduce the chance of fusing the green wire from lightning). Of course, the other "lightning based wiring" rules need to be followed (short straight runs from bonding point to ground rod, soft radius bends where needed, run ground wires on outside walls and not through middle of building--such as array on roof, etc.).

    Note that for lightning, chassis, sheet-metal, multiple grounded devices (motors, outlets, Surge Protection Devices, etc.) are all examples of multi-point grounding... And, many times, where multi-point ground connections are actually a "good thing". And as long as you have a limited maximum current (200 amps or less), you can "mix" the different grounds together as long as you meet the minimum AWG size of 6 AWG for the major lightning grounding points (exterior boxes to remote devices like solar arrays, Main Panel, E-Panels, etc.).

    I am not sure this answers your question(s) directly--But gives you the idea of why and how the general design rules should be applied.

    Does this help, or did I miss your question?

    -Bill


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rfandy
    rfandy Registered Users Posts: 1

    How do inverter, shore power, GFCI, bonded and/or floating neutral all come into play?  If running off-grid, shouldn't I have neutral and ground bonded in trailer?  If I'm plugged into shore power, neutral and ground shouldn't be bonded as it'll be bonded by the shore power circuit.  Do I put GFCI breakers in my trailer's panel? Will they work properly? Do I need a switch that bonds/un-bonds neutral to ground??  Guidance would be appreciated.

  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    The inverter manual should detail wiring. Most likely, shore power in goes to the inverter and it handles it with an internal transfer switch. You likely don't want to add a switch to bond/unbond neutral. You might want to add a shore power polarity warning light though as that sometimes helps flag flaky shore power wiring.

    AFAIK, a GFCI should work ok. Personally, I'd probably just put a GFCI outlet if near the galley/head sinks, but I think a GFCI breaker should work too. There can be issues with false trips for loads far away (>~150') but that's not likely to be a problem in a trailer.

    Bill covered the grounding piece pretty well.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • schoolbusmagic
    schoolbusmagic Registered Users Posts: 1
    When you plug in a device and hear a buzzing in your case; I believe you are hearing a “ground loop”.

    When you ground the battery bank separately from the inverter you create multiple paths to ground that interact. Feedback from your AC current induces current in the closed ground loop and causes that buzz. 

    In turn, the circuit feedback can lift the ground potential between the two circuits and cause voltage spikes and other nasty effects.

    Look it up on Wikipedia, there’s a bunch of pretty standard fixes for it. I’m wrestling with the same thing in my RV solar array as we speak. I noticed a 3V spike in a 13.6V float... lol... I disconnected that circuit pretty fast.

    Im going to try connecting my grounds to my inverter and the inverter to the chassis. I’ll let you know how it goes. This way there should be just one ground path.
  • stmoloud
    stmoloud Registered Users Posts: 111 ✭✭
    My inverter chassis and battery are grounded to the same vehicle chassis point. The AC which comes from the inverter goes to a switchboard which is grounded to another place on the vehicle chassis. The principle being that under no circumstances should DC and AC wiring ever be in direct contact or use the same wiring under any circumstances. I don't get any buzzing. 
    760W panel array, 4 x 6v 220 ah Crown batteries, Tristar TS-45 PWM controller,  no name 600 PSW inverter.