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Thread: Grounding an RV

  1. #1

    Default Grounding an RV

    I'm curious what folks have done regarding grounding in RV systems. I'm helping some friends out on a small PV system for their travel trailer, and I notice that the wiring schematic (or what's left of it) specifies that it is "grounded" to the chassis. Pardon the ignorance on my part, but I've never quite understood what that actually does in terms of safety... wouldn't that be essentially energizing the chassis (and in this trailer the "skin" as well)?
    The basic system we're looking at is the following:
    -generator/utility input feeding AC loads and/or 30amp Iota charger
    -Iota charger and or PV feeding battery
    -battery supplying DC loads and inverter/AC loads

    This would be a mobile system, so we can't just put in a permanent ground rod or anything. I appreciate any help or advice you can give me on this
    HB

  2. #2
    Join Date
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    Default Re: Grounding an RV

    Rubber tires have a fiar amount of carbon in them, so that takes care of most of the "static" issues. Tires can't carry a household ground.

    The inverter is what you have to be careful of, grounding one side of the 110V is generally bad, as often, one side of the battery is tied to ground too.
    http://tinyurl.com/LMR-Solar
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    Grundfos 10 SO5-9 with 3 wire Franklin motor (1/2hp 240V 1ph # 214505 ) on a timer for 3 hr noontime run - Runs off PV
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  3. Default Re: Grounding an RV

    confusing terminology.

    in this case you are talking about a frame ground and not an earth ground. That provides a common referent for electrical devices attached to the RV and reduces noise problems. In low voltage systems, the frame ground is often used as a return path in circuits to reduce wiring costs. Current practice tends to be getting away from this and towards using frame ground strictly as an electrical reference. That reduces problems of ground loops, poor connections, noise sources and also provides a point of reference that can be used for safety reasons like GFI.

    The only time an RV gets an earth ground, or needs one, is via its connection to grid power. When connected, the earth ground and the AC electrical neutral to ground connection occur at the service entrance.

  4. #4
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    Default Re: Grounding an RV

    Primary concern is making sure the ground of any A.C. 'shore' cord connection is grounded to RV chassis.

    Within your RV, it is prefered to keep A.C. neutral (wide AC prong) floating (versus grounded) as this is what is required for small inverter/generators like Honda and Yamaha units.

  5. #5

    Default Re: Grounding an RV

    Sorry for the confusing terminology. I'm certainly confused by the whole idea of RV frame grounding. The common reference point makes sense, hadn't thought that the trailer frame could accomplish this. I'm still unclear as to how this accomplishes the basic safety goals of grounding; if there was a loose conductor or something that energized a component of the system would this "frame ground" not simply allow those currents to then energize the whole trailer?

    My poor brain is getting tired trying to visualize how this frame grounding affects the system as a whole; can I still make a system ground by bonding the DC negative to a ground wire and connect that wire to the frame? I'm trying to make sure that I don't develop any ground loops within the system. I'm pretty sure that the AC circuit of the trailer is indeed bonded to the frame and probably the DC circuit as well (I'll need to verify both of these, as the wiring is quite old and it's hard to say "what was" and "what still is" without a more thorough inspection). What else am I overlooking or missing here?
    Thanks,
    HB

  6. #6
    Join Date
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    Default Re: Grounding an RV

    Generally, the idea that one wire (whether the neutral of a an AC system or the negative of a battery system) is grounded to the frame/chassis/any exposed metal pieces.

    Then if there are any exposed "hot" wires (broken insulation, an electrical component touches the case of a metal box, etc.)--an electrical connection causes high current flow and causes the upstream fuse/breaker on the "hot lead" to pop.

    This also prevents somebody from touching an ungrounded metal box (toaster, electric drill, etc.) that has a short to hot and touching a metal sink/faucet/etc.) form getting a shock/electrocuted.

    With ground referenced systems (like AC power into the home... You could be standing in a puddle outside and touch the trailer siding/door handle and get shocked if the the trailer chassis was not connected to the electrical safety ground (metal box energized with respect to ground).

    A "floating system" like a battery or a transformer coupled AC system is not ground referenced until one of the leads is connected to earth. So--you could touch the positive OR negative lead of a battery or either output lead of a 120 VAC isolation transformer an not get shocked--In fact, this is commonly done on purpose to prevent shorts (using isolated power). But isolation is not done with normal household power.

    Both ground referenced and floating systems have their advantages and disadvantages. Following convention is usually a good idea in the end as it prevents mistakes.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  7. #7

    Default Re: Grounding an RV

    Quote Originally Posted by RCinFLA View Post
    Primary concern is making sure the ground of any A.C. 'shore' cord connection is grounded to RV chassis.

    Within your RV, it is prefered to keep A.C. neutral (wide AC prong) floating (versus grounded) as this is what is required for small inverter/generators like Honda and Yamaha units.
    Ok I think this makes sense: in this system we're planning on sharing the same outlet for either the Honda input OR utility/shore power. We have an Iota automatic transfer to isolate the inverter from the main AC input and Iota charger, so we could run a ground wire from the main AC input to the frame and not ground the AC circuits or the inverter's AC to the frame.

    Quote Originally Posted by BB. View Post
    Generally, the idea that one wire (whether the neutral of a an AC system or the negative of a battery system) is grounded to the frame/chassis/any exposed metal pieces.

    Then if there are any exposed "hot" wires (broken insulation, an electrical component touches the case of a metal box, etc.)--an electrical connection causes high current flow and causes the upstream fuse/breaker on the "hot lead" to pop.

    This also prevents somebody from touching an ungrounded metal box (toaster, electric drill, etc.) that has a short to hot and touching a metal sink/faucet/etc.) form getting a shock/electrocuted.

    With ground referenced systems (like AC power into the home... You could be standing in a puddle outside and touch the trailer siding/door handle and get shocked if the the trailer chassis was not connected to the electrical safety ground (metal box energized with respect to ground).

    A "floating system" like a battery or a transformer coupled AC system is not ground referenced until one of the leads is connected to earth. So--you could touch the positive OR negative lead of a battery or either output lead of a 120 VAC isolation transformer an not get shocked--In fact, this is commonly done on purpose to prevent shorts (using isolated power). But isolation is not done with normal household power.

    Both ground referenced and floating systems have their advantages and disadvantages. Following convention is usually a good idea in the end as it prevents mistakes.

    -Bill
    Let me make sure I follow this all correctly. In the first statement are you suggesting that if I have the AC side grounded that I would not want to ground the DC side to the same frame? Or if I were to ground the AC input only, and not the AC circuits in the trailer would I then want to ground the DC side as well?

    If you're suggesting that I don't want both the AC and DC to ground to the same frame, then I'm a bit confused by the latter part of your post regarding following convention. Sorry for being dense, I'm not sure what you were suggesting in terms of following convention. Or were you merely comparing how grounded vs ungrounded circuits function in general. I would very much like to follow convention in terms of grounding, and keep this part as simple and clean as possible.

    Thanks again for your help everyone,
    HB

  8. Default Re: Grounding an RV

    Sorry for the confusing terminology. I'm certainly confused by the whole idea of RV frame grounding.
    you are not alone.

    When connected to grid power, the ground lead is connected to an earth ground at the service entrance. The neutral power lead is connected to the ground lead at that point also. Your RV is considered a sub-panel and only carries these leads through to the outlets as is. The only earth to ground and neutral to ground lead connections are at the service entrance.

    Your RV will likely have the battery negative connected to the chassis permanently.

    On a small genset or inverter (under 5 kw with plugs), just plug your RV in and leave it to the genset or inverter as to such issues. The chassis ground on the genset or inverter will get connected to the RV chassis ground via the ground lead in your power umbilical. The hot and neutral will probably be floating and isolated from any ground reference.

    For a larger genset or inverter that is wired in with a transfer switch, the transfer switch should leave the chassis ground as is and switch the hot and neutral as appropriate. That means straight through when to grid but connecting chassis ground and neutral when off grid. That connection, the neutral to chassis ground bond, is usually done in the genset wiring so the transfer switch doesn't have to worry about it.

    Then there are the special cases, especially with older equipment. Read the manual and follow the instructions!

    Do not worry about earth grounding in RV power systems. There is no need to try to install a ground rod or whatnot every time you set up camp off grid.

  9. #9

    Default Re: Grounding an RV

    Bryan,
    Thanks so much, the dense fog in my brain just started to clear up. I knew that I was over-thinking things here and missing the obvious! That all makes a lot of sense. The one thing is that there is not currently a battery in the trailer at all, so I'm not totally sure what we'll find as far as the DC side. There is a DC circuit already in place, so perhaps that is already bonded to the frame... at this point I'm pretty tempted to just yank the whole DC circuit out as it's only a couple of lights and it may be cleaner and tidier to just start the whole DC loads from scratch. I'd love to do that with the AC circuit too since the wires are probably almost 40 years old now, but I think that it would likely be far to messy and destructive to mess with that circuit at all.
    Cheers,
    HB

  10. #10
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    Default Re: Grounding an RV

    I am sorry--it is confusing because there are two perfectly valid ways to make your system "safe".

    One is to ground reference the power source--then any shorts to ground trip the breaker fuse.

    The other is to "float" the power source--then any single short still does nothing (no way to complete a circuit).

    With North American Power--it is always ground referenced back at the "main panel) in one place. So--for an RV, one would expect to see three wires (120 VAC) connections that are straight through. The Hot to Load, the Neutral to Load, and the Ground to metal chassis, outlets, etc...

    When you have an inverter or small genset--the AC power output is generally floating. So, you have a choice of grounding or letting it float. Generally, I would just let it float (plug the shore plug into the genset or use a transfer switch that switches both the hot and neutral lines).

    With batteries it gets more complex. The chassis is usually grounded directly to the battery "-" terminal. And the chassis may be used for the return lead of the loads (small radios, fans, etc.) instead of using a negative return wire. For all metal vehicles (like old cars) there is usually no issue. For RV's and such build with wood and aluminum (or fiberglass) siding and such... You probably have to ground your 12 VDC appliances with a ground wire back to the chassis frame or back to the common ground connection.

    And with inverters, it becomes even more complex. Just to be clear, not grounding the "neutral" wire from the inverter is the default safest pick. That means you cannot ground the neutral in the RV's AC wiring (if connected to shore power, the RV's wiring will be earth grounded anyway back at the RV park's grounding setup).

    The short answer is some Inverters (typically MSW) outputs are Battery Reverenced (the +/- DC leads are, at times, directly connected to the AC output). If you both negative frame ground the battery bank, an attempt to frame ground the Inverter's neutral wire--you will get a short circuit through the inverter and let out the magic smoke.

    You can with some inverters (mostly TSW type) ground reference the Neutral without any problems (see manual--some will recommend neutral to ground bonding--just follow their instructions) -- but it is not necessary for safety. And, again, to let the Inverter's AC output float is the default "safe" condition.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

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