Why don't RVs need an earth ground for AC circuits

[Joel_T]

I've been told AC always needs an earth ground, DC not. An AC earth ground, as opposed to a DC type chassis ground, saves lives. To run any 120v stuff in an RV, the shore cord is supposed to provide that earth ground. When I plug that shore cord into a generator or invertor I don't have an earth ground. (I “may” have a bonded neutral to ground. How’s that safe? Seems that'd still be dangerous if there was a short.) When I plug into that unbonded generator or invertor I have to "bypass" my EMS device because it appropriately gives an open ground error. One of my invertors (Morningstar) doesn't even have an AC ground output terminal...just a hot and neutral, with neutral to be sent to a DC "earth" ground. RV's don't have an earth ground. When camping why shouldn’t I need a relatively short earth ground rod to attach to the RV's chassis ground and/or generator ground lug. Then I read neutrals are aren't supposed to be sent to chassis grounds just earth grounds. WTF! I'm not an electrician and would love a safer understanding of all this. Is there one?

[time2roll]

When connected to the grid, an RV is a subpanel with no Neutral-Ground bond.
When disconnected from the grid, the power source is independent and the bond is optional.
It is what it is. And it changes depending on your mode of camping.

[Joel_T]

"It's against NEC code for the trailer to have neutral and ground bonded together." ....and yet all trailers do?? Morningstar requires it for their inverter? What do they say?

I'm on page 7 of that 41 pages and realize whatever I do will in some way be wrong.

[ktmrfs]

It's against NEC code for the trailer to have neutral and ground bonded together. Neutral/ground bonding is ONLY allowed at the main feed breaker panel, nowwhere else down the line. The main feed panel is either the panel in your house, or the panel at the CG. The trailer is just like another thing plugged into a wall outlet, neutral/ground bonding prohibited.

If you open up the pedestal at a campground, I suspect you would also find that at the pedestal neutral and ground are not bonded either. the neutral bus bar is likely insulated from the metal housing. At least that is what NEC code will want.

In fact, if the trailer has a neutral/ground bond an upstream GFI will trip and not supply power since newer GFI will trip either on a hot neutral imbalance or a neutral/ground bond.

One can come up with all kinds of cases where trailer neutral/ground bond will be dangerous or protect you. Likewise without a neutral/ground bond.

However, I'll leave it to NEC to decide what is best, and they say not allowed.

now this does cause an issue with those of us using a portable generator with an EMS system that will shut down with an "open" ground, which is what it will detect since a portable generator in compliance with NEC as a portable device doesn't have a neutral/groud bond. There are mutliple solutions, one common one is a plug with the neutral side shorted to the ground side and plugged into one of the outlets on a duplex outlet, thereby making the EMS think there is a neutral/ground bond.

This condition does not apply to most onboard generators because they usually do have a ground/neutral bond with the generator being the main power source.

[time2roll]

Sometimes I just trust the professionals and the code book.
I let it go at that.

[SoundGuy]

Joel_T wrote:
I'm not an electrician and would love a safer understanding of all this. Is there one?

Sure ... wanna read 41 pages of discussion on the subject of portable gensets, bonding, earth grounding, etc, then have at it 😉 ... but don't expect to come away with any "safer understanding of all this" when even the "experts" who participated in that discussion can't agree! :S

[Terryallan]

It works. that is all I need. Don't make it harder than it has to be. :B

[Salvo]

There should be a sticky for this question. A search will show lots of threads.

The short answer, shore power is referenced to ground. Portable generators or inverters have no reference to ground. They don't even have a neutral. They have two wires, either one could be defined as "Hot" or as "Neutral". You could bond the "neutral" to generator chassis/receptical ground. It won't make the MH any safer. In fact, the MH will become less safe. With no bond, it takes two faults to get shocked. It takes just one fault to get shocked when bonded.

[road-runner]

My first thought is that it's a compromise. It would be ideal if an RV's frame and skin was always earth grounded. More developed campgrounds provide this via the shore connection. For less developed campgrounds they could provide ground connections at each site, with a lot of cost to install and maintain them. When moving down the road an earth ground is obviously impractical. So the code allows the vehicle frame to "act" as the earth ground. What about shock risk? Shore power always has one of its conductors earth grounded, so an accidentally hot frame or skin could give a full-blown shock. Earth grounding is therefore important when on shore power. The portable or vehicle mounted generator has all of its conductors isolated from earth ground, so there should be no current path for a shock to earth ground. There is sometimes a conduction path from the vehicle frame to ground, so the vehicle frame may or may not be connected to some degree to earth ground. If the generator isn't bonded, there's no current path to earth ground. If it is bonded, any current path from the frame to earth ground will put them at the same potential, meaning no current path for a shock.

[NORM_WADDELL]

This is NOT to answer your question, but it might interest you.

Navy combat ships do NOT ground any system. The reason
is that, with our 'normal' HOUSEHOLD grounded systems, if the HOT
wire touches ground the breaker trips or the fuse blows. In combat,
this is obviously NOT acceptable !............in place of the grounded
system, a GROUND DETECT system is used to detect grounds, and they
are removed as soon as possible.

I have long thought that this should be extended to homes and businesses.

[MrWizard]

it Real Simple
the utility power is connected to earth ground
therefore circuits connected to utility power need an earth ground as a safety
if something shorts in an appliance or device, and earth ground completes the circuit and trios the breaker, rather than the device/appliance/tool being electrically HOT and you being the ground to complete the circuit

the RV genset or portable genset, is NOT connected to earth ground, if something shorts in the RV there is NO electrical circuit between RV and earth or portable tool/appliance and earth so power cannot flow between RV or tool and you to the ground and back to the generator

but it can with utility power

RV genset or portable genset is "isolated power"

if you want to drive a 10ft ground rod and ground your generator
go ahead and do so, but you are safer with out it

[Old-Biscuit]

When connected to shore power you have ground protection thru 'ground' lug and back thru power pedestal.

When connected to generator it acts like an isolation transformer. Since the power source is isolated, there's no way for part of the current to flow back to the source through the ground and there's no shock hazard from the voltage to ground.