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

Ok the reason you need an earth ground when hooked to shore power but NOT when using a generator or inverter.. That is the real question, how come?

Simple.. At many many points the NEUTRAL in your SHORE POWER system si bonded to ground.. In fact at one time, due to metal shortages, they ran a single wire from the pole to your house.(the hot lead) and relied on the Ground to act as the "Return" IN SOME PLACES THEY STILL HAVE HOUSES WIRED THAT WAY.

So imagine you are holding oh, say a hand grinder (Skill to be specific) and there is a hot-case short.. Now YOU become a return path when hooked to shore power.

BUT if you are running off the inverter, or generator and it is NOT bonded to earth ground, Then using that self same hand grinder.. You would be protected,, Because you are NOT a return path, there is no "path to ground" at either end.

Why did I choose a skill hand grinder? Used one professionally for a bit. It had a case to hot short.. I had to wear gloves to use it (Got my own Dremel (low end) to use in it's place. Dang thing lasted 20 years before it smoked Still have a Dremel here).

But the reason a earth ground saves lives when on shore power is because it shorts the current to ground (if there is an internal short) and blows the fuse/breaker instead of the user.. BUT if on Generator or inverter. There is no path to or from the ground so you are protected by "Double Insulation" technique (Which is used on many brands of tolls with plastic houses and 2 wire plugs)

Joel_T wrote:
I've been told AC always needs an earth ground, DC not.

Strictly speaking, this is incorrect. The earth ground is actually a safety ground, and not required if you have ground fault interrupters.

"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?

First, no RV has neutral and ground bonded- the only place it will be is if it has on board power source. The inverter is a power source, as are generators. The neutral and ground are bonded *at the source*, but when you disconnect and go to shore power, they must be separate.

NORM WADDELL wrote:
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.

I've often felt the same, but too many would never get fixed because it still worked.
Factories used to use an isolated system with their 480v 3ph buss duct, but even there it often did not get repaired.

Dusty

ktmrfs wrote:
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.

OK, I'll bite ... what are all those "multiple solutions" you're referring to? :h The only practical solution I know of when an EMS interprets a genset's floating neutral as an "open ground" and won't output voltage to the RV is to bond the genset's neutral to it's ground. Enlighten us. ;)

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.

"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.

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.

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

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

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