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)
