There's lots of lively discussion going on about any potential difference between neutral and earth ground.

But what is earth ground and how do you define or measure the potential between it and anything else?

Remember also that this potential may vary from place to place and time to time.

 

Notice that the quote merely says, "by some other path other than the neutral wire." Unstated but implied is that "the neutral wire" refers to the specific neutral wire that goes through the GFI. The next part, about "a ground path back to the neutral source" is merely given as an example, not a requirement (that's the big difference between e.g. and i.e.).

I'll give you a simple example of where a completely ungrounded 2-wire system would trip a GFI.

You have two lamps (think homebuilt lamps where you just have a simple edison base mounted on a board that you are using for some photosensitive project) on your workbench. One of them is plugged into Outlet A, that is GFI protected. The other is plugged into Outlet B, which may or may not have one, but which is NOT downstream of the GFI protecting Outlet A.

You grab both bases to adjust the spacing between them and contact the hot screw of one and the neutral of the other (perhaps one of them is plugged in backwards so you thought you were grabbing a part of each that was tied to a neutral). What happens? Current flows from the hot of one outlet to the neutral of the other outlet and, bam, the GFI trips.

 

There's lots of lively discussion going on about any potential difference between neutral and earth ground.

But what is earth ground and how do you define or measure the potential between it and anything else?

Remember also that this potential may vary from place to place and time to time.

See post #6.
http://forum.allaboutcircuits.com/showthread.php?t=87868
Max.

 

Actually my definition of an earth, which works for any of the three uses we put an earth to is

An earth is a circuit element that does not change potential regardless of the current flow into or out of it.

This definition also allows use in circuit theory eg Kirchoff etc.

Unfortunately real world earths do not fully possess this characteristic, like other real world components or elements.
So depending upon circumstances the earth potential may vary from place to place or time to time, thus leading to a varying potential measured to other circuit points.