[AT] : Ice Storm

[Henry Miller]

Excellent question Charlie, the short answer is in an ideal world if the GFCI 
is not connected to earth, you can't get a shock and so the GFCI does 
nothing.  Sometimes the world is not ideal, and something gets connected to 
the earth, and then the GFCI protects you.

There are two different things we mean by ground.   One it an actual 
connection to the Earth, and the other is the ground wires run all over your 
house.  I did not specify (and in fact rarely does anyone specify), but the 
two are very different.   We don't think of them as difference because the 
ground wires are normally (but not always!) connected to the earth at some 
point.

Power comes always comes in 2 sides.   We say hot and neutral, but in truth, 
both wires are equally hot - the only difference is one wire is connected to 
the ground wires (which are normally connected to the earth), and the other 
is not.   We call this hot wire neutral, but there is no significant 
difference (note that your house is 220, and there is a difference, but not 
in any way that is important to this discussion so I'm going to ignore that 
case to avoid confusion).

For electric current to flow there must be a connection between the two hot 
wires.   If there is no connection no current flows.   If there is no earth 
connection and you grab a hot wire nothing happens - you don't get a shock 
because there is no way for power to get between you and the other hot wire.   
Connect one wire to a ground rod though, and grab the other and you will get 
a shock because current can flow through you, through the ground, and back to 
the other wire.   

If you understood the above you will now be asking "Why do they have ground 
rods if they just make electric shock possible?"    The reason is the real 
world is messier, and an ungrounded system can burn your house down in some 
situations where a ground would not.   If a mouse chews on the wires in your 
walls (which happens all the time), you can have a bare wire providing a poor 
earth connection.   If a second mouse chews the the other wire elsewhere, a 
small amount of current can flow - not enough to blow fuses, but plenty to 
start a fire in the walls.     By adding a ground wire and earth connection 
odds are much better that enough current will flow to blow a fuse long before 
a fire can start if there is a problem.

As you see, the earth/ground system is for fire protection, not human safety. 

So if your generator is not earthed, why the GFCI?   The problem is you cannot 
be sure that the entire system isn't earthed.   An internal problem may 
connect one wire to the frame, which touches the earth.   Now you have a 
grounded system.   A GFCI will protect you from trouble now, just like it 
will in hour house.

Note that a GFCI does NOT require a ground/earth connect.  The GFCI just 
measures the current in the two hot wires.   If there is any difference, the 
rest of the current must be getting back some other way - by which it assumes 
the earth via your body, so trips to protect you.   Code allows (as in 
strongly discouraged but sometimes there is no choice) a GFCI protected 
outlet to have a ground terminal that is not connected to anything.

So I say use a GFCI outlet on your portable power generators because they will 
protect you in case something is wrong.    There is a small possibility that 
something could go wrong and cause a fire, but since this is a portable 
power, you should be right there to see problems before they become big.    

When connected to your house, use the house's ground/earth system for fire 
safety.  


On Sunday 16 December 2007 19:57:01 charlie hill wrote:
> Henry,  I've already admitted that I'm no electrician but I have to
> question one thing you are saying.  How is a ground fault interupter going
> to work if it's not connected to a ground?  If you are using any sort of
> tool that has a three prong plug and you don't have an earth ground on the
> generator YOU are going to become the ground if something goes wrong inside
> the tool.