[AT] : Ice Storm

[Henry Miller]

On Friday 14 December 2007 12:45:15 charlie hill wrote:
> Mike you are probably 90% ok since you are grounded through the transfer
> switch.  I was primarily talking about the units that are sitting on
> trailers and connected via drop cords.  If you plug something into the drop
> cord in that application  you have no ground.
>
> I'm not qualified to talk about your situation because I'm not an
> electrician by any means BUT my friend said something about a ground
> potential difference between the ground in the house and the generator unit
> and said that the unit it's self should be grounded.  Maybe someone else
> can explain that.  Like I said, I don't really know other than what I was
> told. Electrical stuff is one of my very weak points.

Grounds are VERY complex.   They seem simple, but they are the most complex, 
most dangerous part of an electrical system.  

Summery for those who don't want to know the whys:  When using a generator as 
portable power, plug into a GFCI outlet and ignore the ground connections (if 
your generators doesn't have a GFCI get one installed).   When using a 
generator to power your house, the house provides the ground, any earth 
connection at the generator could be a DEADLY second ground!  

Long explanation follows:

The safest system has NO ground connection at all, and perfect wiring.   If 
there is no ground connection you can safely grab a live wire.  Unfortunately 
things are not that simple, it only takes one mouse (every house has mice) in 
the walls to turn your perfect wiring into wires with problems.   One problem 
isn't too big a deal, as you shouldn't be touching the wires anyway.    When 
there is a second problem it can burn your house down!  Fuses won't protect 
you because these problems don't have to be big enough to blow a fuse to burn 
the house down.    So fire code requires (and rightly so) that one wire has a 
good connection direct to ground.  The hope (and this mostly works out) is 
that if there is a problem, the good ground will make it more likely that a 
fuse will blow.   There are many different cases of a bad ground and several 
electrical problems burning a house down, so make sure your house has a good 
ground.

Note that I said A good ground, as in singular!   Strangely enough, two 
grounds can kill you just by walking in the wrong place.   (Sometimes a house 
will have several different ground rods, but these are carefully connected so 
they act as one ground)  So if your have your generator wired to a transfer 
switch in your house you should not have a ground connection on the 
generator, as your house is already grounded.   If your generator has a 
ground point (and all non-plastic generators should), it should be connected 
not to the earth, but to the ground connection in the house.   (By rights 
your generator should have a switch so you can select if generator is 
grounded at the house or in the generator, but I've never seen a generator 
with this, most likely because the switch wouldn't be connected right 
anyway.)

For generators on a trailer (meaning not used to power a house) the situation 
is different.   The rules are simple: Grounds are for fire protection, and 
not human protection.  GFCIs are for human protection.   So for your outdoor 
use make sure you only plug things into GFCI outlets (Don't forget to test 
all GFCIs monthly by hitting the test button and making sure all outlets you 
think should be protected really turned off).   The GFCI will take care of 
everything you want a ground for, and avoid the hassle of getting a good 
ground connection.  

If you can't use a GFCI outlet, double insulated tools and good cords (inspect 
them as you unroll them, and get rid of any that are getting frayed) are good 
enough.  Equipment that has a ground connection is a risk when run from a 
generator.

Note, if you have a pacemaker things are more complex yet, because they are 
affected by tiny currents that you wouldn't notice.   I recommend you check 
with your doctor and pacemaker manufacture to see what the rules are, before 
you use any generator or welder.