I don't understand some of Denker's comments concerning grounded
distribution systems and GFCIs.
* * * *
He said the ground-return system is less expensive because it uses one
wire instead of two wires. This is not true. First of all, it is a two
wire system. For residential wiring the "return" to the center-tap of
the transformer is a wire. Also, from pole to pole, the HV is along the
top, and a ground wire also runs from pole to pole as the lowest wire on
the pole: a true two-wire system.
In addition, at each pole, the ground wire is referenced to the earth
with an additional wire that runs down the side of the pole and into the
ground. Therefore, the grounded system requires considerably more wire
and considerably more labor to install than a two-wire system without
ground. The grounded system is more expensive.
* * * *
John said a GFCI can work on a two-wire system but not on an
OK, but we do have a two-wire system. There isn't supposed to be any
current in the dirt. The current is all carried by wires, one of which
is referenced to the dirt.
The GFCI works exactly as I described.
* * * *
John said the GFCI detects a "gross fault to ground."
Actually a GFCI does not detect a fault to ground. It detects a
difference in the outgoing and return current. The name is misleading.
Typically, when the out/return currents are different, it is because of
a ground fault, but it does not need to be a ground fault. All that is
required is a difference.
The difference is not a gross difference. It is about 2 mA. The "can't
let go" current during electrocution at 60 Hz is about 16 mA. A GFCI
must trip at much less than this in order to be approved. How fast the
GFCI trips depends on the difference. The GFCIs I have tested trip in
about 0.5 second with a difference of 2 mA. They trip in about 0.1
second with a difference of 5 mA. The trip is essentially instantaneous
(about one cycle) if the difference is more than 5 mA.
Yes, you will probably feel a shock when the GFCI saves you. But it
should be a minor and short-duration shock.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton, OH 45817