[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: grounding question
Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
At 10:15 PM 10/10/2006, Tesla list wrote:
Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
Hi Gary,
Your post is very interesting. Our house also has the breaker panel
grounded to the copper pipe bringing water into the house. An
electrical contractor for our area says that this connection is no
longer done and instead the breaker panel is grounded via a rod
pounded into the ground at the breaker panel. Seems like a step
back according to your measurements. I wonder if the reason for the
change has to do with electrolysis of the copper pipe.
Nope.. has to do with the fact that water mains are not always metal
any more. Plastic is more and more common.
Anyway, if a grounding rod is good enough for the breaker panel
then why not good enough for a TC RF ground.
Lots of reasons. the breaker panel ground is designed for line
frequency grounding, primarily for human life safety. If a live wire
inside something shorts to the case, and the case is grounded, then
someone standing on the floor (also grounded) won't get a shock. The
goal of the electrical safety ground (green wire) is two fold: a)
keep cases of things at "ground potential" and b)provide a low enough
impedance path so that the overcurrent protection (fuse or circuit
breaker) pops. (notable failure of the latter: MGM Grand hotel fire)
At the breaker panel, one of the AC service conductors (the "neutral"
) is connected to ground, to insure that the entire system stays
within a reasonable voltage of "ground" (so that insulation breakdown
doesn't happen, for one thing). The neutral is called the "grounded
conductor" in code-lingo (as opposed to the green wire, which is
"grounding conductor"). In the U.S., in 3 phase systems, for
Wye/Star, the center point is the Neutral/Grounded conductor. For
delta connected, often the centertap of one phase is grounded (just
like single phase 120/240 services).
None of these are designed or intended for decent RF grounding.
The RF choke between RF ground and safety ground is probably fine
for the fundamental RF (say 100KHz), however if a secondary strike
hits an object that is earth grounded, the RF choke may do little to
keep the return current out of the house wiring.
I would think that in a "RF isolated" system (such as contemplated
here) you'd really, really want to avoid strikes to things NOT
connected to the RF ground; i.e. you want strikes ONLY to the
counterpoise, and if you have a "grounded" electrode to draw strikes,
you want it connected to the counterpoise, not "earth ground".
Can't do anything about transients induced in surrounding circuits, of course.
The return current will probably travel up the green wire in
parallel with the RF grounding rod to some extent. At the very
high frequencies associated with an impulse current, the RF choke
probably looks like a capacitor.