On 12/30/2010 8:36 AM, Jim Hoge wrote:
> What does the NEC have to say about the number of ground rods used in a single
> point grounding system where the building is surrrounded by a ground wire?
NEC has remarkably little to say about the spacing of ground rods. It
requires a minimum of one rod, and and if the resistance to earth
exceeds 25 ohms, a second rod is required. NEC also requires that ALL
grounds in a given building be bonded together.In other words, you can
have as many grounds as you like, but they must all be bonded together.
The word "bond" in this context means a low impedance conductor that can
carry any current that may be imposed, and that is physically robust.
That means short and fat.
> Is there a required minimum and maximum spacing between the ground rods?
There is,considerable published science on the topic of the spacing of
ground rods. A good "rule of thumb" is that they should be spaced a
distance roughly equal to their length. That's because lightning is a
high frequency event, and the inductive coupling between adjacent rods
reduces their effectiveness.
> Is there a required depth the perimeter wire is to be buried?
In general, deeper is better so that the wire has a better chance of
being in contact with moist earth, and because it is less likely to be
disturbed by a gardener. Since I don't use them, I don't know what, if
anything, NEC has to say on the subject.
In all of this, remember that the primary function of a connection to
EARTH is lightning protection. The proper bonding of all such
connections, and of all wring within a building, are a critical part of
that protection system, and also provide protection from electrical
shock and fire hazard.
Connections to EARTH do NOT generally make antennas work better, nor do
they generally reduce noise or RFI. There are, of course, a few
exceptions -- Beverages needs either an earth connection or radials,
and an earth connection can be helpful as the shunt element with a
common mode choke on a telephone line.
73, Jim Brown K9YC
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