Jim Lux wrote:
At 06:31 PM 12/6/2004 -0500, Gary Schafer wrote:
But is a good RF radial grounding system really "one of the best
lightning grounds you can get". A raft of small wires might well be
a worse lightning ground than a few nice big wires or rods. The
smaller wires may fuse with the lightning current. Say you get a
20kA strike and you've put in 60 wires. That's 300+ amps into each
wire (if the current divides equally, which it probably won't). 300
Amps is a ballpark fusing current for AWG10 wire in air (admittedly,
that doesn't take into account the short duration of the lightning
However it would not hurt to install a few ground rods connected to
the radials in addition.
I'll say that this is true. For just the reason described above.
What's standard practice in the broadcast industry?
#6 wire is often recommended to carry the current to ground.
#10 will carry 2.5 times less.
#18 will carry 6.4 times less than #10.
#18 will carry 16 times less than #6.
Would this be a recommendation from the broadcast industry? The code
(NEC-2002 at least) requires a somewhat larger conductor as a buried
ground (i.e AWG4, 20 fet long), The low voltage part of NEC requires
AWG10 (copper) for the "grounding conductor" (i.e. the wire from
whatever to the "grounding electrode")
I don't think that they mention multiple radials. If you are using only
one conductor as a buried conductor then that conductor is being asked
to carry the full current.
Don't get hung up on NEC codes. They have blanket codes that try to
cover many situations. Often they are way overkill. Sometimes they fall
So if you were to use #18 for your radials, 16 of them would have the
current carrying capacity of a single #6 wire.
Yes the current does divide pretty much equally in the radials.
Only if the RLC circuits going to the radials from the "lighting point"
happen to be identical. I've seen a variety of ground radial systems on
ham antennas, and the lengths of the wire from the "bonding point" to
the point of entry to the soil vary quite a bit (for instance, there's
those nifty square plates with all the attachment points). Using the old
1 uH/meter for a single conductor approximation, and considering the
lightning di/dt as 10 kA/microsecond (20 kA avg stroke, 2/50 waveform),
I get a voltage difference of around 10kV/meter. If the lengths of the
wires vary, say, 6", you're looking at more than a kilovolt difference
in voltage drop along those little wires. Without getting into gory
details of mutual inductance, stray capacitance, etc, I think it's safe
to say that any assumption of equal current distribution is
unrealistic. Off the cuff, I'd probably go for a factor of 10 between
highest and lowest, though. So, instead of 300 some amps, you might see
1000-3000 amps in the highest current conductors (the ones with the
shortest, most direct connection to the soil).
Current is what we are concerned with. Of course all will not be exactly
the same. But the whole idea here is current sharing.
It is even better with radials than just parallel wires as the radials
afford more dissipation to ground being spread out. The ground does
not get a chance to saturate as it can with only one or a few ground
Soil is a resistor. It does not "saturate". There is a recommendation
that the maximum current density per electrode be limited to avoid
"smoking rods". For 8 ft rods, it works out to a maximum current of
around 500-1000 Amps in typical soils. Lightning is short duration
compared to other grounding requirements, and there's a square root of
time factor in the recommendation which might result in a factor of
100-1000 increase in a lightning kind of application.
Oh the ground is much more than just a resistor. It has capacitance and
inductance as well. It also has propagation delays.
I have never heard of a ground rod being smoked but it is common for the
soil around a rod to turn to glass during a large strike because of
arcing in the ground.
I don't know where the 500-1000 amps per rod comes from. What do you
think happens to the current in a 10ka strike if there is only 1 rod?
Yes the ground does saturate around a ground rod! A given area of earth
around a ground rod can only dissipate so much energy in a given amount
of time. That is one of the reasons for rods being spaced twice there
lengths. The effective area of a ground rod is a diameter and depth
approximately equal to its length.
A fact is that "a good lightning ground makes a good rf ground", "but
a good rf ground does not always make a good lightning ground". (as in
We know that just ground rods do not make a good rf ground in most
cases. They don't make a good lightning ground either.
Good lightning grounds do NOT necessarily make a good RF ground. The
requirements are totally different. No practical RF ground is going to
be asked to carry a current of kiloamps. A lightning ground might have
a DC (low frequency AC) resistance (defined in kind of a funny way, I
grant you) of 10 ohms (NEC allows 25 ohms), and be perfectly good for
lightning protection where the goal is to conduct the stroke current
somewhere "safe" (i.e. it doesn't result in high induced voltages or
flashovers to neighboring conductors).
The requirements are not totally different.
If a lightning ground is not a good rf ground then it is not a good
lightning ground! A lightning ground may be a marginal one that
satisfies the "code" but that doesn't necessarily mean that it is a good
Lightning can not be thought of as just a DC current with a little AC
component in it. It must be treated as DC and rf. The rf portion of it
is very substantial at 1mhz and extends up into the vhf region.
So if your lightning ground doesn't work very well as an rf ground on
160 then it is not a real good lightning ground.
Lightning propagates just like rf. It takes time to dump all the
energy. If you try to do it all at one point the ground saturates and
the voltage will rise high. With a radial system it allows the energy
to dissipate as it travels. Radials are lossy transmission lines.
Soil does not saturate. The voltage rises because of resistance and the
stroke current and/or inductance and stroke current rate of change
(di/dt). The overall system is basically a big RLC... C in the
cloud/earth, R in the stroke itself and your grounding system, L
likewise. Your contribution to the system is basically the bottom tiny
part of a giant voltage divider. The lower the impedance, the lower the
voltage. Changing your impedance (either R or L) isn't going to change
the time waveform of the stroke a bit.
Lightning is modeled as a current source. A certain amount of current is
available in a particular strike. It does not matter what the resistance
of the path is, The stroke is still going to develop that strike current
amount in the path.
By the way the cloud/earth path is not a capacitive one. It is a plasma
path that actually has negative resistance. Thus the current source.
Yes the ground system has R and L. But it is also a transmission line.
It has time delay. That is why Tom has large differences in potential
between his ground systems.
If you are uncomfortable with using only the buried radials for a
lightning ground then attach some ground rods also.
Unless those radials are a lot bigger than the usual ham radial wires,
you'd better put in that rod, because otherwise it won't meet code.
Regardless of whether the code defines an "adequate" lightning
The "code" doesn't know ground radials from rain spouts.
In a common lightning ground system installation it is recommended
that ground rods be placed around the tower and separate radials run
out to each ground rod from the tower. Additional ground rods would be
installed at approximately the distance of twice their length on each
radial to the same wire.
Is the recommendation a "generally accepted industry practice" or an
actual recommendation from a standards body (like NFPA or IEEE or
EIA/TIA??) I am curious if there is an actual published standard (I've
been looking for one, but haven't found it, but that doesn't mean that
it's not out there).
A generally accepted industry practice. At least it is becoming so.
The recommendation that ground rods be spaced at least twice their
length IS embodied in several standards (IEEE 142, for instance) and is
based on both analytical models and field measurements (closer spacings
don't provide as much reduction in ground resistance).
Closer spacing allows for ground saturation and the second rod is of
You want as many connections and directions from the tower that are
practical. I.e. a radial system.
The recommendations that I've seen talk about a ring around the base of
the tower and several (not 60) ground rods. Perhaps half a dozen. And
the rods spaced twice their length apart.
A "ring" connecting ground wires is a waste of wire. Think about what
happens during a strike. The energy travels out away from the tower in a
straight line. It does not make bends at the ring to go over to another
ground wire or rod. All ground leads leaving the tower are at more or
less the same potential as the stroke propagates. So the points that the
ring is attached to are at the same potential already whether the ring
is there or not.
There's also a requirement that "every down conductor must be connected,
at its base, to an earthing or grounding electrode. This electrode needs
to be not less than 2ft from the base of the building" (p118, IEEE
Are we grounding buildings here or towers. Building down conductors are
also required to be bonded to anything near by on their way down. Pipes,
staircases railings etc.
A ground rod is really a radial in itself. It runs down rather than
parallel to the earth.
Am broadcast stations depend on the radial system for lightning
grounds. In some cases where soil conditions are poor it has been
found that adding ground rods at distances along some of the radials
If you were to use ground rods along some of the radials you would
want to use heavier wire for those radials rather than #18 or so. But
you don't need to go the #6 if you have a large number of radials as
the current is going to be divided in all runs.
Also you mentioned before about corroding buried copper.
Most ground rods are copper coated to prevent the steel rod
inside form corroding. Copper oxide is a good conductor.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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