|Subject:||Re: [TowerTalk] Ground Radials Insulated or Not|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Mon, 06 Dec 2004 16:20:05 -0800|
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 impulse, etc.)
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")
So if you were to use #18 for your radials, 16 of them would have the current carrying capacity of a single #6 wire.
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).
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 rods.
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.
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 elevated radials)
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).
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.
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 protection strategy.
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).
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).
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.
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 142-1991)
A ground rod is really a radial in itself. It runs down rather than parallel to the earth.
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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