On Thu, 19 Jan 2006 22:19:04 -0800, Richard Clark
wrote:


I found a much more compelling report in:
UNITED STATES DEPARTMENT OF AGRICULTURE
Rural Electrification Administration
REA BULLETIN 1751F-802
SUBJECT: Electrical Protection Grounding Fundamentals
Which is vastly more comprehensive and directly answers these
questions when viewed in the terms of the resistivity of the earth
connection.

Thanks Richard, I have read the document quickly overall, and a bit
more detailed in some key areas.

It covers similar material to documents I collected when studying
power earthing and lighting protections in years past, but it is a bit
more comprehensive... so a good read and a good reference document.

One good pickup was the functions for predicting the low frequency
resistance of shallow buried radials (which is relevant when radial
wires are required to provide a level of power / lightning protection.
I created a graph to add to an existing web page from the functions
for 3mm (bare) wires buried 0.1m, the graph is at
http://www.vk1od.net/post/earth02a.gif . (For avoidance of doubt, this
graph does not predicting the RF characteristics of the radials.)

Tks...

Owen

PS apologies for the delay in responding, I have been up to the big
smoke (the city... Sydney) over the weekend... reminds me of why I
left there thirty something years ago.
--

"Owen Duffy" wrote
One good pickup was the functions for predicting the low frequency
resistance of shallow buried radials (which is relevant when radial
wires are required to provide a level of power / lightning protection.
__________________

For those using buried radials as one terminal of a vertical monopole...

The only path consisting of physical conductors that can exist between a
series-fed vertical monopole and buried, uninsulated radials is through the
PA output, and the antenna matching network at the tower base. This is not
adequate to control/prevent system damage from lightning.

Three added techniques are used in most MW broadcast applications:

1. A "static drain choke" is installed between the tower base and the
junction of the radials.

2. An arc gap is installed across the base insulator and set to flash over
at some margin above normal peak voltage

3. The tx contains r-f phase sensors that kill tx output for a few
milliseconds after an arc is sensed, so as not to sustain it.

RF

"Owen Duffy" wrote
One good pickup was the functions for predicting the low frequency
resistance of shallow buried radials (which is relevant when radial
wires are required to provide a level of power / lightning
protection.
I created a graph to add to an existing web page from the functions
for 3mm (bare) wires buried 0.1m, the graph is at
http://www.vk1od.net/post/earth02a.gif . (For avoidance of doubt,
this
graph does not predict the RF characteristics of the radials.)
=========================================

Owen, I assume the curves in your graphs have been obtained by
treating the conductors as transmission lines. As far as I am aware
there's no other way of doing it. Except perhaps EZNEC
number-crunching mathematical modelling methods.

At VLF the inductance of the conductors and the capacitance due to
relatively high permittivity of the dielectic material (soil) can be
neglected.

This leaves only conductor resistance and conductance (or resistivity)
of the soil. It is then quite simple for single wires.

To predict performance at RF it is necessary to take inductance and
capacitance into account. What is unknown is the way in which soil
permittivity and resistivity change with frequency. But this hardly
matters as the uncertainty at 60 Hz is sufficient to swamp it.

I won't ask you what you did about calculating the effects of multiple
radial wires, and the interaction between individual wires, which
causes "The Law of Diminishing Returns" to be followed.

There is sufficient information in your graph to demonstrate that
Magician Marzipan's magic high number of 120 is never necessary for
amateur purposes.
----
Reg.

On Mon, 23 Jan 2006 15:55:52 +0000 (UTC), "Reg Edwards"
wrote:


"Owen Duffy" wrote
One good pickup was the functions for predicting the low frequency
resistance of shallow buried radials (which is relevant when radial
wires are required to provide a level of power / lightning
protection.
I created a graph to add to an existing web page from the functions
for 3mm (bare) wires buried 0.1m, the graph is at
http://www.vk1od.net/post/earth02a.gif . (For avoidance of doubt,
this
graph does not predict the RF characteristics of the radials.)
=========================================

Owen, I assume the curves in your graphs have been obtained by
treating the conductors as transmission lines. As far as I am aware
there's no other way of doing it. Except perhaps EZNEC
number-crunching mathematical modelling methods.

Let me quote again:
(For avoidance of doubt, this
graph does not predict the RF characteristics of the radials.)

The graph uses the functions in the paper identified by Richard (
http://www.usda.gov/rus/telecom/publ...s/1751f802.pdf
). Looking at the functions, I think they just calculates the DC /
low frequency resistance of the electrodes immersed in the soil which
is a high resistivity medium, by modelling the geometry of the
equipotential "layers" around the electrode as is done with a single
straight earth electrode. The functions for 6+ radials (or all of
them) may just be a fit to experimental data.


At VLF the inductance of the conductors and the capacitance due to
relatively high permittivity of the dielectic material (soil) can be
neglected.

I think these functions are for the resistance at power frequencies
(ELF?) and are not applicable to RF. Nevertheless, most lightning
protection texts seem to deal with the earth system as a DC resistance
with some lumped series inductance to model the above ground
connection, though clearly, lightning spikes are a double exponential
with components up to VHF depending on the way in which the network
modifies the waveshape.


This leaves only conductor resistance and conductance (or resistivity)
of the soil. It is then quite simple for single wires.

To predict performance at RF it is necessary to take inductance and
capacitance into account. What is unknown is the way in which soil
permittivity and resistivity change with frequency. But this hardly
matters as the uncertainty at 60 Hz is sufficient to swamp it.

I won't ask you what you did about calculating the effects of multiple
radial wires, and the interaction between individual wires, which
causes "The Law of Diminishing Returns" to be followed.

See above.


There is sufficient information in your graph to demonstrate that
Magician Marzipan's magic high number of 120 is never necessary for
amateur purposes.

I am guessing that the magic 120 was from BLE's paper, and it was
talking about performance at 1MHz or so, so it is RF performance that
is being considered.

The graphs I produced certainly suggest that at DC / 50Hz / 60Hz, that
there is insignificant benefit in installing more than 6 or 8 radial
wires. The reasons will be the same as why installing two vertical
electrodes close together achieves almost no improvement.

Owen
--

On Mon, 23 Jan 2006 07:29:11 GMT, Owen Duffy wrote:

I created a graph to add to an existing web page from the functions
for 3mm (bare) wires buried 0.1m

Hi Owen,

Does this discount the proximity of the radials nearest the center?
That is, the graph is not simply a summation of the individual
lengths, is it?

What would nominal be (in other words, actual) for this specific
description above?

73's
Richard Clark, KB7QHC

On Mon, 23 Jan 2006 10:25:26 -0800, Richard Clark
wrote:

On Mon, 23 Jan 2006 07:29:11 GMT, Owen Duffy wrote:

I created a graph to add to an existing web page from the functions
for 3mm (bare) wires buried 0.1m

Hi Owen,

Does this discount the proximity of the radials nearest the center?
That is, the graph is not simply a summation of the individual
lengths, is it?

Richard,

See my response to Reg.

The functions are from the reference paper you identified. I don't
recall that they explained the derivation of the functions, and they
may even be fits to experimental data. They do not appear to do
something as crude as summing the individual lengths.


What would nominal be (in other words, actual) for this specific
description above?

Did you mean "normalised"? You need to multiply the %/m value from the
Y axis by the actual soil resistivity in ohm-metres to get the
resistance of the electrode. For example, if you look the chart up for
3 radials of 5m length, you get 15%, which is multiplied by soil
resistivity (say 50 ohm-metres at a location) to get expected
electrode system "DC/AC" resistance of 7.5 ohms. (The graph is part of
a larger article which explains my "normalisation".)

Interestingly, I note the ref doc recommends galvanised electrodes. I
have been conducting an experiment here where I have recorded the
resistance of several driven earth electrodes over some years, and a
galvanised electrode of 25mm OD performs much worse than a copper clad
electrode of 13mm OD driven just 300mm away from it (both 2.4m long).
(The galvanised electrode is not electrically bonded to the earth
system for reasons of galvanic corrosion).

Owen
--

On Mon, 23 Jan 2006 20:54:19 GMT, Owen Duffy wrote:

Interestingly, I note the ref doc recommends galvanised electrodes. I
have been conducting an experiment here where I have recorded the
resistance of several driven earth electrodes over some years, and a
galvanised electrode of 25mm OD performs much worse than a copper clad
electrode of 13mm OD driven just 300mm away from it (both 2.4m long).
(The galvanised electrode is not electrically bonded to the earth
system for reasons of galvanic corrosion).


Hi Owen,

What method did you use to measure the resistance?

73's
Richard Clark, KB7QHC

On Mon, 23 Jan 2006 13:42:41 -0800, Richard Clark
wrote:

On Mon, 23 Jan 2006 20:54:19 GMT, Owen Duffy wrote:

Interestingly, I note the ref doc recommends galvanised electrodes. I
have been conducting an experiment here where I have recorded the
resistance of several driven earth electrodes over some years, and a
galvanised electrode of 25mm OD performs much worse than a copper clad
electrode of 13mm OD driven just 300mm away from it (both 2.4m long).
(The galvanised electrode is not electrically bonded to the earth
system for reasons of galvanic corrosion).


Hi Owen,

What method did you use to measure the resistance?

I used a Kyoritsu instrument designed for the purpose. It uses the
three wire fall of potential method, and makes its measurements using
an AC waveform of about 800Hz.

Owen
--