RE: [Premium-Rx] Re:_gas_discharge,_lightening,_etcos_and_High_RF_powers_(Andreé_Knott)

[George Georgevits]

Richard,

With respect, I have had a good look at this site. It is certainly
outstanding in terms of how much money it would have cost. However, I have a
couple of technical reservations about the design as presented, and for
anyone contemplating installing an earth system, it is as well to be aware
of these.

Coming from down under, we take into consideration the ground resistivity
when implementing an earthing system design. We have to, because in many
places ground resistivity is very high due to our climate and the
composition of the ground.

The point I am trying to make here is that even a heap of 8' long ground
rods, whilst looking impressive, may not do a lot for you if the ground
resistivity is high.

Lets take a look at some examples.

A good estimate can be obtained for the resistance to earth of a thin rod
driven into the ground by using the following relationship:

Resistance to earth = (soil resistivity)/(length of rod)

needless to say, the soil resistivity and rod length must use the same
units.

Thus for 100 ohm-metre soil (good low resistivity ground) and a 2.5 metre
(8') long rod, the resistance of a single stake comes to 100/2.5 = 40 ohms.

HOWEVER, if the soil resistivity is 1,000 ohm metres (relatively high
resistivity ground), the same stake presents a resistance to earth of 400
ohms! Now I have seen places where the soil resistivity can get as high as
10,000 ohm metres (eg. dry, sandy soil).

To calculate the effect of a number of earth rods in parallel (provided they
are well spaced apart), use ohms law for resistors in parallel.

So, if his soil resistivity is 100 ohm-metres, 18 rods will yield, say, 3
ohms, allowing for some interconnection cable and joint resistances. This is
fine for a lightning protection system.

HOWEVER, if his soil resistivity is 1,000 ohm-metres (commonly found in dry
areas), the same 18 rods yield 22 ohms. You may think this is low, but it
exceeds the 10 ohms maximum permitted by Australian standards (not sure
about US requirements, but I am sure that lightning is the same in the US as
it is here in Oz).

To put this into better perspective, the average lightning stroke has a peak
current of around 20,000 amps, with strokes of 200,000 amps occurring
infrequently. Now if we use ohms law again, the average lightning stroke
will cause a potential rise of some 60,000 volts when it hits a 3 ohm
earthing system, and 440,000 volts for the 22 ohm earth system.

I leave you to draw your own conclusions about this, and also what happens
if you happen to cop a big stroke.

Also, you can see why it is not a good idea to be using a telephone when
your house is hit by lightning - the telephone stays at 0V (because it is
fed from the exchange, a long way away) whilst you and everything else in
the immediate vicinity rises to the 60kV or whatever it happens to be.

One further point worth noting is that it is essential to measure the
resistance to ground of an earthing system, both when the installation is
completed and also after an extended period of dry weather. The former to
confirm you have something in place which works, and the latter to make sure
it is still effective in the most adverse conditions. After this, an annual
check is wise, just to make sure that corrosion is not eating away at the
joints etc.

Finally, for what it is worth, in high soil resistivity areas, burying some
galvanised mesh strips a few feet deep can get you a very long way towards
achieving a low resistance earth, whereas driving in a heap of rods may not.


Regards,
George Georgevits
Power and Digital Instruments Pty Ltd


-----Original Message-----
From: premium-rx-bounces at ml.skirrow.org
[mailto:premium-rx-bounces at ml.skirrow.org]On Behalf Of Richard M. MC
Clung
Sent: Wednesday, 4 June 2003 12:13 PM
To: Premium-RX LIST
Subject: Re: [Premium-Rx]
Re:_gas_discharge,_lightening,_etcos_and_High_RF_powers_(Andreé_Knott)


Here is an outstanding protection system that would be a good "how to"
for doing your whole station.
<http://users.erols.com/n3rr/lightningprotection/>
RICH  WA6KNW


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