[OZAPRS] rf earth

[Ray Wells]

Paul, et al,

Buried conductor, preferably copper strap with a large surface area, in 
a shallow trench is a very effective earthing medium.

I think surface area is a more appropriate term than skin effect, which 
refers to (RF) current flowing only in the outer "skin" of a conductor. 
Would you agree, Paul?

I worked for a county council for over twenty years and quite a bit of 
that time was in the "testing laboratory". Whenever we wanted to test 
zone substation (132kv, 66kv or 33kv in and 11kv out) earthing systems 
we had Telstra/Telecom/PMG (yes, I'm that old) provide a remote earth as 
our reference. That's hardly a practical method for AR operators, however.

To test a buried radial earth mat (44 radial wires connected to a 
central hub) at my previous location I used the electricity supply MEN* 
point earth as the reference. To that point I connected one side of an 
isolating 110v transformer. The other side connected to my earth mat. I 
measured the current flow and the (110v) supply voltage to calculate the 
impedance of my earth map.

I absolutely concur with Paul's safety reminder. I'm a licenced 
electrician, licenced electrical contractor, worked in the zaps section 
of a county council for much of twenty years and (at the time) had 30+ 
years in the electrical trade. If you don't know what an MEN* earthing 
system is you are not qualified to be using the mains earth as a 
reference point.

*MEN - Multiple Earthed Neutral system of earthing where the neutral at 
the supply transformer, and at every switchboard, is earthed (to a 
stakes, or stakes, and the metallic water pipe system). The effect is to 
provide a low impedance earthing system. In suburbia, with lots of 
houses (and MEN points), it usually works very well. In rural areas, 
such as where I live, where there are only four houses fed off the 
transformer, and the ground is not particularly conductive, it's not so 
effective although it is still the safest system in the world. Even 
within an installation you can find multiple MEN points. E.g., my  
remote garage is supplied by a sub-main from the main switchboard on the 
house. The switchboard in the garage has another MEN point where the 
neutral connects (effectively) to an earth stake.

As Paul said, connections can be broken. When doing voltage 
investigations (responding to customer complaints about supply quality) 
I went to one house with low voltage issues. As I pulled up I noticed 
the Neutral at the point of connection to the house was firmly connected 
to the surrounding air and the house was relying on the MEN earth. The 
customer, one very lucky family, admitted to getting "tingles" when in 
the bath. They were lucky to not be killed. It's interesting that they 
were concerned about the TV picture shrinking (common problem with valve 
TVs and low mains voltage) but not the tingles in the bath.

I'm not sure that I agree with Paul's suggestion to keep the RF and 
mains earth separate. A bond between the mains earth stake (NOT THE MEN 
POINT IN THE SWITCHBOARD) is something I would recommend. That 
arrangement is providing just an equi-potential bond, without 
interfering with the mains installation. I'm a strong believer in 
bonding everything together to minimise voltage differences, especially 
for lightning protection. It's interesting to note that even 
Tesltra/Telecom/PMG used to require a separate earth (using violet 
coloured wire) for telephone system earthing at (larger) customers' 
premises. Even they now subscribe to the "bond everything" philosophy.

An interesting topic of conversation.

Ray vk2tv

Paul Mullins wrote:
> That should work ONLY if the steel sections are in cased in ground, its all
> about surface contact area, skin effect is the word.
> This is the reason why I buried bare copper wire in the ground between
> stakes.
> Also burring wire in a garden is great moist ground is going to work heaps
> better than dry , I know bashing the bible again.
>
>
>
>
> To test the earthing system what we did in 'ETSA' the local power supplier.
>
> You need about 10 meters of hookup wire, a low ohms meter ie has to read 2
> ohms or better, with the hookup wire connected as a loop to the meter zero
> the meter.
> Connect one end of the hookup wire to your earthing system, "make sure there
> is NOT MAINS VOLTAGE PRESENT" believe it or not some stobbie poles are alive
> remember safety first people. I DON'T WANT TO HEAR OF PEOPLE GETTING HURT, I
> mean this.
>
> So one end is connected to the 'earth system' to be tested, now run the long
> wire out in a direction that there is NO EARTHING SYSTEM, drive a long steel
> stake into the ground, we used a stainless steel stake or screw driver,
> needs to go about 1 foot (30 cm) then measure the earth resistance, good
> earthing should be below 1 ohm, if you feel up to it and you check your
> mains earth system , extreme caution, as a sparky I have found broken earth
> bonds and all sorts of potential life shorting bad connections.
>
> My house when I bought it was getting boots from the hot water system in the
> shower, now my hot water is gas right. Should not get any thing, I measured
> with an ANALOGUE meter digital ones don't work for this part that I had 12
> volts AC. This is enough to make a lot of hertz, a sparky joke.
>
> I found my house main earth to be substandard, fixed it with a new copper
> stake just below the meter box, the original one was connected to the mains
> water, I re did the complete earthing system, new stake, bonds from the
> earth bar to the cold water and the hot water, a long time ago the gas
> supply pipe to people was steel and this use to get bonded, mines steel and
> it now got a wire, people with plastic incoming gas mains could bond the
> steel part IE the consumers side as that is normally steel / copper.
>
> NOW ANOTHER WORD OF WARNING.
>
> Only licensed people should play around with 240 volts mains, this is what
> the law states, if you are competed I think that's the right word, you could
> make a great earthing system, ALSO you need to keep the two system apart
> easy said. Also beware of potential voltage differences on the two earthing
> systems, As I said my RF earth is better than my mains earth, so be aware .
>
> Old motto from school days.
>
>
> TEST BEFORE TOUCH.
>
> If you keep a level head and plan out you will live.  
>
> Any way I got my booting from 12 volts AC down to 50mV I can live with that
>
>
>
>
> I am sorry about the long winded way of explaining, but I have to make sure
> people are going to do it safely.
> Remember that RF is a power source so make sure no radios are TXing, RF
> burns are worse than power burns trust me, they take months to heal up,
> nasty stuff.
>
>
> STAY ALIVE
>
> Paul
>
>
>
>
> -----Original Message-----
> From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
> Behalf Of Rodney Mitchell
> Sent: Tuesday, 27 April 2010 1:17 AM
> To: Australian APRS Users
> Subject: Re: [OZAPRS] rf earth
>
>
>   
>> Hi All,
>>
>> I intend on using shed as an RF earth since it is all bonded together
>> and is in the ground with steel "C" section.
>>
>> Is this a good idea or is it better to use earth rods.
>>
>> thoughts or suggestions would be much appreciated
>>
>> cheers
>> Ben
>> VK5JFK
>>
>>     
>
>
> Hi Ben,
>
> What are you doing? Antenna earthing to simulate a the other half of a
> vertical 1/4 wave?
>
> Regs, Rod.
>
>