Re: [Electronics_101] Re: Textbook mistakes - literally
There used to be a process called "proof reading". They used to have a second person read the book and make sure everything is correct. But that means somebody has to pay this person and it seems they just don't proof read anything anymore. Books are now like first DRAFT - not first edition.
On Apr 28, 2013, at 9:14 AM, "Kerim F" <ahumanbeing2000@...> wrote:
> To be fair, we have to remember always that professors, as all of us here, are not perfect
- I did some reading and low frequencies (1Hz or less) are indeed used
to measure earth impedance in some cases.
So your memory did not deceive you and I learned something new again.
Might be because of the skin effect, the earth does become a rather
large cross section conductor rather quickly.
I was told after a certain (relatively short) distance the resistance
between two grounding rods does not increase significantly if you add
more distance because the cross section gets so large only the nearby
area really counts.
Not sure what good it does to measure at low frequency rather than
50Hz if you then inject 50Hz fault currents into the earth....
The 4 prongs are needed in ground resistivity measurement because two
inject the current and the other two measure voltage. Otherwise the
(rather unreliable) contact resistance would render the measurement
useless. When testing grounding rods you also need two (sometimes
three) temporary test rods, otherwise it would be impossible to
measure the ground impedance of the (hopefully much better) permanent
Modern ground impedance testers are much easier to use. They look like
a clamp-on ammeter (which they are) but there is also a coil to inject
the current into the conductor. It's all contact-less and no fiddling
around with temporary ground stakes and long wires.
On Sun, May 5, 2013 at 9:59 PM, Randy <randy@...> wrote:
> I should have explained myself more clearly, sorry.
> It seemed to be applying an AC voltage with a frequency much less
> than 1 Hz and measuring that between the ground rod and what the
> ground/neutral wire on the pole, I "assume".
> It's been dozens of years, but I do recall some sort of 4-prong affair
> in one of the publications for poking around your yard to measure
> soil resistance, or resistivity, or conductivity, or... something.
> Sorry for the confusion.
> (*disclaimer: Of course, I could be *completely* wrong about all of the
> above, except as noted with the words "appeared to", "assume", and other
> ambiguous and ill-defined things...)