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grounds, measurement
OK, A while back there ws some discussion about 'how to measure a ground'.
What i'm going to cite is 'lightning based', but the relavant frequencies are
in fact, pretty close.
Electrical Protection Guide for Land Based Radio Facilities,
Joslyn Electronic Systems
(Found my copy in a used book store). NEVER actually gets to discussing
measurements. good biblio.
ANSI/IEEE standard for lightning arrestors & grounds, C62.1
Lighytnign Protection for Electric Systems, Bexk
Lightning Observations in Buried Cables, Bell Sys Tech Journal,
April '49
Grounding Electric Circuits Effectively, Bulletin 25t2,
James G Biddle Co.
Impulse and 60 Hz Characteristics of Driven Grounds, AIEE Transactions,
1942. [I THINK I have this....]
and on....
They do discuss some typical systems, which will be, i think, familiar:
Buried square of wire, 20 ft on a side, #2 AWG, 1.5 ft down.
Same, ground rods. about 6ft, 8 total. First config vices 80ish
ohms, second drops it to 60ish, either is ok, lower is better.
Elsewhere (I've posted this before), they discuss influence of soil tmeperature
on grounds. They cite a study giving the following (30 meters, #2AWG, 0.5meter
down (obviosuly soil type dependent)
Soil resitivity (not resistance) Temp(F)
82 60
135 33
330 31
1230 (!!) 20
(Cite is to Earth Conduction Effects in Transmission Systems, Sunde, Dover pubs
Dover reprints olde books, be nice if they still had this,,,)
Second Reference:
Lightning Arrestor Grounds
'electric service supplies', ca 1920
Another used book store, find, a little pamphlet. Discusses installation
and measuring. On install trick: for pipe, hammer one end closed to semi
chisel point...
They measure by measuring to another point, either a known good ground
(eg, extensive water pipe) or other electrodes driven for the purpose.
(I would think that using multiple rods, temporarily disconnected from a
ring bus would do. Or, for measurement purposes only, using the "power
service ground".)
They do the measurements at 60Hz, or DC. [i'll 'handwave' about this
below] They reccomend using realtively high currents, to avoid stray effects
from stray currents, and to emulate operational conditions. The preferred
method is to use an isolation (1-1) transfomer from the power line.
CAUTION: YOU CAN KILL YOURSELF THIS WAY!
Enuf Said?
Suitably fuse and instrument the secondary side. Measure E and I (both AC)
simultaneously while driving current from the ground under test to the
reference ground.
If no ground reference (eg water pipe) available:
minimum of three electrodes needed.
Denote one as "R" (the one to be measured)
Two random grounded things, at some distance (not specified.), call them
A and B.
Using technique above, measure all three resistances:
R to A r1
R to B r2
A to B r3
Rground = ((r1+r2)-r3)/2
They discuss essentially similar methods of measuring ground with bridges or
ohmeters, the technique is the same, tho the use of 10-12v or so, minimum,
to avoid odd insulating effects of low voltages. (ferinstance, i would
NOT use any common ohmeter...)
They never come right out and say what a 'good ground' is, numerically.
The EXAMPLES (measured) tend to run between 30 and 60 ohms. (this implies
an amp or more if 110vac is used.
They include a curve of resistance vs temperature that matches the figures
from the other reference, more or less.(No freezing, 78-32 ohms range.) The
relavance is that the ground WILL change if the ground freezes and will change
again if a coil starts pushing amps into it, heating it.
Speculation 1)
It might be interesting to do this WITH RF, say, coupled off the primary, down
to some manageble voltage. I some nice test equipment discussed here, the
proper shunts and voltmeters should allow characterization of 'ground' AT
OPERATING FREQUENCIES. (or do it with a signal generator. less fun....)
Speculation 2)
Somewhat ad rem the thread on ball lightning.
(I saw the Corum's presentation on this at ITS Con in '90(?)
They tried for BL from Bill Wysock's coil, i left somewhat early, i am told
they 'may' have gotten some later.)
Anyway, my point about base current, firstly WHICH base current, when?
With the coil running, but not broken out?
(As Tesla may have planned on operating)
OR with sparks flung about?
First case:
I submit that base currents will be HIGH. If the secondary is
running as a resonant 1/4 wave, it has a voltage PEAK at the top and
a current peak at the bottom. There is current multiplication at the
base even as there is a voltage multiplication at the top.
Second case:
Broken out. hmmmm I'm unclear here. Likely things change. I've
seen 'series resonant at 10-20 MHz' proposed/measured... hmmmm again.
Suppose its parallel resonant, with the loop closed by the spark?
That means spark current (large) returning by way of the ground
connection, with the secondary/electrode/arc, as the whole is a
paralle (tank) circuit with large circulating currents. Handwaving.
I could easily be wrong.
regards
dwp