Measuring Ohms Resistance

On Thu, 09 Oct 2003 10:54:39 GMT, indago Gave us:
With the relatively short distance between a dog's paws, I hardly think he'd notice any difference at all. Especially as the ground became more and more conductive.
Reply to
DarkMatter
Loading thread data ...
First of all, the technique you use for measuring the ground electrode resistance is critical. This is because what you are measuring is the effective resistance between the electrode under test and a theoretical zero resistance earth reference at some distance from the electrode. Reference electrode(s) installed at some distance (usually more than 100 feet, depending on local regulations )from the one under test are required.
Even if you position the reference electrode properly, an ohm meter may give an erroneous reading because it uses too low a test current. Part of the path under test is the contact between the ground electrode and the soil. The interaction at this interface is rather complex, involving some electrochemistry and as a result, may exhibit non-linear behavior with respect to the current flow. A proper measurement method should specify a minimum test current as well as the test electrode setup.
Reply to
Paul Hovnanian P.E.
BEM=electrician? Never heard of it. Why would an engineer have tools? An IBEW wireman would never carry anothers tools. But back too the point, is the only method of increasing a ground fields conductivity achieved by pissing on it? Come on guys someone knows the answer. I'll post the answer tomorrow if no one does. Remove RATSandSCABS to reply Impeach George Bush.
Reply to
Mr. Sandman
---------snip--------------->
--------- An ohmmeter of the normal type (i.e. a Fluke) is absolutely useless for measuring grounding resistance. There is a strong voltage related non-linearity as the low voltage of an ohmmeter is simply not enough to break down contact resistance between the rod and the soil or between soil particles. There has to be sufficient potential applied to get an appreciable current flowing several ampers at least. There are specific techniques used to measure ground resistance - the recommended one is to pass current between a remote rod and the rod under test and measure the voltage with respect to the rod under test at points in between using a third rod as a probe. The voltage will reach a plateau somewhere and this voltage and the current may be used to determine the resistance (if no plateau then the remote rod is too close). There are also special "meggers" designed for ground resistance measurements. However with the above technique these are not needed. -- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer
Reply to
Don Kelly
-----------------skip----------------
-------------- It's not often that I agree with you but you are absolutely right regarding the higher voltage than that of a DVM. By reading voltage on a shunt resistor - are you implying use of a third rod as a probe?. If so - fine -otherwise, please clarify. -- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer
Reply to
Don Kelly
The ONLY way too perform this operations is too measure the conductivity! This is measured in siemens. The fluke multi megger xj596 ought too do you the trick. Good luck and be careful. Remove RATSandSCABS to reply Impeach George Bush.
Reply to
Mr. Sandman
Standard techniques I'm familiar with are;
On new installations - increase the square surface area of the grounding electrode material in contact with the earth, concrete encase the grounding electrodes to increase the surface area in contact with the soil, Ufer grounds in the bottom of concrete foundations, etc.
To increase conductivity of existing ground fields;
The addition of conductive, or moisture retentive minerals (bentonite is one commonly used) to the sub-soil around the grounding electrode system, grade the soil surface to provide natural pooling of surface water over the ground field, or till and garden (flowers or otherwise) in the soil above the ground field.
One of the most clever solutions to the problem of poor performing ground fields, in a very dry, rocky, climate known for problematic ground resistance, was the installation of large copper conductors buried in the soil below septic drainfield trenches before the rock was added (moisture frequently replenished :-] ).
Louis-- ******************* Remove the two fish in Address to reply
Reply to
Louis Bybee
On Fri, 10 Oct 2003 02:13:41 GMT, "Don Kelly" Gave us:
A DC power supply that is well regulated, and a DVM, and a 0.1 ohm resistor. Pass a known voltage between two separated rods with the resistor in series with the circuit. Read amperage across the resistor in milliamps per millivolt. Calculate resistance of the remaining segment of the circuit (the earth in question) with ohm's law using the amperage reading and known input voltage in the equation. Subtract for the minuscule resistor drop if you really think it is important. Don't forget to detach the system lead from the main ground rod, if possible.
Reply to
DarkMatter
Booger Eatin' Moron You've heard of it. don't lie.
Reply to
Johan Lexington
I'll add to that. Using DC current can give inaccurate readings due to the chemical interaction that you referred to. Also, there are stray AC currents in the test path. Earth resistance testers use AC current at a non-harmonic frequency and filter out the unwanted currents.
The measurements are made with a kelvin setup, and proper placement of the potential electrode is critical. A two-wire measurement with any type of instrument is just not going to give the correct result.
Ben Miller
Reply to
Ben Miller
All perfectly reasonable, except a 0.1 ohm resistor wouldn't result in direct correlation between mA and mV. One mV would imply 10mA. Or perhaps you meant to use a 1.0 ohm resistor?
daestrom
Reply to
daestrom
message
--------- Thank you. You are substituting a resistor and DVM for an ammeter, nothing more. No problem there. The problem is that you are measuring the effective series resistance between the particular rods and cannot separate out the resistance to ground of either rod. What is wanted is the resistance to "true ground" for the rod under test.
One way to make this work is to use 3 well separated rods and measure the resistance between pairs of rods, then use a delta-wye transformation to determine the resistance to the "star point" which is assumed to be true ground.
Another way is to measure current in a 2 rod system and measure the voltage from the rod being tested and a third rod. move the third rod to various points in between and determine where the rate of change of voltage is 0 or near 0. (i.e. end effects at the rods are eliminated) This voltage and the current will give the resistance to "true ground" and the method is, as far as I recall, the method recommended in IEEE.
-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer
Reply to
Don Kelly
non-harmonic
Right on. In addition, the current distribution in the ground for AC (even at 60Hz) is generally different from that for DC.
-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer
Reply to
Don Kelly
For a description of a related chemical/electrical effect I once encountered go to
formatting link
then go to the 6th paragraph (or search for the word "resistivity").
Reply to
Guy Macon
Is that right? Could you please tell me how to measure antenna impedance with an ohm meter? Reactance?
If you think that you can measure ground conductivity with a regular ohm meter, you are far more deluded than I thought.
Why don't you offer up some kind of proof of this? Given your 40 years in farming, pyro, physics, electrical engineering, and computers, when did you have time? Hope I didn't leave out any of your "careers". Oops, that's right, you're a hydrogen fusion expert too. Sorry, I almost forgot about that.
Well they did Harry, now what have you to say? Oh that's right, nothing as usual. Hit and run Harry strikes again. BTW, are you still telling people to shut their computers down with the power switch, instead of that proper shutdown "nonsense" that everyone else in the industry recommends?
Reply to
Anthony Fremont
On Sat, 11 Oct 2003 04:50:58 GMT, "Don Kelly" Gave us:
Yes, it allows one to determine any gradation in conductivity in the top 8 feet of a large 3-d space. A wad of earth. hehe
You are correct. 100% Ufer slabs read pretty low back east.
Out here, in the dessert, pretty dry base.
Reply to
DarkMatter
Evidently you missed the fact that the question concerned ground resistance, not impedance.
I'll assume that you grasp the distinction, between resistance, impedance, and rectance. If not, I'll be happy to explain it to you.
Anthony, you can easily demonstrate that the ground resistance is below some limiting value with an ohm meter, since electrode potential and other electrolytic effects will act to drive the ohm measurement to a higher than actual value. Also, since most VOMs have current measuring capability, galvanic action, if present, is easily detected.
How. Want me to describe the installion and certification used in the installation of ground systems for the a.m. radio station vertical towers that I have installed? (Hint: These typically employ 120 quarter wave radials made of bare #12 or #10 wire radiating outward over 360 degrees from the base of the tower. You certify their ground resistance using an ohm meter, follow by masurment of their ground impedance by typically employing a General Radio Model 916A r.f. impedance bridge excited at the design frequency.)
Since you probably are unfamiliar with things like this, you may want to read Edmund A. Laport's book: "Radio Antenna Engineering" published some years ago by McGraw-Hill. It cover the constuction and certification of grounding systems in detail.
Your no doubt equally unfamiliar with the class r.f. bridges. This citation may help you familiarize yourself with these instruments, and notably the General Radio 916A which for many year was a standard of the industry before the "no operator skill required" computer based bridges hit the market.
formatting link
Well, even some of us poor, dumb, New Jersey farm kids are pretty quick reads and learn fast. I earned by FCC First Class Radio Operators license while in highschool (my ham ticket when I was 12 or 13), and had earned a B.S. in physics and an M.S. in electrical engineering by 24. I worked as the chief engineer of WBUD (Trenton, NJ), and as a staff engineer at WCAU and WFIL in Philadelphia which paid for my professonal education.
What I had no time for was drinking, parties, sports, and chasing women.
You missed the fact the from 1972 to 1980 I worked as manager of system engineering at General Railway Signal Company in Rochester, NY where I managed the development and installation of the control systems for both the Washington Metro and Atlanta transit systems (WMATA and MARTA) while raising 3 children, but still managed to shoot professional fireworks displays for relaxation and recreation.
Hardly an expert, but I did manage to earn a few bucks (and learn a few things) as a COOP student working on the constructon of both the Particle Accelerator and Stellerator project at Princeton's Forrestal Research Center.
Not much, other than the fact is simply that likely less than 0.01% of the existing ground systems have had such certification, nor in most venues are they required to. The responders made some very good points, but for the most part largely academic. Had you some actual experience, you should realize that most ground systems are just one or more copper rods coupled togeter and driven in the ground to the point where they strike ledge and can't be driven any further. Sometimes a superficial test will be conducted, but more often than not the electrician or lineman driving the ground will declare that based on his experience, "that's good enough for government work." This is reality!
Anthony, now that you have shot your wad and I have responded, how about telling newsgroup readers a bit about yourself, including both your credentials, years of experience, and accomplishments that you can at least take partial credit for?
My guess is that from the content of your posts, you earn your living as a 3rd shift computer operator at some networking firm or ISP, you lack a professional education and have no degrees in engineering or science, and more than likely have only a limited level of expertise in either programming or the design of computers (or for that matter anything electronic), and you probably consider yourself to be some sort of an expert hacker. On top of that, you have a serious psychological problem with anyone who has achieved or accomplished more than yourself, and have a difficult time dealing with it.
Prove me wrong. Tell the group about yourself.
Harry C.
Reply to
Harry Conover
message
--------- It does quite a bit better than this as the voltage probe is carrying negligable current The process looks for the "0" equipotential surface just as one could probe the field between two charges ----------------- . >
------- No more of a problem than in parts of Quebec where the lakes are more conductive than the granite which is barely below the surface. Hence a power station ground was made of a grid which was tossed into the forebay and T-lines had continuous counterpoises as well as overhead ground wires. -- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer -
Reply to
Don Kelly
Not at all. I was referring to your open ended statement about "25 Ohms is 25 Ohms on any resistance measuring unit".
Perhaps not so well as thou, but I've tinkered.
I would accept that you could measure it with a large enough voltage source and a current indicator, but not with "any" or even most VOM's.
If you're talking about a full fledged AM broadcast antenna, that wire probably wouldn't last a year in the soil. You must be referring to amateur radio type antennas for HF.
formatting link
I've tuned a "few" antennas in my life Harry.
What a life.
I supose that's true if you include all residential ground rods, and I do mean all.
This is precisely the kind of thing you said to me the first time we conversed. I suspect that's why I hold you in such high esteem.
Never said that I did. But then I wasn't telling the OP how to incorrectly check his grounding either. I find that if you don't know the NEC inside out, it's probably not safe to be guessing about things like power and grounding around here.
Oh I have a couple years worth of experience with computers and electronics. ;-) Epert hacker though, that's pretty good Harry. I've hacked a few things I guess, you must have done your homework before responding.
Oh you've definitely got me pegged Harry, I didn't know that you were a physchologist as well.
I doubt the group cares to hear me brag, it seems that few people here do that. Besides, I could hardly put on a performance like you. You already know my credentials as we've been thru this before when you were bantering about something that you effectively know nothing of any significant detail about. Let's see you pull something out of Google demonstrating me espousing a false set of credentials.
Reply to
Anthony Fremont
On 11 Oct 2003 14:02:58 -0700, snipped-for-privacy@yahoo.com (Harry Conover) Gave us:
"drive"??? Hahahhaahah... A higher value resistor passes LESS.
The simple fact is that it is not a straight line on the graph and the current characteristics for the ground at the test potential a DVM makes is not going to match what a 60 volt loop or 120 volt loop would exhibit under observation. Ergo; you would get a non-calibrated errant reading!
Reply to
DarkMatter

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.