More experiments on current flowing in earth

[mivey]

As for taking an earth resistance measurement, I don't necessarily have the "approved" equipemnt to do that. I did go outside today and do some more testing.

I'm not that convinced that "approved" equipment is all that necessary as they are fancy voltage and current meters. What in the world did we do before the fancy equipment was available?

You have the means to run a current (I) through the ground. Now run the current between two ground rods with a separation of at least 60X the rod depth. Drive two more rods in a line between the current rods at a separation of A cm, keeping the rods evenly spaced, and measure the voltage drop (V) in the soil. Be sure the rod separation for all of the rods is at least 20X greater than the rod depth (i.e. 4 rods => 3 spaces => 3 spaces * 20X separation = 60X separation end-to-end).

The ohms resistance (R) is V/I. The soil resistivity (r) in ohm-cm to the depth of your rods is: r = 2*Pi*A*R

 

[steve66]

.

Tons of paths everywhere and who knows where current was traveling...

That was one of my thoughts. And 10 amps seems like a lot.

The other was the voltage gradients caused across the ground. If someone was standing near one of the electrodes (or maybe even 100' away - its kind of hard to guess what a safe distance would be), and they didn't have shoes on, is it possible there could be a voltage between their feet?

Even with an isolated supply connected to two ground rods, most of the current would run between the two pipes, but some of it would take a longer path. Again, who knows where it might go.

 

[mivey]

Okay.... how about if I drive another pipe about 2 feet from one of the existing pipes, and then attach a voltmeter between the pipes. Then I will energize one of the pipes and see what voltage exists to the other pipe.

Very very interesting question, and it applies directly to the question of connecting equipment to earth to prevent electrical shock.... connecting the equipment to earth puts the earth and the equipment at the same potential(?) Some have said that it doesn't help at all. I am figuring it depends on the local conditions. If the soil/pipe resistance is very low, then the potential difference between the two should be low.

So go ahead and try the resistivity test.

Let's suppose you have a 500 ohm-cm resistivity (very good). Now use a 1 ft depth and a 10 amp current. The minimum separation is 20 ft (609.6 cm), so:
500 = 2*pi*609.6*V/10 => V = 0.1305 * 10 = 1.31 volts

add: I guess while we are at it:
Let's suppose you have a 8000 ohm-cm resistivity (high for clay). Now use a 1 ft depth and a 10 amp current. The minimum separation is 20 ft (609.6 cm), so:
8000 = 2*pi*609.6*V/10 => V = 2.0886 * 10 = 20.89 volts

 

[crossman gary]

Mivey, my first problem is going to be that my supply is grounded. I'll have to use a xfmr to isolate that so the building electrode doesn't interfere with the readings. I'll dig around and see what xfmrs I have laying around.

I may be able to do the test on Saturday (if I can find a suitable xfmr)

So if I go 2 feet deep on the pipes, then I need 4 pipes spaced 40 feet apart? And a total distance of 120 feet?

I'm also going to need a more accurate ammeter. I'll have to see if I have a digital around here that has a 10 amp capability.

 

[mivey]

Mivey, my first problem is going to be that my supply is grounded. I'll have to use a xfmr to isolate that so the building electrode doesn't interfere with the readings. I'll dig around and see what xfmrs I have laying around.

I may be able to do the test on Saturday (if I can find a suitable xfmr)

So if I go 2 feet deep on the pipes, then I need 4 pipes spaced 40 feet apart? And a total distance of 120 feet?

I'm also going to need a more accurate ammeter. I'll have to see if I have a digital around here that has a 10 amp capability.

Correctamundo on the spacing.

 

[zog]

Uhhhh.....I'm not sure if your power supply is grounded, & I'm not sure what you are using for a return, but isn't that like really, really, really dangerous to anyone who walks by??

Just put up a sign that says, dangerous step potentials, hop with feet together.

 

[mivey]

Just put up a sign that says, dangerous step potentials, hop with feet together.

If you are a barefoot giant with a 30 foot stride. :grin:

 

[gar]

090724-1942 EST

crossman:

Your experiments are useful information for you. When I have commented on putting 120 V on an isolated ground rod for this type of experiment there have been numerous responses that this is not safe. The experiment needs to be done where you have complete control of the environment around the experiment, you have some idea of the voltage gradients that may occur, and you are protected relative to these gradients.

Your DC resistance measurement with the Simpson 260 on the low ohms range only uses the 1.5 V cell in the meter. There is an 11.2 ohm resistor between the 1.5 V cell and one meter terminal. The other cell end goes to the other meter terminal. The meter with some series resistors is across the 11.2 ohm resistor as a voltmeter. When the meter leads are shorted for zero calibration full cell voltage is applied to the voltmeter and a 10,000 variable resistor is used for the zero adjustment in the series resistor string. At half scale on the meter the external resistance under test should be 11.2 ohms. This you can verify by looking at the meter scale for the 50% point on the DC range. Mine looks like it points to 12 ohms. Thus, besides the 11.2 ohm resistor Simpson must assume another 0.8 ohms internal resistance in the leads inside the meter.

Your 2 ohm reading corresponds to about (1-0.84)*1.5 = 0.24 V across the 2 ohm resistance being measured. If for any reason there is an external voltage source, dissimilar materials in an electrolyte in the 2 ohm path, then you can have an error in the resistance reading. This is because the source voltage is not 1.5 anymore.

A DC measurement of soil resistance or a conductive liquid is not a good method and can result in large errors. Try this glass of tap water test. Nearly fill the glass. Insert two pieces of bare #14 copper wire in the water on opposite sides of the glass. Mount so they do not move much. In my experiment I used the Rx100 range on the Simpson 270 (same as a 260). Connect the Simpson ohmmeter to the two electrodes. My initial reading was about 1500 ohms. This quickly changed to about 5000 ohms. Now switch the selector back and forth between + and - and the measurement characteristic will repeat. This results from gas bubbles on the electrodes or other electrolytic action. Using AC for the excitation produces a lower resistance reading. At 1 kHz on my bridge I read about 900 ohms.

When you make earth resistance measurements always do a voltmeter check between the electrodes with no excitation current. This is to look for stray currents that might cause an error in your readings.

To get more controlled results I believe you should use AC excitation. I use 60 Hz because it is convenient. The areas where I have run tests the stray currents produce less than 0.25 V. Using a 28 V source the residual stray current effect it insignificant compared to the accuracy required.

If you are really as low as 2 ohms, then at 28 V you need a transformer with 20 A capability.

Screwdriver probes with a high impedance meter are adequate for measuring voltages in the earth at the surface from stray currents.

.

 

[crossman gary]

When I have commented on putting 120 V on an isolated ground rod for this type of experiment there have been numerous responses that this is not safe. The experiment needs to be done where you have complete control of the environment around the experiment, you have some idea of the voltage gradients that may occur, and you are protected relative to these gradients.

Absolutely. The value of "the knowledge" can only be determined by each individual. The value of said knowledge must be weighed against the risks involved in obtaining it. Such is the dilema of the inquiring mind. I am willing to subject myself to personal danger for the sake of spreading "the knowledge" upon these pages for the benefit of others.:wink:

Your DC resistance measurement with the Simpson 260...

I understand what you are saying about the stray voltages and the possible effect on the Simpson reading. I'll check that if I do this experiment again. Thank you for the detailed information, I enjoyed reading it and it made me think more deeply about the situation.

When I checked the resistance from the pipe to the brass spigot, I wasn't really trying to get an accurate reading, I was just making a preliminary check so I would have a ballpark figure of what kind of current flow to expect when I did the actual current flow experiment. When I got the reading of 2 ohms, I wasn't equating this to any actual earth resistance. The only reason I even mentioned it is because it seemed pretty low.

Thanks again for the detailed info.

 

[hillbilly]

This is some pretty interesting reading, and applies to this subject.

http://tdworld.com/mag/power_western_measures_steptouch/

steve

 

[hurk27]

Gary HERE is another site which has some great info , on earth resistivity, and some resistance of different types of soil, and grounding and test methods, that might be of an interest to you and others, make sure to also click on the links at the bottom of the page.