kwired
Electron manager
- Location
- NE Nebraska
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- EC
And it doesn't need to be ultra low resistance to accomplish that.
HOW FREQUENTLY GROUNDING RESISTANCE NEEDS TO BE INSPECTED ??
- Thread starter Anwar Pasha
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electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
No one replied to this, I'll take a stab at it. The lower resistance electrode certainly did lower touch potential with a lost neutral. I believe this is relatively insignificant because It didnt drop it much, from 30 to 24 Volts, and this represents a fairly specific scenario: A lost neutral, and then someone has to be in the fairly unlikely position of standing on the ground in bare feet touching a powered grounded object.
I guess there are some possible scenarios where an elaborate grounding electrode system could help in a lightning event. I think we have to be very specific though. This is the phrase you used:
I dont see how a low resistance GES will prevent or reduce excessive voltage from entering the facility. How does it enter? What are the two points with different potential? Be specific. I dont claim to be an expert, but to have a meaningful discussion we need to be very specific about these scenarios and not just assert vague things like "grounding will limit surges". If the lightning hits or induces a surge on the power supply to the building, I dont see how grounding will mitigate that. Maybe if it hits the neutral, a lower resistance ground will help "clamp" or "divert"that voltage a little - maybe. I think bonding, surge suppression, and maybe a lighting protection system are what will do 95% of the work.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
And many of us have an issue with some of these (some of this is basically what I just stated in my previous post). I think the "line surges" one is hogwash. Only way I really see that is if the "surge" is on the neutral/grounded conductor. I am not really sure what would cause that.
"stabilize voltage to ground" is indeed true, problem is people tend to read too much into what "stabilize" means and under what conditions.
As I was watching the video, where Mr. Holt tested for touch potential, I asked myself it's a shame Mike did not preform the touch potential test when he first started his ground resistance testing. I think he would have discovered the risk of a potentially lethal deadly electrical shock hazard.
What would the touch potential have been with the first 10ft ground rod test? The ground resistance measured 265 ohms. I believe Mike measured 0.24A in his step potential test.
I also wondered, after watching the video, what the grounding electrode ground resistance, for the service, was before he started his testing. I wonder what it is for the homes of his surrounding neighbors? Who tests ground resistances for houses?
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gadfly56
Senior Member
- Location
- New Jersey
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- Professional Engineer, Fire & Life Safety
He did measure the potential at the very beginning. About 1 foot away from the rod IIRC it was about 100± with current at .42 amps. So, lean against the box and you'd have about 20VAC between you and the ground.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
I haven't seen the video mentioned here, but you should have different results from an isolated system (say an off grid system) then you will have with the interconnected MGN of the grid. That single rod on an isolated system with a resistance of say 50 ohms is 50 ohms.
The same rod also connected to the large grid network is parallel to many other electrodes. It gets more complex though because of voltage drop in the MGN if it is carrying current.
This doesn't necessarily change the resistance to earth of that individual electrode, but the fact it is in parallel to many others changes how much current/voltage gradient you will see compared to if it were more isolated.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
With regard to lightning induced current (not necessarily the strike current itself) the high frequency nature of the pulse train makes the inductance of the grounded conductor path back to earth potentially significant.
The local GES may well have a lower inductive impedance, enough to make it useful. However the NEC does not put any hard restrictions on the length and routing of the GEC.
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The local GES may well have a lower inductive impedance, enough to make it useful. However the NEC does not put any hard restrictions on the length and routing of the GEC.
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I don't think the local GES has any real value in a lightning event and it certainly has no value in terms of safety.
Not just the box.... If I understood Mike correctly the siding on the structure is steel.
As for your post, the test you described has nothing to do with the touch potential tests Mike preformed later in the video. (Time marker 20:56).
Though the test you referred to was a step potential test where mike measured, I belive, 104Vac. More than enough to deliver an electrical shock or worse. Mike even mentioned the possible danger of the test.
Go to time marker 20:56 on the video, Touch Potential.
https://www.youtube.com/watch?v=Yg6G5VUSsWA
Using one ground rod with the hair dryer set at high Mike measured 30Vac. Ground rod is 50ft deep. Ground resistance? I believe the 50' rod that mike used for the test was the one that measured 16.4 ohms.
Note, with both rods in parallel, (total ground resistance was 10.8 ohm), the touch voltage measured 20Vac. What did mike say? The lower the ground resistance the lower the touch potential, voltage.
Correct.
Using the same touch potential test experiment mike used, instead of using the hair dryer use an 8 ohm resistor in its' place? What will the touch potential, voltage, measure, if the ground resistance of the 10' ground rod is 265 ohms? At 265 ohms any guess what the entire outer metal skin of the building voltage was above ground? The amount of touch potential, voltage, would depend on if there was a heavy dew the night before.
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(NOTE: Mike may have only said the siding was made of steel as an example for running his tests, and it actually is not.)
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
The idea is to limit the voltage to ground that might be seen on any of the power wires coming into your house. That voltage might be higher than normal for any of the reasons stated in the code. Grounding does a fair, but very imperfect job of that. Anything much beyond that is in fantasyland.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
It gives the grounded conductor one more reference point, the as long as there is a ground reference somewhere else on the system ungrounded conductors will have the same voltage to ground with or without the electrode.
To bleed off surges one needs a clamping device between conductors and the electrode.
Marty Lustig
Member
- Location
- Prairie Village, KS
Type of meter to measure earth resistance?
Type of meter to measure earth resistance?
Type of meter to measure earth resistance?
Can one simply use a standard ohm-meter to measure earth resistance between electrodes? If the resistance is not linear with voltage applied to measure the resistance (and I don't know if it is linear or not, but I don't see any reason to assume linearity) it would seem there could be different readings if using different ohm-meters. Any thoughts? Practical experience?
kwired
Electron manager
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- NE Nebraska
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- EC
That gives you the resistance of the path between the two measured points. If you assumed both had identical resistance then you could say half your reading is resistance of one of the electrodes. They likely are not going to be equal.
wwhitney
Senior Member
- Location
- Berkeley, CA
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- Retired
If I have three grounding electrodes with ground resistances R1, R2, and R3, can I use a regular ohm-meter to determine the ground resistances via taking the 3 measurements of R1 + R2, R1 + R3, and R2 + R3, and then solving for the individual values? Or is there something about a regular ohm-meter that precludes its use in measuring the resistance of a circuit that includes a path through the earth?
Cheers, Wayne
Cheers, Wayne
You cannot use a regular ohm meter to test ground resistance.
When using the proper test equipment all 3 rods should be connected together before preforming the test. The 3 rods become one grounding electrode.
Note in the video Mike had one 50ft ground rod that measured 16.4 ohms and the other 50ft ground rod measured 21.7 ohms.
16.4 + 21.7 = 38.1 ohms / 2 = 19.05 ohms.
When mike tied both rods together and measured the two rods as one electrode he measured 10.8 ohms.
https://www.youtube.com/watch?v=Yg6G5VUSsWA
(Start at time marker 18:50)
oops, I did that wrong.
Parallel.
1/16.4 + 1/21.7 =
0.06097 + 0.04608 = 0.1070529
1/0.1070529 = 9.341 ohms
wwhitney
Senior Member
- Location
- Berkeley, CA
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- Retired
OK, why? If one ground rod has a ground resistance of 60 ohms, and the other ground rod has a ground resistance of 75 ohms, why won't my ohm meter read 135 ohms when I test between the two ground rods?
Cheers, Wayne
kwired
Electron manager
- Location
- NE Nebraska
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- EC
I think you will read DC resistance. AC impedance won't necessarily be the same value though.
My understanding of clamp on ground testers is they induce an AC voltage on the conductor being tested and at same time measure how much current flows. Knowing both those values it can determine the impedance. That method does need a loop for the induced current to flow through, the other end of the loop is through the earth back to other electrodes (presumably the multiple electrodes of the utility system and presumably at a very low impedance through those) leaving the bulk of the circuit impedance in the electrode being measured.
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- Retired PV System Designer
The voltage versus current curve will be linear unless there is a semiconducting surface layer.
But when the rods are within each other's zone of influence the R values you calculate will not correspond to the idealized resistances between each isolated electrode and "the earth". Which also means that you cannot get the resistance of the triad from those results.
If the three electrodes in question are miles apart and not near any conductive elements embedded in the earth your method might work.
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