3 point fall of potential test question
- Thread starter PowerdT
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ATSman
ATSman
- Location
- San Francisco Bay Area
- Occupation
- Electrical Engineer/ Electrical Testing & Controls
Are you referring to the 3 point Fall of Potential test using a Ground Resistance Test Set?
Yes
It’s Megger DET3EC earth ground resistance tester
- Location
- Illinois
- Occupation
- retired electrician
No matter what type of system you are talking about, it always goes back to the source. If there is no path to the source, there is no circuit and no current flow.
ATSman
ATSman
- Location
- San Francisco Bay Area
- Occupation
- Electrical Engineer/ Electrical Testing & Controls
The GROUND resistance tester you mentioned applies a low DC voltage to the area of the earth being tested then measures the current that flows between the probes. With the voltage and current known it uses Ohm's law and calculates the resistance of the earth (Ohms.) Not sure what you mean by voltage spike. You may be confusing this tester with the "Megger" insulation tester that roger mentioned that sends out a high voltage to measure the INSULATION resistance between conductors (that also uses Ohm's law) and indicates Meg-Ohms (1,000,000 Ohms.)
I think I am mixed up with the Megger insulation. However, you answered my question. Another question related to this.
When you measure earth electrode resistance, would that make big difference on the test result when the rod is connected and disconnected?
ptonsparky
Tom
- Occupation
- EC - retired
Connected to what?
ATSman
ATSman
- Location
- San Francisco Bay Area
- Occupation
- Electrical Engineer/ Electrical Testing & Controls
Yes of course it will make a difference. The current only flows when the rod is connected to the test set probes. If disconnected there is no current flow and open circuit means no current and no measurement.
Although this is not the manual for your instrument, this is the benchmark on ground testing theory by Biddle (now owned by Megger/ AVO.)
Although this is not the manual for your instrument, this is the benchmark on ground testing theory by Biddle (now owned by Megger/ AVO.)
Getting Down to Earth: A Manual on Earth-Resistance Testing for the Practical Man : Biddle Instruments : Free Download, Borrow, and Streaming : Internet Archive
Getting Down to Earth... a Manual on Earth-Resistance Testing for the practical man. Electrical grounding systems, Earth resistivity. Fourth Edition, April...
archive.org
hillbilly1
Senior Member
- Location
- North Georgia mountains
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- Owner/electrical contractor
I believe the op is talking about the electrode that is driven into the ground, not a “voltage spike” as in actual voltage. Yeah, “Megger” has been synonymous with insulation testers, as that is what they are famous for, but they do make other products that are not as well known.
Sorry I was not clear. Let’s see an example of earth electrode resistance test using 3 point fall.
We have service entrance panel with GEC to the rod. We want to test the resistance of the building from service entrance.
You have 2 stakes, probes, everything connected and you test the rod at the service entrance and result 0.5ohms. Call it a day!
However, I believe the requirement to test resistance using 3 point fall is by disconnecting the service entrance rod, if this is correct, would it make big difference on the resistance at the rod?
Joethemechanic
Senior Member
- Location
- Hazleton Pa
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- Electro-Mechanical Technician. Industrial machinery
Are you asking if you shoild test your ground electrode with the electrode connected to the panel? If the electrode is connected to the panel, and there is an intact neutral-ground bond, you will have other grounding electrodes coming into play because of the connection to the multi grounded neutral
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
For your future reference, we perhaps improperly use “megger” to mean megometer like bandaid. You used it to represent brand. You used it correctly, but we would not likely understand.
- Location
- Bremerton, Washington
Yes you must disconnect the electrode under test
There is a clamp on ground resistance tester where the electrode goes not have to be disconnected
There is a clamp on ground resistance tester where the electrode goes not have to be disconnected
Yea I figured. I didn’t realize that till now
ATSman
ATSman
- Location
- San Francisco Bay Area
- Occupation
- Electrical Engineer/ Electrical Testing & Controls
Ok, it really depends on how the spec is written and what type of grounding system you have. It can be a building with separate ground rods driven around the perimeter of the foundation and bonded together and connected to the GEC that stub-ups at the service entrance. Or can be the separate ground rods connected to a Ufer Ground. A Ufer ground is a steel mat made of re-bar and built into the concrete foundation of the building. The spec may state that all components have to be disconnected and individual ground measurements taken or may just want one measurement with the whole system connected. Can you post a copy of the testing spec?
https://www.google.com/search?q=ufe...QABiABBixA8ICCxAuGIAEGMcBGK8B&sclient=gws-wiz
ATSman
ATSman
- Location
- San Francisco Bay Area
- Occupation
- Electrical Engineer/ Electrical Testing & Controls
This is so typical of the posts on MH forums. When taking the time to try and help people, they never respond back. 
Sorry for late reply. It’s from the earth ground tester manufacturer
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ATSman
ATSman
- Location
- San Francisco Bay Area
- Occupation
- Electrical Engineer/ Electrical Testing & Controls
Not what I was asking for. The job contract submittals or specifications describing what tests are required to be performed.