Mike Dolt Enterprises
Mike Dolt Enterprises
User: sfav8r, Location: San Francisco Bay Area
Asked on: Saturday, April 9, 2005 at 1:10 am EDT
Answered on: Thursday, May 9, 2024 at 12:46 pm EDT
Question: I've been looking into these meters. How exactly do they work? I have not found any good documentation explaining how it is possible to check the resistance of a ground rod by simply clamping around the conductor.The really puzzling part is that you don't need to disconnect the secondary ground (ie: water pipe) to measure the resistance of the ground rod to earth.Any thoughts? Any links to good articles?FYI I am looking at the Amprobe DGC-1000 and the AEMC 3711.Links to both meters are below.AmprobeAEMC 3711THANKS!
Operation Principle of Clamp Ground Resistance Meters
Clamp ground resistance meters utilize the principle of loop impedance measurement. They measure the total loop impedance between the ground rod and a secondary ground reference (e.g., water pipe or building steel) without the need for disconnecting any connections.
Simplified Circuit DiagramProcedure
1.
Clamp the Meter
Open the clamp and wrap it around the ground conductor (ground rod or bonding wire).2.
Connect Reference Probe
Insert the reference probe into the secondary ground.3.
Measure Loop Impedance
The meter injects a small alternating current (AC) signal into the loop and measures the resulting voltage drop.
Calculation of Ground Resistance
The ground resistance (Rg) is calculated using Ohm's Law:
where: V is the voltage drop across the loop I is the current injected into the loopRg = V / I
Measurement Technique
The loop impedance measured by the clamp is not the same as the ground resistance. The meter compensates for this difference by applying a correction factor based on the length of the ground rod and the distance between the reference probe and the ground rod.
Benefits of Clamp Ground Resistance Meters
Non-invasive: No need to disconnect connections or drive additional ground rods. Quick and convenient: Can be used on energized or de-energized conductors. Measures total loop impedance, providing a more accurate representation of the grounding system performance.
Additional Notes
Influence of Secondary Ground
The accuracy of the measurement can be affected by the resistance of the secondary ground. It should be as low as possible.
Calibration
Clamp ground resistance meters must be regularly calibrated to ensure accurate readings.
Recommended Articles: [Electrical Grounding Resistance Testing with Clamp-On Meters](https://cdn.amprobe.com/literature/Application-Notes/AP70260_EGRT_Clamp_On_App_Note.pdf)* [Ground Resistance Clamp-On Meters: Troubleshooting Applications](https://www.aemc.com/catalog/assets/files/grounding-and-bonding/Ground_Resistance_Clamp-On_Meters_Trouble_Apps.pdf) Flag for review