CSLB C-10 Electrical ContractorSafety and TestingMedium
During an electrical installation in a commercial building, a qualified electrician needs to verify that the newly installed grounding electrode system has sufficiently low resistance to earth. Which test method is most commonly used and required by industry standards for this purpose?
- AFall-of-Potential (Three-Point) Test
- BTwo-Point (Dead Earth) Test
- CClamp-on Ground Resistance Test
- DContinuity Test with an Ohmmeter
Show answer & explanationAnswer & explanation
Correct answer: A. Fall-of-Potential (Three-Point) Test
The Fall-of-Potential (Three-Point) Test is the most accurate and widely accepted method for measuring the resistance of a grounding electrode system to earth, as it effectively nullifies the resistance of the test leads and auxiliary electrodes.
Why the other options are wrong
- B. The Two-Point test is less accurate and typically used for continuity or bonding, not electrode-to-earth resistance.
- C. Clamp-on ground resistance testers are convenient but measure loop resistance, not true individual electrode-to-earth resistance, and require a complete ground loop.
- D. A continuity test with an ohmmeter measures resistance within a conductor or component, not the resistance of an electrode to the general mass of earth.
Fall-of-Potential Test
A precise method for measuring the resistance of a grounding electrode to earth using three test probes, commonly known as the Three-Point Test.
- Requires disconnecting the electrode from the system.
- Uses two auxiliary electrodes (current and potential).
- Measures earth resistance by varying the distance of the potential probe.
Memory trick: Fall of Potential: The Gold Standard for Ground.