CSLB C-10 Electrical ContractorSafety and TestingHard
A qualified electrical worker is performing a ground resistance test on a newly installed grounding electrode system at a commercial building. Using the fall-of-potential method with a three-point test instrument, which of the following is crucial for obtaining an accurate resistance reading?
- AEnsuring the soil is completely dry for minimal resistance.
- BApplying a high-frequency AC current from the test instrument to saturate the soil.
- CMaintaining a proper distance between the current probe, potential probe, and the grounding electrode.
- DPlacing the current and potential probes as close as possible to the grounding electrode under test.
Show answer & explanationAnswer & explanation
Correct answer: C. Maintaining a proper distance between the current probe, potential probe, and the grounding electrode.
For the fall-of-potential test, proper spacing of the probes is critical to avoid overlapping resistance areas and to ensure the potential probe is outside the spheres of influence of both the grounding electrode under test and the current probe. A common rule of thumb is to place the potential probe at 62% of the distance between the current probe and the electrode.
Why the other options are wrong
- A. Soil moisture affects resistance, but ensuring it's 'completely dry' is not a practical or crucial step for accuracy; the test measures existing conditions.
- B. Ground resistance testers use AC current, but the 'high-frequency' and 'saturate the soil' aspects are not the primary crucial factors for accuracy compared to proper spacing.
- D. Placing probes too close will result in inaccurate readings due to overlapping resistance areas.
Fall-of-Potential Test Spacing
The precise placement of current and potential probes in a ground resistance test to ensure accurate measurement of the grounding electrode's resistance to earth.
- Critical for accuracy
- Avoids overlapping resistance fields
- Potential probe typically at 62% distance
Memory trick: Fall-of-Potential: Space Probes Right!