CSLB C-10 Electrical ContractorSafety and TestingMedium
A qualified electrical worker is performing a ground resistance test using the Fall-of-Potential method. The current electrode (P2) is placed 100 feet from the ground electrode under test, and the potential electrode (P1) is placed at 62 feet. If the meter reads 15 ohms, what is the ground resistance?
- A6.2 ohms
- B100 ohms
- C15 ohms
- D150 ohms
Show answer & explanationAnswer & explanation
Correct answer: C. 15 ohms
In the Fall-of-Potential method, the meter directly measures the ground resistance at the potential electrode's position. The given reading of 15 ohms is the ground resistance. The distances are provided to confirm the setup is within typical guidelines for accurate measurement (P1 at 62% of P2 distance for optimal results).
Why the other options are wrong
- A. This value is not directly obtained from the meter reading in this scenario.
- B. The 100-foot distance is for the current electrode, not the resistance reading.
- D. This is an arithmetic manipulation that does not apply to how the Fall-of-Potential meter provides a reading.
Fall-of-Potential Test Reading
The ground resistance reading obtained from a Fall-of-Potential test is directly displayed by the test instrument when the potential electrode is positioned correctly.
- Meter directly measures resistance.
- Electrode spacing is critical for accuracy.
- No calculation required from the meter's reading itself.
Memory trick: The meter's number is the answer, no tricks!