WiremanWireman · Measure ground resistance: troubleshooting for electricians · 2023 NEC ·
Measure ground resistance
Fall-of-potential with stakes, or a clamp-on tester, and what each can and can’t tell you.
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How it works
A grounding electrode’s resistance is how hard it is for current to spread from the rod into the earth around it. You can’t measure it with a multimeter; the earth needs a test current pushed through it to a far reference. Fall-of-potential uses stakes driven in a line from the rod. A clamp-on tester induces a test current around a loop, so it only works where the electrode is one of many parallel grounds, like a service on a utility multi-grounded neutral.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What tester do you have?
- 3-pole or 4-pole tester with stakesCan the electrode be disconnected from the electrical system for the test?
- Yes, it can be isolatedTest: Run the fall-of-potential testExpect: The three readings agree closely.
- The three readings agreeWhat did it read?
- 25 Ω or lessResistance is 25 Ω or less
- More than 25 ΩResistance above 25 Ω
- Readings change a lot as P2 movesStakes are too close: no flat spot
- The three readings agreeWhat did it read?
- No, it stays connectedTest: Use the clamp-on testerExpect: A reading in ohms with little current on the conductor.
- A reading in ohmsWhat did it read?
- 25 Ω or lessResistance is 25 Ω or less
- More than 25 ΩResistance above 25 Ω
- Very low, under about 1 ΩYou measured a metal loop, not the earth
- Open or very highNo return path, or a broken connection
- Significant current on the conductorCurrent on the grounding electrode conductor
- A reading in ohmsWhat did it read?
- Yes, it can be isolatedTest: Run the fall-of-potential testExpect: The three readings agree closely.
- Clamp-on ground testerTest: Use the clamp-on testerExpect: A reading in ohms with little current on the conductor.
- A reading in ohmsWhat did it read?
- 25 Ω or lessResistance is 25 Ω or less
- More than 25 ΩResistance above 25 Ω
- Very low, under about 1 ΩYou measured a metal loop, not the earth
- Open or very highNo return path, or a broken connection
- Significant current on the conductorCurrent on the grounding electrode conductor
- A reading in ohmsWhat did it read?
- Only a multimeterA multimeter can’t measure earth resistance
- 3-pole or 4-pole tester with stakesCan the electrode be disconnected from the electrical system for the test?