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Earth Electrode Resistance Acceptance Criteria — Field Reference

2026年06月08日

How low is low enough? Acceptance criteria vary by application. Here is the field reference.

The Question Every Earthing Job Ends With

After driving the electrode and running the test, the question is always the same: is this value acceptable? Earth electrode resistance acceptance criteria are not a single universal number — they depend on the application, the protective system, and local code. EK engineers compiled this field reference so you can answer with confidence on site.

1. Typical Acceptance Values by Application

  • General building / domestic installation: ≤ 100 Ω (TT system, depends on RCD).
  • Commercial / industrial earthing: ≤ 10 Ω is a common practical target.
  • Substation earthing grid: ≤ 1 Ω, often ≤ 0.5 Ω for large grids.
  • Lightning protection system: ≤ 10 Ω per BS EN 62305.
  • Telecom / sensitive electronics: ≤ 5 Ω, sometimes ≤ 1 Ω.

2. Why the Criterion Depends on the System

In a TT system protected by an RCD, the earth-fault loop must allow the RCD to trip — so the acceptable resistance is tied to the RCD rating. In a substation, the criterion is touch-and-step voltage safety during a fault, which drives the value far lower.

3. Measurement Method Matters

  1. Fall-of-potential (3-pole / 4-pole) is the reference method for a standalone electrode.
  2. Clamp-on (stakeless) is fast for verifying electrodes in a meshed system without disconnection.
  3. Soil resistivity (Wenner 4-pole) predicts achievable resistance before installation.
  4. The EK-C300 supports all three methods in one instrument.

4. Seasonal and Soil Effects

The same electrode can read 5 Ω in wet spring and 25 Ω in dry summer. Acceptance testing should account for the worst-case dry season, or apply a seasonal correction factor documented in the report.

5. Documenting Compliance

  • Record the method, the measured value, and the applicable criterion.
  • Note soil condition and season.
  • State the verdict against the criterion.
  • Attach the instrument calibration certificate reference.

EK Recommendation

For acceptance testing across all these applications, the EK-C300 earth resistance tester provides fall-of-potential, clamp-on, and soil resistivity in one traceable instrument. Talk to EK service teams.