Measuring Earth Electrode Resistance — 3-Point Method and the TT Rule of Thumb

In a TT system (own earth electrode, no direct connection to the PEN of the network), the resistance of that earth electrode is decisive for the operation of the RCD protection. This guide covers the practical measurement of that earth electrode resistance (RA) and the calculation rule that determines whether a measured value is acceptable.

The TT rule of thumb

IEC 60364-4-41 sets the following requirement for TT systems:

$R_A \times I_{\Delta n} \leq 50\ V$

• RA — resistance of the earth electrode plus protective conductor (Ω)
• I∆n — rated tripping current of the RCD (A)
• 50 V — the maximum permitted touch voltage in the event of a fault

Filled in for common RCD values:

Practical limit: 200 Ω

Although the calculated limit for a 30 mA RCD theoretically allows up to almost 1667 Ω, in practice a much stricter guideline value is used: an earth electrode with a resistance above approximately 200 Ω is considered unstable — the resistance of an earth electrode varies with soil moisture, temperature, and season, and a high baseline value leaves little margin for that natural fluctuation. The target is therefore RA ≤ 200 Ω for RCDs up to and including 100 mA, well below the theoretical calculated limit.

The 3-point method (fall-of-potential)

The standard measurement method for earth electrode resistance:

1. Place two auxiliary electrodes in a straight line from the earth electrode being measured, at sufficient distance to prevent mutual interference between the electric fields.
2. Pass a known test current between the electrode being measured and the furthest auxiliary electrode.
3. Measure the voltage between the electrode being measured and the intermediate auxiliary electrode; the resistance follows from this via Ohm's law.
4. Repeat the measurement at least 3 times with the intermediate auxiliary electrode at different positions.
5. The measurements are reliable if they are within approximately 20% of each other; the average of these measurements is the final RA value.

Common mistakes

1. Placing auxiliary electrodes too close to each other or to the main electrode — insufficient distance causes the electric fields to overlap, producing an unreliable (often too low) measured value.
2. Taking only a single measurement instead of measuring at least 3 positions and averaging — a single measurement can deviate from the actual value due to local soil variation.
3. Using the theoretical calculated limit (for example 1667 Ω at 30 mA) as the practical standard — the practical guideline value of approximately 200 Ω accounts for seasonal fluctuations that the calculated limit ignores.
4. Confusing earth electrode resistance measurement with loop impedance measurement (Zs) — one specifically measures the electrode-to-earth resistance in a TT system, the other measures the full fault current loop including the network; these are different measurements with an overlapping but not identical purpose.

Related

• §411 — Protective Conductor & Earthing
• Loop Impedance Measurement (Zs)
• §708 — Camping and Caravan Sites