11.4 Earth Electrode Resistance Testing

11.4 Earth Electrode Resistance Testing

Core Testing Principle: A TT installation normally relies on its earth electrode and an RCD for fault protection. Verify the earthing, applicable resistance criterion and required disconnection time together. The criterion does not clamp actual fault touch voltage to 50 V before the protective device operates.

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Earth Electrode Systems in TT Installations & Generation

Under BS 7671 Regulation 542.2, acceptable earth electrode configurations include:

• Earth Rods: Copper-bonded high-tensile steel core rods (typically 15 mm or 16 mm diameter, driven in 1.2 m or 1.5 m extendable sections).
• Earth Plates & Lattice Mats: Copper plates buried vertically or horizontally in high-resistivity or rocky ground.
• Earth Tapes & Rings: Bare copper tape (25 mm × 3 mm) buried horizontally in perimeter trenches at depths of 0.5 m to 1.0 m.
• Foundation Earth Electrodes: Structural metalwork or reinforcement steel bars encased in concrete foundations (BS 7671 Regulation 542.2.2).

The Nature of Earth Resistance

The electrode resistance ($R_E$) comprises three physical components. For the TT protection assessment, $R_A$ additionally includes the protective-conductor resistance:

1. Electrode Material & Connection: The metallic resistance of the copper rod, cable lug, and connection clamp (typically negligible, < 0.01 Ω).
2. Contact Resistance: The interface resistance between the metal surface of the rod and the immediately adjacent soil compacted against it.
3. Soil Spreading Resistance: The resistance of the concentric hemispherical shells of soil surrounding the rod extending outwards into the earth mass. As distance from the rod increases, the available conducting area grows and the additional resistance of successive soil layers decreases. Most resistance is near the electrode; the simplified resistance-area model has no sharp physical boundary at which soil resistance suddenly becomes zero.

  EARTH ELECTRODE RESISTANCE SHELLS (SPHERE OF INFLUENCE)

               [ Earth Rod Driven into Ground ]
                            │
               ═════════════╪═════════════  Ground Level
                  ( ( ( ( ( │ ) ) ) ) )
                ( ( ( ( ( ( │ ) ) ) ) ) )
               ( ( ( ( ( ( (│ ) ) ) ) ) ) )
               Shell 1    Shell 2    Shell 3
               (High R)  (Medium R)  (Low R ──► Negligible incremental R)
               ◄──────── Sphere of Influence ────────►

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Method 1: The 3-Terminal Fall-of-Potential Test (BS EN 61557-5)

The fall-of-potential method is an established way to measure an electrode separately. 643.7.2 requires the relevant measurement; BS EN 61557-5 sets test-equipment requirements rather than prescribing only this arrangement or one spike geometry.

Dedicated Test Instrument Characteristics

Use a suitable earth-resistance tester and its specified leads/connections. Instruments normally use AC at a selected frequency to limit interference and avoid electrode polarisation. Follow the maker's procedure and assess interference or excessive test-spike resistance rather than assuming every tester uses one frequency range.

  FALL-OF-POTENTIAL (3-TERMINAL) TEST CONFIGURATION

      Disconnection
      Clamp OPEN
        │
        ▼          P2 Potential Spike               C2 Current Spike
     [  E  ] ─────── (At 61.8% of D) ───────────────── (At Distance D)
     [ Rod ]
        │                    ▲                                ▲
        │                    │                                │
        │ E / C1 / P1        │ P2                             │ C2
     ┌──┴────────────────────┴────────────────────────────────┴──┐
     │           Dedicated Earth Electrode Tester                │
     └───────────────────────────────────────────────────────────┘
     ◄─────────────────────── Distance D (30 m - 50 m) ──────────►

Step-by-Step Testing Procedure

1. Make disconnection safe: Isolate and secure the installation supplied by the electrode before interrupting its earthing connection. Identify other sources and possible diverted/parallel current; a main switch OFF position alone does not make an earthing or bonding conductor safe to disconnect. Follow the Chapter 2 precautions and stop if safe conditions cannot be established. Separate the electrode from parallel installation paths at the appropriate test link/connection, then connect the tester to the electrode side. Reconnect and verify earthing before restoring supply.
2. Position the Spikes:
   • Connect instrument terminal $E$ (or $C_1/P_1$ combined) to the isolated earth electrode.
   • Drive the Current Spike ($C_2 / H$) into the ground at a distance $D$ from the electrode in a straight line (typically 30 to 50 metres).
   • Drive the Potential Spike ($P_2 / S$) into the ground along the exact direct line between the earth electrode and $C_2$, at a distance equal to 61.8% of $D$ ($0.618 \times D$, based on golden ratio electrical field distribution).
3. Perform the Three-Point Verification:
   • Reading 1 ($R_{\text{nom}}$): Measure resistance with $P_2$ at the nominal 61.8% position (e.g. 31 m for $D = 50$ m).
   • Reading 2 ($R_{-10\%}$): Relocate spike $P_2$ 10% of $D$ closer to the electrode ($51.8\% D$, e.g. 26 m) and measure.
   • Reading 3 ($R_{+10\%}$): Relocate spike $P_2$ 10% of $D$ further towards $C_2$ ($71.8\% D$, e.g. 36 m) and measure.
4. Evaluate the Resistance Plateau:
   • Compare the three readings using the instrument procedure’s specified tolerance or accuracy calculation. Substantially agreeing readings support a flat part of the resistance curve for that method; this is not a zero-potential point.
   • Record the electrode resistance determined by the procedure, using the mean of substantially agreeing readings where specified. For the TT assessment, $R_A$ also includes the protective-conductor resistance to the exposed parts.
   • If the readings continuously rise without a plateau, the distance $D$ is too short; the resistance area of the electrode overlaps the resistance area of $C_2$. Increase $D$ to 70 m or 100 m and repeat.

  FALL-OF-POTENTIAL: CHECK FOR A FLAT RESISTANCE REGION

  Measured resistance
       ^                                    /  approaching C2
       |                                   /
  Re   |          ________________________/
       |         /
       |        /   readings around the selected potential-spike position
       |_______/
       +--------------------------------------------------> P2 distance
       Rod        about 52%D   62%D   72%D               C2 at D

  No satisfactory flat region: increase separation and repeat the checks.

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Method 2: Earth Fault Loop Tester Method (TT Systems)

Where driving test spikes into the surrounding ground is physically impossible (e.g., dense urban environments, asphalt pavements, multi-storey city centres), BS 7671 Regulation 643.7.2 permits an alternative method: using an Earth Fault Loop Tester.

What the Loop Measurement Includes

The measured TT loop includes the source/line path, the earth return through the source electrode and the consumer electrode, and the relevant protective connection. It is not a pure electrode-resistance measurement. Account for the source contribution: the IET lists 21 Ω as a typical declared TT external value excluding the consumer electrode.

With the outgoing installation isolated and secured, the competent tester can arrange a controlled live test between the supply line and the separately connected electrode conductor, following the instrument instructions. Establish safe conditions before disconnecting that conductor from the MET, exclude parallel paths, and restore/verify earthing before energising the installation. Opening the main switch alone does not remove bonded-pipe paths. With parallel paths excluded and a suitable test arrangement, the measured loop can provide a conservative assessment of the electrode/protective path because it includes additional series impedance. Label and record the actual method and quantity. In this terminology $Z_e$ excludes the consumer electrode; the complete measured loop includes it.

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MaximumAcceptable Earth Electrode Resistance Values ($R_A$)

TT earth-loop impedance often makes overcurrent protection alone unable to achieve the required disconnection time. An RCD is therefore normally used. Where an overcurrent device provides fault protection, its actual loop/disconnection requirements must instead be satisfied.

RCD Protection under 411.5.3

For the RCD arrangement, verify the required disconnection time and:

$R_A\times I_{\Delta n}\le50\text{ V}$ $R_A\le\frac{50\text{ V}}{I_{\Delta n}}$

Here $R_A$ includes the electrode and protective-conductor resistance to the exposed-conductive-parts, and $I_{\Delta n}$ is the rated residual operating current in amperes. This is the BS 7671 criterion; it does not guarantee a maximum actual fault touch voltage of 50 V. The exposed parts can rise to a much higher potential until the protective device disconnects. Apply any relevant special-location requirements as well.

Values from the Resistance Criterion

Stability: The 200 Ω Note

Table 41.5 says electrode resistance should be as low as practicable and a value above 200 Ω may not be stable. Soil drying and freezing must not raise the resistance above the required value. A single acceptable measurement in damp conditions does not establish year-round suitability.

Assess electrode depth, soil/seasonal conditions, connections and the applicable protection. A value above 200 Ω prompts that assessment; it is not automatically declared unsafe by a universal 200 Ω pass/fail rule. Nor does BS 7671 impose a universal ideal 100 Ω target for every TT installation.

Improving an Unsuitable Electrode Arrangement

Where assessment shows inadequate resistance or stability, options include a deeper electrode, additional suitably spaced rods, or a different electrode/backfill arrangement suited to the ground and corrosion conditions. Extra rods close together have overlapping resistance areas and give less improvement than well-separated rods. Select a suitable arrangement, obtain necessary permissions, check buried services and retest against the actual protection and stability requirements.