Earth ground resistance is the impedance that the soil presents to current flowing from a grounding electrode into the earth. Low resistance means fault currents flow freely — protective devices operate quickly, and the electrode safely dissipates lightning energy. High resistance means the ground system is not functioning effectively — protective devices may not operate, and lightning energy may not be safely dissipated. Singapore's CP 33 (lightning protection) and SS 638 (electrical installation) set maximum acceptable earth resistance values, and testing is required at commissioning and periodically thereafter.
Equipment Required
- Fluke 1625-2 GEO Earth Ground Tester (or Fluke 1630 for stakeless testing only)
- Calibration certificate for the tester
- Test stakes (spike electrodes) — 2× for fall-of-potential method
- Test leads — colour-coded (green for E/Earth, yellow for P/Potential, red for C/Current)
- Measuring tape — for laying out test stake distances
- Mallet or hammer for driving stakes into Singapore soil
- Safety barriers and warning signs
- Digital multimeter for auxiliary checks
Method 1: Fall-of-Potential (3-Pole) — Most Accurate
Use this method when you can access open ground (gardens, unpaved areas, car parks with soil access) near the electrode under test.
Step 1: Determine Stake Spacing
The current probe (C) stake must be driven at a distance of at least 5× the depth of the grounding electrode (or 5× the equivalent radius of an electrode system). For a single 1.8 m driven rod: minimum C distance = 5 × 1.8 = 9 m from the electrode. The potential probe (P) is placed midway between E and C — at 4.5 m in this example. For a full earth grid (typical at Singapore substations), C distance must be much larger — consult the ground resistance tester manufacturer or IEEE Std 81.
Step 2: Drive Test Stakes
Drive the C stake (red) to the determined distance in line with the electrode under test (E). Drive the P stake (yellow) midway between E and C. All three points should be in a straight line where possible — deviations cause measurement errors. In Singapore's urban environment, this may require using a garden or green area adjacent to the building being tested.
Step 3: Connect and Measure
Connect the Fluke 1625-2's green lead to the E terminal and the electrode under test. Connect the yellow lead to the P terminal and P stake. Connect the red lead to the C terminal and C stake. Select "3P Fall of Potential" on the 1625-2. Press the test button and hold until the reading stabilises (typically 2–5 seconds). Record the reading in ohms (Ω).
Step 4: Verify with Additional Readings
Move the P stake to 40% and 60% of the E-to-C distance and repeat. If the three readings (40%, 50%, 60%) are within 2–3% of each other, you are in the "flat region" of the potential curve and the result is reliable. If readings vary significantly, the C stake may be too close to the electrode (mutual coupling) — increase the E-to-C distance and repeat.
Method 2: Selective (Stakeless) — For Multi-Electrode Systems
Use this method when multiple ground rods are installed in parallel (typical for Singapore substations and commercial buildings with multiple earth rods).
- Connect E and P terminals of the 1625-2 to the ground electrode connection point (where the electrode bonding conductor connects to the electrical installation earth bar).
- Select "Selective" mode on the 1625-2.
- Clamp the current clamp (supplied with 1625-2) around the grounding conductor of the specific electrode under test — not around the whole bonding bar.
- Drive one stake (P) into the soil near the electrode.
- Measure — the 1625-2 displays the resistance of the single electrode under test, isolated from the parallel electrode network.
Method 3: Clamp-On (Stakeless) — For Paved Urban Singapore
Use this method when no soil access is possible — typical in Singapore urban buildings where all surroundings are paved.
- Confirm the electrode system has multiple ground rods or a bonded grid (minimum two paths to earth). Clamp-on testing will not work for single isolated electrodes.
- Select "Stakeless" mode on the Fluke 1625-2 (or use the Fluke 1630).
- Clamp the tester jaw around the ground conductor of the electrode under test.
- Press test and record the reading. This measures the resistance of the single electrode in parallel with all other electrodes in the system — not the combined system resistance.
For a combined system resistance result in stakeless mode, use R_system = R_individual / N (where N is the number of electrodes of equal resistance in parallel). This is an approximation — fall-of-potential gives the true system resistance.
Acceptance Criteria for Singapore
| Application | Standard | Maximum Earth Resistance |
|---|---|---|
| Lightning protection | CP 33 | ≤ 10 Ω |
| General electrical system earth | SS 638 | ≤ 1 Ω (main earth terminal) |
| IT/Telecommunications equipment | BICSI / equipment spec | ≤ 0.5–1 Ω |
| Substation earth grids | SP PowerGrid specs | ≤ 1 Ω |
Documentation Requirements
Record: date and time of test, weather conditions (post-rain soil is more conductive — note if recent rainfall), instrument model and calibration certificate number, test method used, stake distances (if fall-of-potential), electrode description and location, and measured resistance value with pass/fail against applicable standard. CP 33 requires test records be kept for the lifetime of the lightning protection system and be available for SCDF inspection.